Benthic Macroinvertebrate
Microhabitat Requirements
and Trophic Structure in
Southeastern Streams:
A Literature
Synthesis
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June 1992
Fisheries and Allied Aquacultures Departmental Series No. 3
Alabama Agricultural Experiment Station
Lowell T. Frobish, Director
Auburn University
Auburn University, Alabama
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Benthic Macroinvertebrate Microhabitat
Requirements and Trophic Structure in
Southeastern Streams: A Literature Synthesis
E. Cliff Webber
Michael R. Struve
David R. Bayne
Department of Fisheries and Allied Aquacultures
June 1992
Alabama Agricultural Experiment Station
Auburn University, Alabama 36849
Lowell T. Frobish
Director
Auburn University
Alabama 36849

CONTENTS
Page
INTRODUCTION ................................................... 3
SOUTHEASTERN STREAMS ........................................... 3
The River Continuum and Southeastern Streams ........................ 5
Floodplain Dynamics ............................................. 9
MICROHABITAT REQUIREMENTS OF BENTHIC MACROINVERTEBRATES .. 10
Influence of Local Abiotic Factors ................................. 10
Temperature ............................................ 10
Substrate ............................................. 11
Current Velocity ........................................ 12
Future Needs ............................................ 13
Responses of Benthic Macroinvertebrates to Discharge Fluctuations ......... 13
Future Needs ............................................ 15
Movements of Benthic Macroinvertebrates ............................ 16
Future Needs .......................................... 17
Life Histories of Benthic Macroinvertebrates ........................... 18
Future Needs .......................................... 19
KEY INDICATORS AND HABITAT GUILDS ............................ 19
Future Needs .............................................. 20
TROPHIC RELATIONSHIP BETWEEN FISH AND MACROINVERTEBRATES ... 21
Future Needs ................................................. 22
BENTHIC MACROINVERTEBRATES IN REGULATED RIVERS .............. 22
Observed Effects ................................................ 22
Underlying Mechanisms .......................................... 22
Macroinvertebrate Communities ................................... 24
Future Needs ................................................. 24
ACKNOWLEDGEMENTS ............................................. 25
LITERATURE CITED ................................................ 26
FIRST PRINTING, JUNE 1992
Information contained herein is available to all persons without
regard to race, color, set, or national origin.

INTRODUCTION
A variety of factors regulates the occurrence
and distribution of stream-dwelling inverte-
brates. The most important of these are current
velocity, temperature (including the effects of
altitude and season) the substratum and dis-
solved substances (Hynes 1970). Other impor-
tant factors include the effects of droughts and
floods, food, competition between species,
predation, shade, depth, and zoogeography.
However, sorting out the relative roles of the
biotic and abiotic factors that determine the
distribution and abundance of stream macroin-
vertebrates remains a complicated task.
Recent studies have emphasized the impor-
tance of stream hydraulic forces in regulating
invertebrate communities. Investigations have
included attempts to predict both the distribu-
tion and abundance of invertebrates by measur-
ing the heterogeneity of flow characteristics
within a stream reach (Statzner and Higler 1985;
review by Statzner and Higler 1986; Davis 1986;
Smith-Cuffney and Wallace 1987; reviews by
Gore 1987, 1989; Statzner et al. 1988; Davis and
Barmuta 1989; Heede and Rinne 1990; Gore et
al. 1992). Organismic responses have been
linked to stream hydraulics, within whole catch-
ments or within reaches of different types of
lotic waters (Statzner and Higler 1986). In fact,
many of the factors listed above have been
incorporated into models designed to predict
invertebrate community structure (i.e., distribu-
tion, abundance, diversity) in streams and rivers
(Gore 1978; Gore and Judy 1981; Peckarsky
1983; Wright et al. 1984; Armitage 1989).
The purpose of this paper is to review,
evaluate, and synthesize the scientific literature
on the following topics: (1) microhabitat prefer-
ences of benthic macroinvertebrates in warm-
water streams of the Southeastern United
States; (2) use of macroinvertebrates as key
indicators for assessing instream flow needs in
the Southeast; (3) trophic relationships between
fish and macroinvertebrates in Southeastern
streams; and, (4) macroinvertebrate responses to
stream regulation, particularly in the Southeast.
Our intent was to provide an exhaustive cover-
age of peer-reviewed studies conducted in the
Southeast, and a representative and selective
examination of the "gray" literature (i.e., techni-
cal reports and papers from state and federal
agencies).
The term macroinvertebrate, by definition,
refers to aquatic invertebrates retained on a U.S.
No. 30 sieve (mesh size = 0.595 mm) (Weber
1973). However, the term has also been applied
to organisms retained on sieves smaller than the
No. 30 (e.g., see Hudson and Nichols 1986).
When citing specific findings of researchers,
their terminology is used, but with observations
of a more general nature, the terms invertebrate
and macroinvertebrate are occasionally used
interchangeably.
SOUTHEASTERN STREAMS
The Southeastern United States includes six
physiographic provinces, figure 1: Coastal Plain,
Piedmont, Blue Ridge, Valley and Ridge, Appa-
lachian, and Interior Low Plateaus (Fenneman
1946). In the hills of the Piedmont province,
elevations range from about 366 m near the
mountains to about 183 m at the Fall Line
where the Piedmont merges with the Upper
Coastal Plain. Here the valleys and slopes are
often steep as streams and rivers descend
through the Fall Line.
Stream gradients north of the Fall Line may
range as high as 28 percent (Huryn and Wallace
1987a, 1987b), but only in headwater streams at
the higher elevations in the Appalachian Moun-
tains. More typical gradients range from just
under 1 percent to about 4 percent as found in
the Piedmont (Gordon and Wallace 1975;
Wallace et al. 1977). The substratum in these
streams is generally heterogenous, consisting
predominantly of boulder, cobble, and gravel,
with occasional outcrops of bedrock (Tebo and
Hassler 1961; Malas and Wallace 1977; Huryn
and Wallace 1987a, 1987b) and macroinvertebr-
ate diversity may be high (Lenat 1987). Al-
though many streams north of the Fall Line are
warmwater streams, coldwater streams are
common at the higher elevations, especially in
the mountains, and some support trout popula-
tions (Tebo and Hassler 1961; Flecker and Allan
1984).
South of the Fall Line lies the Coastal Plain
where the topography is such that stream gradi-
ents are usually less than 0.1 percent (Smock et
al. 1989), although gradients in portions of the
Coastal Plain approach 1 percent (Gordon and
Wallace 1975). Substrates are predominantly
unstable, shifting sand (Benke and Wallace
Figure 1. Locations of the major physiographic provinces in the Southeastern United States.
1990) and streams are mostly warmwater, includ-
ing some described as subtropical (Cowell and
Carew 1976; Benke et al. 1984; Wallace and
Benke 1984; Benke and Jacobi 1986).
Distinguishing between coldwater and warm-
water streams is a recent practice that resulted
from the identification of different management
needs for various lotic ecosystems. Winger
(1981), in a review of physical and chemical
differences between the two stream types, stated
that streams with summer temperatures exceed-
ing 20*C generally contain warmwater fish
species. He characterized warmwater streams as
usually occurring at relatively low elevations and
having cool to warm water in summer, quiet
flows, high turbidities, more pools and fewer
riffles, substratum of smaller particle size, root-
ed and floating vegetation and sparse shade and
cover, table 1.
The River Continuum and Southeastern Streams
Traditionally, streams have been classified
based on drainage basin analysis or order
(Strahler 1957). Although this classification
system has limitations (Hughes and Omernik
1981), it provided the structural basis for one of
the most useful conceptual models of stream
ecosystems developed in the last decade, the
river continuum concept (RCC) (Vannote et al.
1980; Bruns et al. 1984; Minshall et al. 1985).
The RCC integrates predictable and observable
biological features of lotic systems from headwa-
ter streams to large rivers. Biotic and abiotic
variables, figure 2, of lotic ecosystems are cate-
gorized by stream size into small headwater
streams (orders 1-3), medium-sized streams, or
midreaches (orders 4-6) and large rivers (orders
> 6). Streams are viewed as longitudinally
linked systems in which ecosystem-level process-
es in downstream areas are linked to those in
upstream areas.
The RCC describes assemblages of organ-
isms, or functional groups rather than focusing
on individual taxa. Functional groups are
general categories into which invertebrates can
be grouped based on similar feeding mechanisms
(e.g. shredders, collectors, grazers, piercers,
engulfer-predators, and parasites) (Cummins
1978). The relative abundances of these groups
(biomass) are predicted to change from headwa-
ters to large rivers, figure 2, in response to
changes in the quantity and quality of particu-
late organic matter (POM) 
inputs (Cummins
1980).
Criticisms of several basic 
assumptions
underlying the RCC appear in 
the literature
(Winterbourn et al. 1981; Rounick and
Winterbourn 1983; Statzner and Higler 1985).
In general, criticism centers on the worldwide
application of the concept that macroinverteb-
rate communities in headwater streams are
dominated by large particle detritivores (shred-
ders) and downstream, functional feeding groups
change along a continuum as coarse particulate
organic matter (CPOM) is reduced in size to
fine particulate organic matter (FPOM).
Winterbourn et al. (1981) described headwater
streams in New Zealand in which shredders
were poorly represented, and the dominant
invertebrates were browsers that fed on FPOM
and stone-surface organic layers. They also
noticed little change in functional groups down-
stream and no temporal continuum of synchro-
nous species replacements as predicted by the
RCC. Rounick and Winterbourn (1983) also
found no shredders in unstable, poorly retentive
headwater streams in disturbed watersheds.
Minshall et al. (1985) pointed out that the
original presentation of the RCC clearly
accomodated these sorts of deviations. Minshall
et al. considered it unreasonable to expect
shredders to dominate if the CPOM supply is
unreliable or insufficient, thus whenever CPOM
enters a stream it is reduced to FPOM and this
activity generally is quantitatively most signifi-
cant in the headwaters of a river system.
Headwater streams in the Southeast are
typical of most in that they are generally influ-
enced by riparian vegetation that limits autotro-
phic production by shading and contributes large
amounts of allochthonous detritus as POM.
Most of the POM is leaf material, or CPOM,
but smaller quantities of FPOM also enter the
stream, especially from the surrounding flood-
plain (Georgian and Wallace 1983; Webster et
al. 1983; Smock and Roeding 1986; Roeding and
Smock 1989).
Woody debris also plays an important role in
low-order streams. Wallace et al. (1982),
Wallace and Benke (1984) and Golladay et al.
(1989) described several roles of woody debris:
dissipation of the stream's energy; retention of
allochthonous organic matter, that can influence
both trophic and nutrient dynamics; providing
Table 1. General Features of Warmwater and Coldwater Streams'.
Stream Type
Characteristic Coldwater Warmwater
Valley shape V U
Temperature Cold ( < 20
0
C) Cool - warm ( > 20'C)
Discharge Low 
Medium - high
Velocity Moderate (high turbulence) 
Moderate to high (low turbulence)
Depth Shallow Medium to 
moderate
Width 1 - 6 m > 3 
m
Substratum Rubble - gravel 
Rubble - sand - mud
Gradient High 
Low
Elevation High Low
Turbidity Clear Clear 
- turbid
Pools (riffles) Short (Many riffles) 
Long (few riffles)
Temporal variability High Low
Aquatic flora Periphyton Periphyton 
and macrophytes
Shade and cover Extensive Sparse
Organic material Coarse particulate organic 
Fine particulate organic matter
matter
Distance from source < 8 Km 
> 16 Km
Stream order Low ( < 3) 
Higher ( >3)
Competition Intraspecific 
Interspecific
Predatory fish Few 
Many
Fish community Salmonidae 
Centrarchidae
Ictaluridae
Catostomidae
Fish diversity Low 
High
SModified from Winger (1981).
METERS)
CPOM
wr
4 4~$~ PRODUCER IP/R 
<1
w (PERIPHYTON)
3 (4-6 
METERS) 
DCR
CU)A
MICROBE CO
W 4 (10METERS)
PRbO ERS 
P/R >1
Cf) RODUERSREDATORS
6 (50-75 METE )opEPHTN
w7
0 MICROBES CLETR
0
<4'
10 
V* COLLECTORS
11 
(ZOOPLANKTON)
2 (7 METERS)
Figure 2. Expected changes in particulate organic matter in parts 
and functional groups along the
river continuum (from Vannote et al. 1980).
habitat (cover) for invertebrates and fish; and,
food for some species of aquatic insects.
In response to heavy inputs of CPOM in
headwaters, shredders and collectors are predict-
ed to be codominant among invertebrate com-
munities (Vannote et al. 1980). Studies in
headwater mountain streams in North Carolina
generally support the predictions of the RCC
(Woodall and Wallace 1972; Haefner and
Wallace 1981; Webster et al. 1983; Huryn and
Wallace 1987a, 1987b). Macroinvertebrate
communities were dominated by shredders,
collectors, and predators, while grazers faired
poorly. However, in headwater mountain
streams in Virginia, Miller (1985) reported that
shredders were only a minor part (<10 percent)
of the total macroinvertebrate biomass and that
collectors and predators dominated the fauna.
In addition, Miller (1985) found similar results
in reviewing ten other studies that were con-
ducted in small temperate zone streams between
1978 and 1984.
Huryn and Wallace (1987a), in addition to
presenting data that supported RCC predictions,
described what they called "mesoscale" habitats.
Mesoscale habitats are areas within a stream
segment characterized by distinctly different
physical environments, and the boundaries
between them can be abrupt and well defined
(e.g., bedrock-outcrops versus pools formed
upstream of woody debris dams). Huyrn and
Wallace found that bedrock-outcrops with low
substrate roughness and rapid current are gener-
ally characterized by entrainment and export of
materials entering the habitats' boundaries.
Conversely, pools upstream of debris dams are
areas of local deposition and can function as
sites of organic matter accumulation. Therefore,
habitats that are quite different physically within
a stream segment may exhibit a quite different
functional structure with their respective animal
communities (Benke et al. 1984; Huyrn and
Wallace 1987a). Thus, a given stream reach may
include mesoscale regions that are analogous to
various sites along the RCC (Huyrn and Wallace
1987a).
In a headwater Coastal Plain stream in
Virginia, Smock et al. (1985) reported that
annual production of benthic macroinvertebrates
was not dominated by shredders, but by gather-
ing collectors (40-53 percent), followed by
filtering collectors (13-34 percent), and then
predators (16-30 percent). Production by shred-
ders was negligible (1-5 percent). As part of the
same study, Smock and Roeding (1986) found
that periphytic algae supported 15-31 percent of
annual production, while only 1-3 percent was
supported by CPOM. Theoretically, secondary
production in fully canopied headwaters should
be based primarily on CPOM processing with
contributions by algae low (Vannote et al.
1980). Smock and Roeding suggested that only
the smallest first-order streams on the Coastal
Plain, where CPOM retention was greater
because of debris dams, have high abundances of
shredders. Smock et al. (1989) showed that
macroinvertebrate density was 10 times greater
in debris dams than on the sandy sediment and
that 85 percent of the CPOM stored on the
channel surface was in dams. Obviously, debris
dams are a critical habitat for shredders.
In mid-sized streams with a more open
canopy, the RCC predicts reduced influence of
CPOM and increased importance of autochtho-
nous primary production (e.g. periphyton) and
FPOM from upstream. Also, because the effects
of riparian vegetation are predicted to be rela-
tively unimportant in medium to larger streams,
woody debris (snags) should play a minor role in
these streams as well (Wallace and Benke 1984).
However, contrary to RCC predictions, research
in mid- to large-sized rivers of the Southeastern
Coastal Plain has demonstrated that autochtho-
nous production is relatively unimportant and
that allochthonous materials are the principal
food items along the entire river. Additionally,
snags serve as important habitat for macroin-
vertebrates and sites of high secondary produc-
tivity (Cudney and Wallace 1980; Benke et al.
1984). Snags afford a relatively stable habitat
compared with the shifting, sandy substrate
found in most Coastal Plain streams. In addi-
tion, certain game fish apparently forage on
macroinvertebrates colonizing these woody
substrates (Benke et al. 1985).
Middle order streams usually possess few
shredders because of reduced quantities of
CPOM, therefore collectors often dominate
because of FPOM from upstream. Grazers
(scrapers) are also predicted to increase in
number because of increased quantities of
periphytic algae and vascular plants (Cummins
1977). In a mid-size Piedmont stream, Nelson
and Scott (1962) described a trophic structure
similar to that predicted by the RCC. In two
mid-sized Georgia rivers located in the Coastal
Plain, the Satilla and Ogeechee rivers, collectors
(gatherers and filterers) comprised the majority
of macroinvertebrate production as predicted,
however grazers contributed little to either
macroinvertebrate production (Benke et al.
1984) or biomass (Benke and Meyer 1988).
Large rivers receive the cumulative inputs of
FPOM from upstream and are often dominated
by filtering collectors, at least on hard substrates
in the river, figure 2. For example, Cudney and
Wallace (1980) reported high densities of filter-
feeding caddisflies on snag habitats in the Savan-
nah River in Georgia, and Mason (1991) found
high-densities of filter-feeding caddisflies on
artificial substrates in the Suwanee River in
Florida. Bottom muds in the Suwanee River
contained primarily mollusks, such as the filter-
feeding Corbicula, Chironomidae and
Oligochaetes. Most of the chironomids and
oligochaetes were gathering collectors.
In summary, the RCC offers a good concep-
tual model regarding the fate, transport and
processing of POM in Southeastern streams,
even though exceptions exist. However, we want
to emphasize a view expressed by James A. Gore
(personal communication) during the review of
this synthesis: "...that, when considering the
evolution and development of lotic systems,
hydrologic conditions and within channel hy-
draulic heterogeneity are the templates upon
which the RCC is superimposed. That is, if
hydraulic conditions are such that the conflicting
forces of pressure and friction drag are compen-
sated for by the shape and behavioral aspects of
the benthic organisms, then foraging (on what-
ever POM happens to be there) with net gains
in energy 
can occur."
Floodplain Dynamics
Another feature typical of low gradient
streams, especially in the Coastal Plain, is that
the channel usually meanders through heavily
forested floodplain wetlands. Floodplain width,
which usually is correlated with stream order,
has been found to range from about 50 m for
first order streams (Smock et al. 1989) to over
1 km for sixth order rivers (Benke and Meyer
1988). Periodic inundation of floodplain forests
liberates nutrients and dissolved humic substanc-
es. Humic substances leaching from peaty sites
may impart a tea-like color to the water result-
ing in so called "blackwater" rivers (Benke et al.
1984).
Interactions between the stream channel and
its floodplain are complex. Recent studies have
shown that inundated floodplains are diverse
systems, important to both macroinvertebrates
and fish (Holder et al. 1970; Welcomme 1979,
1988; Crance 1988; Crance and Ischinger 1989;
Junk et al. 1989). The aesthetic and economic
value to society of "river swamps" was recog-
nized over 20 years ago when Wharton (1970)
described them as "the last environment in the
Southeast providing an accessible wilderness
experience for rural and urban populations."
Floodplains function in a manner similar to
headwater streams of the RCC by serving as an
important source of POM (Sniffen 1981; Smock
and Roeding 1986; Benke and Meyer 1988;
Benke and Wallace 1990). Sniffen (1981) found
this to be true in a Coastal Plain stream in
North Carolina when he estimated that over 90
percent of the annual aquatic area of the
stream-swamp system was floodplain, and only
20 percent of total invertebrate production
occurred in the channel proper. Sniffen suggest-
ed that the proposed channelization of this
stream-swamp system by the U.S. Soil Conserva-
tion Service would have eliminated about 80
percent of the benthic production. The impor-
tance of floodplain contributions of POM is also
illustrated by studies in two mid-size blackwater
rivers in Georgia that revealed that allochthon-
ous inputs of bacteria from the floodplain
comprised a substantial portion of the microses-
ton available to filter-feeding collectors in the
channel (Edwards and Meyer 1986; Edwards
1987).
Stream ecosystems in which productivity is
intimately tied to the exchange of nutrients
between the channel and floodplain are driven
by what Junk et al. (1989) described as the
"flood pulse." They referred to a moving littoral
boundary as floodwaters spread over the plain
liberating nutrients. This boundary facilitates
the recycling of organic and inorganic materials
between the river and its floodplain and is
characterized by high productivity. The water
behind this moving front is often alive with
rotifers, copepods, algal blooms, and other
organisms (Welcomme 1988).
The flood pulse concept appears to be a
logical extension of Odum's (1969) concept of
pulse-stabilized communities. According to
Odum, a regular, but acute, physical perturba-
tion imposed on the system can maintain the
ecosystem at some intermediate point in the
developmental sequence. The systems Odum
described were relatively fertile and highly
productive, and the life histories of many organ-
isms were intimately tied to this periodicity, or
pulse (disturbance). Odum cited estuarine and
intertidal areas as examples, but applying the
analogy to channel-floodplain systems appears
appropriate. If changes are not too sudden,
communities can adapt to the disturbance
(pulse). However, major physical alterations
(e.g., highly modified flow regimes below a dam)
can devastate the biota (Junk et al. 1989).
MICROHABITAT REQUIREMENTS
OF BENTHIC MACROINVERTEBRATES
Because factors such as depth, velocity, and
substrate are known to influence the micro-
distribution of macroinvertebrates, a common
view in past studies of stream ecology has been
that abiotic factors are far more important than
biotic factors (Hart 1983; Teague et al. 1985).
However, this view has recently been modified
to account for the abiotic regime as either
harsh, or benign, to predators (i.e., the "harsh-
benign" hypothesis of Connell 1978). In harsher
abiotic regimes, predation impacts are predicted
to be low, while impacts of predation are pre-
dicted to be high in benign abiotic regimes
(Peckarsky et al. 1990).
The literature on microhabitat requirements
of macroinvertebrates was organized into topic
areas suggested in the review by Power et al.
(1988) that included: (1) the influence of local
abiotic conditions on invertebrates; (2) inverte-
brate responses to discharge fluctuations; (3)
movements of lotic invertebrates; and, (4) life
history requirements of invertebrates. Our
emphasis was on studies conducted in the
Southeastern United States.
Influence of Local Abiotic Factors
Physical variables often mentioned as regu-
lating macroinvertebrate communities in South-
eastern streams are temperature, substrate,
current velocity, and food availability. Tempera-
ture, substrate, and current velocity will be
covered separately, but food availability 
will be
discussed within the context of each of these
variables.
Temperature. Temperature is related inti-
mately to latitude, altitude, and, in spring-fed or
lake-fed (or reservoir-fed) streams, to the dis-
tance from the source (Hynes 1970). In addi-
tion, although both the seasonal patterns and
ranges of temperature for streams vary geo-
graphically, the amount of heat accumulated (i.e.
degree days) on a yearly basis seems to be
predictable when streams of similar size are
compared over a range of latitudes (Vannote
and Sweeney 1980).
Variations in the size of individuals have
been correlated with either temporal or spatial
temperature gradients. For example, increased
body size has been related to reduced tempera-
tures in aquatic insect species (Cudney and
Wallace 1980; Kondratieff and Voshell 1980;
Sweeney 1984). In a study of the life history
and ecology of Stenonemna modestum, a multivol-
tine mayfly, Kondratieff and Voshell (1980)
found that adults of winter-spring cohorts were
significantly larger than those of summer co-
horts. Hauer and Benke (1987) developed
predictive equations relating growth of black fly
larvae (Simulium spp.) to temperature; one for
use during early stages of flooding, the other
during late flood and low flow conditions.
Sweeney (1984) summarized the results of
studies, primarily conducted in the laboratory,
that reported the response of certain life-history
characteristics of macroinvertebrates to tempera-
ture. However, of the many field studies that
have shown correlations between seasonal
patterns in life-history characteristics and tem-
perature, Sweeney noted that few have com-
pared populations in two or more habitats
differing in temperature. He also pointed out
the difficulty in differentiating between the
relative importance of temperature and nutri-
tion, because temperature affects not only larval
metabolism, but the quantity and quality of
food.
In a spring-fed stream in Kentucky, Minshall
(1968) identified temperature and substrate as
the environmental factors most influencing
macroinvertebrate diversity and abundance.
Gordon and Wallace (1975), in a study along
the length of the Savannah River, found that
stream size (drainage area), increased amounts
10
of FPOM and minimum dissolved oxygen (DO)
concentration were the most important factors
affecting the distribution of net-spinning
Trichoptera. However, they found that size and
DO were highly correlated with altitude and
temperature. Gordon and Wallace reported that
the oxygen requirements of certain macroinver-
tebrates suggested that these species will be
restricted to streams where temperatures remain
below some maximum level. Apparently the
tolerance ranges (i.e., of temperature and dis-
solved oxygen) of many species in Piedmont and
mountainous areas of the Southeast are relative-
ly narrow (Lenat 1983) compared to those for
many species found in Coastal Plain streams
(Penrose et al. 1982). Gordon and Wallace
(1975) indicated that if streams at the higher
elevations (i.e., Piedmont and mountainous
areas) were exposed to some stress (e.g., warmed
surface releases from a flood-control dam) that
caused a rise in temperature and/or a lowering
of dissolved oxygen, shifts in species could occur,
resulting in a fauna similar to that in Coastal
Plain streams. However, the effects of a similar
disturbance on mid-size or large Coastal Plain
streams is unknown because macroinvertebrates
in these systems are already acclimated to higher
temperatures and lower dissolved oxygen than
species upstream (Gordon and Wallace 1975).
The influence of shading on temperature has
also been studied in small headwater streams in
the southern Appalachian mountains. Following
logging and removal of the dense canopy shad-
ing the streams, summer water temperatures
were several degrees higher than those found in
control streams draining adjacent, forested
watersheds (Swift and Messer 1971; Webster et
al. 1983). However, field studies of this type
have yet to separate the effects of elevated
temperatures on stream invertebrates from other
abiotic changes such as increased streamflow,
increased food availability, and increased sedi-
mentation.
Substrate. In high-gradient streams, substra-
tum particle size has been described as a major
determinant of the distribution and abundance
of benthic macroinvertebrates (Crisp and Crisp
1974; Rabeni and Minshall 1977; Gurtz 1981;
Reice 1981; Gurtz and Wallace 1984; Huryn and
Wallace 1987a). Each of these studies reported
higher densities of macroinvertebrates on larger-
sized substrates, such as cobble, than on the
smaller particle sizes like sand.
The type of substrate material also influences
macroinvertebrate distributions. Gurtz (1981)
and Gurtz and Wallace (1984) found substrate
types that included moss-covered boulders or
bedrock, cobbles, pebbles, sand and associated
wood or leaf debris. Gurtz found differing food
quantity and quality (measured as FPOM and
algae) associated with the various substrate
types, plus a current-velocity gradient. Moss-
covered rocks trapped more FPOM and had
more periphytic algae than other substrates.
Highest current velocities were found on the
moss-covered bedrock and boulder substrates
(erosional zones), while lowest velocities were
detected on sand substrates (depositional zones).
He found increased macroinvertebrate densities
on bedrock and boulders, compared to the other
substrates, especially in a stream with extensive
logging on the watershed. Gurtz attributed the
increased densities to greater food availability,
greater protection from scour and less sedimen-
tation because of higher current velocities over
the moss-covered substrates.
Recent analyses of stream hydraulics
(Statzner et al. 1988) indicate that Gurtz (1981)
found variable importance of velocity, depth and
substrate because these factors interact simulta-
neously but "variably," in complex hydraulic
equations for Reynolds number, shear stress or
boundary sublayer thickness. Statzner et al.
(1988) suggested that invertebrates "prefer" a
complex hydraulic condition that matches the
hydrodynamic character of their body size, shape
and behavior. Other studies have also described
the importance of hydraulic variables in deter-
mining macroinvertebrate distributions (Statzner
and Higler 1985; Statzner 1988; Davis and
Barmuta 1989; Gore and Bryant 1990; review
by Heede and Rinne 1990).
Although current velocity is positively
correlated with substratum particle size (Reice
1981), Rabeni and Minshall (1977) demonstrat-
ed that particle size has a far greater effect than
current velocity on animal distributions in
streams. They found that velocity-effects were
insignificant compared with substratum-related
trapping of detritus, a view that differs from that
of Statzner et al. (1988) in which substrate
characteristics were less important than mean
velocity and complex hydraulics in explaining the
distribution of lotic macroinvertebrates.
11
In low-gradient streams throughout the
Coastal Plain, the predominant substrates usual-
ly differ greatly from those found in Piedmont
and mountainous areas. Smock et al. (1989)
identified two types of Coastal Plain headwater
streams, differentiated primarily on the basis of
channel composition. One type, called "bottom-
land" streams, has channel substrates composed
of clay and silt and often covered by a layer of
organic matter. Streams of this type may be
characterized by heavy siltation that can degrade
bottom habitat, and reduce diversity, abundance,
and productivity of benthic invertebrates
(Cooper 1987). A second type, called "sandy"
bottom streams, is characterized by shifting-sand
substrates. The majority of Coastal Plain
streams (regardless of size) have relatively
homogenous, unstable shifting-sand substrates
(Soponis and Russell 1984; Robertson and
Piwowar 1985; Scheiring 1985; Keup 1988;
Smock 1988; Bain and Boltz 1989), although
headwater streams exist that have coarse sub-
strates of boulders, cobble, and gravel (Cowell
and Carew 1976). Additionally, outcrops of
sandstone bedrock, limestone, and siltstone
occasionally occur in Coastal Plain streams.
Wood debris (snags) has been recognized as
the most stable substrate for macroinvertebrates
inhabiting sandy-bottomed streams in the South-
east (Van Arsdall 1977; Cudney and Wallace
1980; Benke et al. 1984; Wallace and Benke
1984; Smock et al. 1985; Thorp et al. 1985;
Roeding and Smock 1989; Smock et al. 1989;
Benke and Wallace 1990). Woody debris usually
remains in a stream for long periods of time,
and the major instability for macroinvertebrates
colonizing snag habitat includes periodic expo-
sure to the air as water levels fluctuate (Benke
and Wallace 1990) and scouring by floods and
sediment.
Coastal Plain streams with snags along the
banks typically possess three major microhab-
itats in which macroinvertebrates can be found:
(1) the shifting sandy substratum of the main
channel; (2) the muddy, depositional substratum
of backwaters; and, (3) submerged wood, or
snags (Benke et al. 1985). A fourth benthic
habitat exists during flood stage, when the
stream inundates all or part of its floodplain for
periods up to 3 or more months per year.
Benke et al. (1985) found that among the three
major habitats, snags had by far the highest
animal diversity, standing stock biomass, and
secondary production per unit of habitat surface.
Current Velocity. Hydraulic stream ecology
is an approach to lotic studies proposed by
Statzner et al. (1988) that attempts to relate
macroinvertebrate distribution and abundance
with various complex characteristics of flow
(e.g., kinematic viscosity of water, Froude num-
ber, shear velocity), rather than just discharge or
mean current velocity. These authors provided
an excellent review of how lotic organisms
(mainly benthic macroinvertebrates) react to
current. Other equally good reviews on benthos
and hydraulics occur in Davis (1986) and Heede
and Rinne (1990).
Current velocity is considered an essential
factor in microhabitat selection by filter-feeding
caddisflies. The retreats and nets constructed by
these organisms are generally adapted for partic-
ular current speed ranges enabling closely
related species to partition available food and
substrate resources (Edington 1968; Freeman
and Wallace 1984; Wallace 1975; Wallace et al.
1977). Cudney and Wallace (1980) found that
net-spinning Trichoptera in the Savannah River
appear to occupy different microhabitats on the
submerged branches and roots. They studied six
species at each of three current speed ranges:
10-15 cm per second, 25-50 cm per second, and
50-75 cm per second. Production and biomass
estimates for all but one species were highest at
the medium current velocity range. Cudney and
Wallace stressed that the production estimates
were not necessarily on discrete populations
found at each velocity range because larvae can
drift and change habitats during their life cycle.
However, these data suggested that net-spinning
hydropsychids are significantly influenced by
current velocity.
Cudney and Wallace (1980) also suggested
that the spatial heterogeneity of submerged
moss-covered snags provided a diversity of
microhabitats with respect to current velocity;
therefore, the ability of filter-feeders to occupy
such microhabitats was possible because of the
large differences in catch-net mesh dimensions
found among the species. They suggested that
these differences in catch-net mesh size were
important to the caddisflies because of limited
space (microhabitat) availability rather than
limited food. Thus, large numbers of filtering
species, each feeding in different microhabitats
12
and on different size particles, should result in
more efficient utilization of drifting seston.
Cudney and Wallace (1980) reported no
evidence of temporal differences in life cycles
among the species they studied, although in an
Appalachian mountain stream, Benke and
Wallace (1980) found distinct temporal differ-
ences that probably minimized interspecific
competition for space and food. Although the
study by Cudney and Wallace (1980) demon-
strated that caddisfly species have current veloci-
ty preferences, the various instars were generally
found over the entire range of current speeds
present in the river. However, last instar catch-
net mesh dimensions of the six caddisfly species
in the Savannah River differed greatly. If these
mesh size differences are maintained among
species throughout successive larval instars, then
temporal similarity in life cycles may reduce
overlaps in mesh size between species. Thus,
the ability of individual species to occupy differ-
ent microhabitats could be enhanced by such
similarity in life cycles.
Future Needs. Limited experimental data
exist from field studies of the effects on biota of
modified temperature regimes. Will changes in
thermoperiodicity patterns (e.g., below dams or
from pollution) disrupt life cycles events, per-
haps leading to elimination of some macroinver-
tebrate species (Ward and Stanford 1987)? Do
suboptimal thermal conditions reduce fecundity,
thus placing certain species at a competitive
disadvantage?
There are also limited data available on the
intricacies of flow variables and the controls
they exert on biota (Davis 1986; Statzner et al.
1988; Gore et al. 1989), even though mean
current velocity and discharge are usually mea-
sured in stream studies. Can the hydraulic
variables that induce drift in stream macroinver-
tebrates be identified? Do stream gradient and
substrate influences on flow variables result in
different drift patterns over coarse substrates
compared to other stream 
substrates (Layzer et
al. 1989)?
Although several of the studies cited here
reported good correlations between distribution
and abiotic factors, there is a strong need for
additional experiments that adequately charac-
terize the flow environment preferred by macro-
invertebrate communities. For example, Teague
et al. (1985) reported that the caddisfly
Dicosmoecus gilvipes preferred large substrate
particles, but in the same geographic region,
Lamberti and Resh (1979) reported that D.
gilvipes preferred the smallest available sub-
strates. However, neither D. gilvipes study
accounted for substrate heterogeneity, or fre-
quency distributions of the various substrate
classes (personal communication, James A.
Gore). Several studies have suggested that, even
in free-flowing streams, certain hydropsychids
may preferentially colonize microhabitats to
maximize food delivery rates (i.e., seston trans-
port), and there is considerable evidence that
the enhanced levels of secondary production at
such sites can be attributed to higher food
quality, rather than quantity of the seston
(Benke and Wallace 1980; Georgian and
Wallace 1981; Parker and Voshell 1983; Ross
and Wallace 1983; Voshell and Parker 1985;
Smith-Cuffney and Wallace 1987).
Another important consideration when
assessing the distributions of macroinvertebrates
with respect to physical variables is that, in
many cases, distribution is mediated by interac-
tions with other organisms. For example,
McAuliffe (1983) found certain midges quickly
colonize shallow, recently inundated substrates
that lack competitively dominant caddisflies.
Caddisflies have also been found to restrict
distributions of a grazer population (Hart 1985).
Finally, drift rates of benthic insects from pool
habitats in logged areas were found to be lower
than those from forested pools because of
greater light penetration to the stream, thus
predation by trout was enhanced in the logged
reaches (Wilzbach et al. 1986).
Responses of Benthic Macroinvertebrates
to Discharge Fluctuations
Seasonal fluctuations in discharge are critical
events in the life histories of most mac-
roinvertebrates. As water levels fluctuate,
stream and river habitats expand and contract,
resource availabilities shift, certain habitats (e.g.,
riffles and pools) may become isolated from
others and flow regimes may change, thus alter-
ing other physical variables (Power et al. 1988).
In addition, extreme events (e.g. scouring floods,
droughts) can eliminate much of the biota and
lead to changes in community composition or
succession (Fisher 1983). Thus habitat require-
ments, especially at the microhabitat level,
13
should include appropriate spatial and temporal
components (Minshall 1988; Gore and Bryant
1990).
Natural disturbances, such as floods, are
common in lotic systems, yet little is known of
the actual response of benthic macroinvertebra-
tes to dynamic flows of varying intensity and
duration.(Gore 1989). Few studies have focused
on the importance of natural physical distur-
bance to the structure and distribution of stream
communities (Hemphill and Cooper 1983),
although Fisher et al. (1982) documented chang-
es in stream communities following floods in
desert streams. However, successional events
caused by seasonal changes were not distin-
guished from successional events following the
disturbance.
Stream ecologists have recently begun em-
phasizing the role of disturbance as a central
theme in community organization (Resh et al.
1988). These authors used a general definition
of disturbance from Pickett and White (1985): "a
disturbance is any relatively discrete event in
time that disrupts ecosystem, community, or
population structure, and that changes resourc-
es, availability of substratum, or the physical
environment." Emphasis has also been directed
at examining the role of various ecological
theories in predicting recovery from disturbance
(Gore et al. 1990).
Historically, the null hypothesis for commu-
nity structure has been an equilibrium model
that assumes a constant environment (Resh et
al. 1988). Biotic interactions are also considered
the primary determinants of community struc-
ture, "all else being equal." These authors
interpreted this phrase to mean that the envi-
ronment remains "constant" or that the commu-
nities are adapted to some degree of variability.
The equilibrium model was used by Minshall
et al. (1983) to argue that if the time interval
between spates in streams was long enough,
equilibrium conditions would prevail, and then
density-dependent processes should dominate.
This rarely disturbed community was contrasted
with "opportunistic" species that were associated
with frequently disturbed communities.
Another stage in the evolving concepts of
disturbance theory was the intermediate distur-
bance hypothesis developed by Connell (1978)
to explain the high species diversity observed in
tropical rain forests and coral reefs. The inter-
mediate disturbance hypothesis presumes that a
competitive hierarchy of species exists in the
absence of frequent disturbances, and superior
competitors eliminate inferior ones and species
richness is reduced. The superior competitors
(resident species) are the more efficient occupi-
ers of habitat. With frequent disturbances, or
one of great magnitude, colonizing species (poor
competitors) gain a foothold and the resident
competitors are displaced. This loss also lowers
species richness. At some intermediate level of
disturbance, members of both groups coexist
and, in this manner, maximum species richness
occurs.
A second role for disturbance was proposed
with Huston's (1979) "dynamic equilibrium"
model. The key to this hypothesis was that the
recurrence interval of some disturbance event
(e.g., flood, drought, anthropogenic input, etc.)
was shorter than the time necessary for competi-
tive, or predator-prey, interactions to result in
the elimination of species. Thus, species that
are poorer competitors would persist in the
system and increase species richness. Huston
concluded that diversity is determined not as
much by the relative competitive abilities of the
competing species as by the impact of the envi-
ronment (disturbance) on species interactions.
Portions of each of these two models have been
applied to stream invertebrates (Peckarsky 1983;
Ward and Stanford 1983). Resh et al. (1988)
felt the dynamic equilibrium model was the most
generally applicable hypothesis, although there
are limited data available to support it. They
also stated that the intermediate disturbance hy-
pothesis has its applicability, but its acceptance
in stream ecology will require adequate dem-
onstration of competitive hierarchies and or-
dered dominance sequences among stream
organisms.
Little information 
exists in the Southeast
that defines the role of disturbance and the
effects of disturbance frequency, or intensity, on
stream communities. We will focus on two
classes of natural disturbances common in
streams, high water (floods) and low water
(drought), then consider research by Reice
(1985) in which he tested the species-specific
responses of macroinvertebrates to a disturbance
(i.e. tumbling patches of cobble) in a North
Carolina stream.
Droughts reduce the total available habitat,
14
but the change is gradual. Under natural
drought conditions, macroinvertebrates migrate
into the wet portions of the channel or down
into the hyporheic zone to minimize mortality
effects. Benke et al. (1985, 1986) suggested that
invertebrates migrate on snag habitats in re-
sponse to changes in water height as long as the
change is slow. This response to drought does,
however, concentrate the biota into reduced
livable habitat and can increase the impact of
competition and/or predation on species. Final-
ly, hydraulic conditions probably become less
important as streams become more drought
prone. That is, in intermittent streams, hydrau-
lic factors may be secondary to factors that allow
retreat to refugia (Matthews 1988).
Floods or spates usually occur suddenly in
the Southeast (often in the span of a few hours
or days), and little is known about how quickly
lotic species can respond to increasing current
velocity (Reice 1985) or the rising water level.
Benke et al. (1984) observed that simuliids
quickly colonized recently inundated snag habi-
tat before other species (e.g. hydropsychids) had
a chance to colonize the new substrate. There-
fore, most blackfly production apparently oc-
curred as water levels rose, but it was unclear
whether this finding was a response to predation
pressures, crowding, or microhabitat conditions.
Physical changes in the stream channel are often
pronounced during floods; while the timing of
floods may be quite predictable in snowmelt-fed
mountain streams, in runoff and spring-fed
streams in the Southeast, floods are unpredict-
able. The predictability of a disturbance will
certainly affect the community's ability to adapt
to it (Reice 1985; Resh et al. 1988).
Floods can have a devastating effect on
benthic communities confined to the stream
channel. Yet, stream organisms seem well
adapted to this type of disturbance because
populations rebound quickly (often within a few
days or weeks after the most severe floods).
Recovery may result from the high fecundity of
lotic organisms and, in part, from individuals
that take refuge in the hyporheic zone. Others
escape injury in protected places and backwat-
ers. Recolonization mechanisms are poorly
understood, but may include several activities,
including swimming, upstream crawling, drifting,
or egg laying (Reice 1985).
Reice (1985) also addressed the question of
whether the intermediate disturbance 
hypothesis
applies to macroinvertebrate communities 
and
he found no evidence of a competitive 
hierarchy
in the North Carolina stream. Hemphill 
and
Cooper (1983) reported that the caddisfly,
Hydropsyche oslari, outcompeted the blackfly
Simulium virgatum under stable flow conditions
in their California stream, but Sinulium was
maintained in the system by its ability to recolo-
nize quickly following disturbance. Reice (1985)
considered these observations by Hemphill and
Cooper (1983) to include species which were
always present in the system and not species
which had been excluded by the superior com-
petitors. Reice found no increase in rare spe-
cies on disturbed substrates, relative to undis-
turbed substrates. Also, species diversity and
richness did not change as disturbance frequency
increased. He concluded that the intermediate-
disturbance hypothesis did not apply to stream
communities.
Future Needs. Given the diversity of micro-
habitats and variation in discharge in streams,
the relative importance of different factors that
regulate community structure should be expect-
ed to vary in both space and time. Efforts to
devise research and expand the findings to real-
world situations have been hampered by insuffi-
cient consideration of the scale of various inter-
specific interactions. On a single rock, or snag,
competition may be important at one point,
while predation is critical at another. Studies
such as those of Reice (1985) suggest that biotic
interactions will be more important at small
scales. As scale increases, interspecific interac-
tions should be weaker even though migrations
among habitats are common. Research is
needed to test these disturbance hypotheses
further, especially on a larger scale.
Studies are also needed to assess the avail-
ability of refugia for species during vulnerable
life history stages. In Coastal Plain streams,
floodplains are inundated at high flow, and may
persist as lentic, and often isolated habitats,
during low flow periods (Welcomme 1988).
These inundated habitats are nurseries and
refugia for many species. They provide opportu-
nities for comparative studies and manipulative
field experiments of temporarily isolated compo-
nents of the biota of larger streams. What are
the exact makeups of these communities? What
are the roles of these communities in the dy-
15
namics of channel-floodplain interactions? With
more information on the mobility, ecological
tolerance, and habitat requirements of taxa, it
should be possible to learn much more from
biotic responses to hydrologic "disturbances."
Movements of Benthic Macroinvertebrates
Complexbiotic-abioticinteractionsregulating
benthic macroinvertebrates may be difficult to
measure, or interpret, if careful thought is not
given to the spatial range of activities of the
organisms under study. This applies to enclo-
sure or exclosure experiments, or other density
manipulations. This synthesis will concentrate
on drift movements because drift is one of the
biological processes that describes the popula-
tion dynamics of invertebrates (Benke et al.
1986). Production, which is closely linked with
drift, is another basic process that will be dis-
cussed in a later section. Drift dynamics in
tailwater habitats will be covered in the section
on regulated streams.
Drift can be initiated by biotic interactions as
well as by a variety of physical factors (Waters
1961; Bishop 1969; Hynes 1970; Reisen and
Prins 1972; Cowell and Carew 1976;
Stoneburner and Smock 1979; O'Hop and
Wallace 1983; Soponis and Russell 1984; Benke
et al. 1986; Obi and Conner 1986; Keup 1988).
Several excellent reviews exist that detail many
of the results of drift studies (Waters 1972;
Miller 1974; Wiley and Kohler 1984).
Drift refers to the downstream transport by
the current of benthic animals found on or in
bottom substrates, and confusion still exists over
the role of behavior in determining periodic
drift activity. The terms "active drift" and "pas-
sive drift" came to characterize two competing
hypotheses about why animals drift (Wiley and
Kohler 1984). Waters (1972) described drift
activity as catastrophic, behavioral, and constant,
although he noted that distinctions are not
always clear among these types because of
overlap and interactions. According to Wiley
and Kohler (1984) these terms actually distin-
guish temporal characteristics of drift (cata-
strophic = pulsed; behavioral = periodic; con-
stant = continuous). Furthermore, these au-
thors concluded that drift is neither exclusively
active or passive, but a "composite phenomenon"
resulting from a variety of causes, some of which
are unknown. Pulsed catastrophic drift can be
initiated by a variety of abiotic disturbances,
including floods, droughts, fluctuating water
levels below dams, high temperatures and pol-
lutants. Behavioral drift refers to consistent
periodic drift activity occurring often at night,
especially with aquatic insects. Waters (1972)
defined "the continuous stream of representa-
tives of all species, in low numbers and occur-
ring at all times" as constant drift.
Drift studies in warmwater streams are
relatively scarce and few have been conducted in
larger streams (> 5th order) of any kind. Drift
investigations have also concentrated on the
highly predictable diel patterns of night activity
found in aquatic invertebrates, especially the
insects (see reviews of Waters 1972; Wiley and
Kohler 1984; and Keup 1988). Little work has
been done on the role that drift plays in popula-
tion dynamics as a dispersal mechanism, on the
habitat origin of drift, on the contribution of
drift as a part of total seston transport (e.g.,
O'Hop and Wallace 1983) or on the contribu-
tion of drift to the diet of fish (e.g., Allan 1978).
The processes that maintain upstream popu-
lations of benthic insects as downstream losses
occur are subject to debate. The exact influence
of benthic density on drift density remains
unclear, because there is evidence for both a
density-independent and a density-dependent
relationship (Hildebrand 1974; Walton et al.
1977). While mayflies often dominate stream
drift, Hildebrand (1974) demonstrated in artifi-
cial streams that drift of four mayfly taxa was
not density dependent, but was related to food
availability. Hildebrand suggested that increased
activity in searching for food at the lower food
level contributed to increased drift. Walton et
al. (1977) found drift to be density dependent on
stones and cobble substrates, but density inde-
pendent on gravel. The observed density-depen-
dent relationship was attributed to competition
for interstitial space which was less prevalent on
the gravel substrates.
Macroinvertebrate drift, both the species
drifting and the number of individuals, exhibits
seasonal fluctuations in addition to predictable
diel periodicity. Diel periodicity apparently
results from several factors that serve as phase-
setting agents (Wiley and Kohler 1984). Waters
(1972) and Miller (1974) thoroughly reviewed
these environmental factors. Light intensity,
however, is apparently the phase-setter usually
16
involved, triggering increased insect activity as
incident radiation decreases to some threshold
level. This threshold level is about 1 to 5 lux
(Waters 1972) although Bishop (1969) recorded
levels as low as 0.001 lux in carefully controlled
artificial streams. The mechanisms that result in
diel drift periodicities remain unclear, although
changes in position and activity levels have been
proposed by researchers (Elliott 1967; Bishop
1969; Wiley and Kohler 1984). Allan (1978)
suggested that fish predation on drift may
influence drift patterns. He found that larger
nymphs were more prone to drift at night than
small nymphs, and considered this an adaptation
to avoid predation.
Seasonal periodicity appears closely related
to periods of maximum growth, prepupation,
and emergence activities (Elliott 1967; Reisen
and Prins 1972; Stoneburner and Smock 1979;
O'Hop and Wallace 1983). For example,
Stoneburner and Smock (1979) observed that
seasonal fluctuations in the abundance of exuvi-
ae in the drift were correlated with fluctuations
in larval drift density. These observations
suggest a close relationship between life history
stages of macroinvertebrates and their presence
in the drift.
The majority of drift studies conducted in
more northerly regions of temperate climates
indicate that maximum drift density occurs
during the summer months (Waters 1972). In
contrast, studies conducted in the Piedmont
Province of South Carolina reported larval drift
densities relatively constant throughout the year,
but peaks occurred during the spring and fall
(Reisen and Prins 1972; Stoneburner and Smock
1979). A study conducted in subtropical Florida
indicated peaks in drift density during the winter
and early spring (Cowell and Carew 1976) which
generally corresponded to periods of rapid
growth, prepupation events, and emergence
activities for the given areas.
Benke et al. (1986), studying drift in a sixth-
order stream located in the Coastal Plain of
Georgia, found high drift rates throughout the
year, although drift densities were slightly higher
during summer months. They concluded that
seasonal abundance patterns for total drift were
not evident in Southeastern warmwater streams
because of active feeding and growth throughout
most of the year.
Understanding temporal variability in South-
eastern rivers is a complex issue 
because of
changes in taxonomic composition throughout
the year. For example, higher drift densities of
certain macroinvertebrates (e.g., Trichoptera)
observed during the summer and fall probably
reflected seasonal variations in life cycle
(Stoneburner and Smock 1979). However,
Benke et al. (1986) found certain Diptera (e.g.,
simuliids), with short generation times, abundant
in the drift throughout the year, and their
abundance was highest during high discharge.
These authors concluded that season and dis-
charge were the important factors that affected
both community production and drift dynamics.
Studies have shown that snag habitats, which
are small in relation to benthic habitats,
contribute significantly to invertebrate produc-
tion in large warmwater streams (Nilson and
Larimore 1973; Cudney and Wallace 1980;
Wallace and Benke 1984; Smock et al. 1985;
Thorp et al. 1985; Benke et al. 1986). It has
been demonstrated in Coastal Plain streams that
the high invertebrate biomass and production on
snags contributes to the high drift densities
observed (Benke et al. 1986). Finally, in larger
streams in the Southeast, macroinvertebrates
have been reported to drift long distances
(Benke et al. 1986; Obi and Conner 1986).
Future Needs. Information on the mobility
of macroinvertebrates is essential to an
understanding of how life history stages of lotic
species respond to changes in their environment.
Little is known, for example, of the actual
response of benthos to dynamic flows of varying
intensity and duration. Studies are needed that
assess drift as it relates to changing natural
flows in a variety of streams and below dams
(Gore 1989; Layzer et al. 1989) in tailwater
habitats:
(1) What is the habitat origin of most of the
invertebrate drift in Piedmont and Coastal
Plain streams?
(2) Do the ecological mechanisms that deter-
mine behavioral drift differ in mid-sized, and
larger, Piedmont and Coastal Plain streams?
(3) Is drift density-dependent or density-inde-
pendent in mid-sized, and larger, Coastal Plain
streams?
(4) Is the nighttime periodicity of drift an
adaptation to avoid fish predation?
Little information is available on the role
17
drift plays as a dispersal mechanism, although
this area of study will require better techniques
of tracking individuals. Another solution to
tracking movements of small organisms might be
experiments conducted in artificial streams
(Kohler 1985), where initial densities from
source areas are known and thus, effects of
factors such as flow, substratum suitability,
crowding, or food availability on movement can
be investigated. Finally, newly available habitat
could be monitored to determine spatial rela-
tions between colonizable habitat and organism
sources. These studies would provide informa-
tion on species mobility and recolonization
potential (Gore 1979).
Many of the questions raised in this section
can be answered only as we develop and clearly
test appropriate hypotheses. Experimental
evidence is essential to complete this process.
There is a strong need for more experimental
work in artificial streams, both in laboratory
microcosms and in mesocosm-type channels that
allow whole-ecosystem manipulations in a more
natural setting.
Life Histories of Benthic Macroinvertebrates
The field of stream ecology still lacks critical
life-history knowledge that is essential to current
research on the structure and function of aquat-
ic communities and ecosystems (Rosenberg
1979). In addition, microhabitat requirements
of benthic macroinvertebrates may change in
response to disturbances, depending on the type,
magnitude, and frequency of the disturbance.
The changes may involve alteration of certain
life history traits for selected taxa in the stream.
In fact, Wallace (1990) stated that life history
traits may influence the rate at which communi-
ties recover following disturbances. Butler
(1984) described the biological features covered
by the term "life history" as, "events that govern
the reproduction (and survival) of a species or
a population, including fecundity, development,
longevity, and behavior."
Several questions raised by Power et al.
(1988) illustrate the importance of life history
information in attempts to measure the distribu-
tion and community structure of lotic species,
particularly with respect to understanding how
factors such as substrate, depth, current velocity,
and temperature influence these organisms.
What cues initiate life history events such as
oviposition, hatching, larval development, diapa-
use, emergence, and the onset of reproduction?
What habitat requirements and resources are
necessary for macroinvertebrates to complete
their life histories? How have macroinvertebra-
tes adapted various life history characteristics to
cope with resource limitations, predation, or
disturbance?
Current physical habitat-based models for
assessing stream flow needs are utilizing more
biological information (Gore and Nestler 1988;
Gore et al. 1992). One measure of aquatic
invertebrates now being suggested is secondary
production (Orth 1987). Adequate life history
data are essential in calculating secondary
production, whether for individual taxa or the
community as a whole (Resh 1977; Benke et al.
1984; Orth 1987). Secondary production, more
than density or biomass, is a more direct reflec-
tion of utilization of the organic resources in
streams by invertebrates. Production of aquatic
invertebrates represents a large portion of the
food that is available to those fish species that
are important for food and recreation. In fact,
aquatic insects often comprise the major compo-
nent of what fishery biologists refer to as "fish
food" (Benke 1984).
Resh (1977) and Benke et al. (1984) recom-
mended measuring production by major micro-
habitat types to derive estimates for the stream,
whether for a particular taxon or a functional
feeding group. Resh suggested that production
estimates that are based on a few replicate
samples for one specific microhabitat do not
adequately consider either sampling variability
or the possibility of differential habitat produc-
tion. These studies and others conducted in
Southeastern streams illustrate the importance
of adequate life history data, whether quantify-
ing biomass turnover (Haefner and Wallace
1981; Benke et al. 1984; Huryn and Wallace
1987b), assessing microdistribution preferences
(Wallace and Sherberger 1974; Cudney and
Wallace 1980) or determining food preferences
and life cycles information (Wallace 1975;
Manuel and Folsom 1982; Parker and Voshell
1982).
Studies concerning life history cues (e.g.,
what initiates emergence) have revealed that
many lotic species use combinations of light,
temperature and flow as stimuli. The impor-
tance of temperature as a factor in the life cycle
18
of temperate macroinvertebrates has been
extensively reviewed by several authors (e.g., see
Ward and Stanford 1982). Much of the evi-
dence for temperature as a critical factor in the
life history of benthic invertebrates has come
from studies in the altered thermal regimes in
regulated rivers (Ward and Stanford 1979).
Light (i.e., photoperiod) as a stimulus has
seldom been investigated apart from tempera-
ture (Power et al. 1988). Bishop (1969) investi-
gated the effects of light on drift activity in an
artificial stream and found that a threshold
value existed for incident radiation when in-
creased light suppressed drift activity, but, when
decreased below the threshold value, higher drift
rates occurred. Temperature was held constant
in this study.
Future Needs. For most macroinvertebrate
species in the Southeast, knowledge of the
habitat and resource requirements necessary for
various life history stages is generally lacking,
especially with respect to filter-feeding macroin-
vertebrates. The production studies cited earlier
in this section include efforts to relate habitat
and food preferences to various life stages of
macroinvertebra tes.
The relative importance of various potential-
ly limiting factors that may occur during differ-
ent life history stages of an organism is poorly
understood, even for salmonid fishes that have
been intensely studied (Power et al. 1988).
Efforts to identify limiting factors for lotic
macroinvertebrates are complicated by the lack
of adequate taxonomy for all life stages, species
migrations, the relative importance of mortality
and growth as limiting factors that constrain
population production, and synchrony versus
asynchrony of life cycles (Butler 1984).
Innovative experiments are needed to help
clarify the intricacies of factors controlling life
histories and the consequences such interactions
hold for determining distribution and communi-
ty structure. Larger streams (orders 4-6) have
not been widely studied (Webster et al. 1983),
particularly in areas such as the Piedmont and
Coastal Plain provinces. These studies should
consider differences in production statistics
among taxa in addition to estimates of abun-
dance and biomass. This approach should allow
a more accurate understanding of the ecological
roles of coexisting invertebrates (Benke et al.
1984).
A thorough assessment of secondary produc-
tion in streams also must include appropriate
study of the spatial distributions of macroinver-
tebrates among habitats (Resh 1977). One
habitat that merits considerable study is snag
habitat (Benke et al. 1984; Webber et al. 1989).
Finally, knowledge of the distribution of inverte-
brate production among functional groups and
habitats says much about stream function,
however analysis of food consumption by inver-
tebrates is essential before the type and source
of food actually supporting the production can
be determined (Benke et al. 1984). Food sourc-
es and types for invertebrates in Southeastern
rivers remain as important unanswered ques-
tions.
KEY INDICATORS AND HABITAT
GUILDS
Stream ecosystems are complex hydraulic
systems containing many species and, as such,
efforts at predicting ecological relationships are
necessarily based on mechanistic models of a
few key processes or species, and their interac-
tions (Power et al. 1988). It is not uncommon
to find certain species whose removal from the
community or ecosystem will cause measurable
changes in structure, distribution, and function
(Paine 1980). Standing stock estimates (num-
bers and biomass), production, and behavior of
appropriate species, or "key indicators," [i.e.,
Minshall's (1988) "keystone species"] can explain
much about the structure and function of lotic
communities, and thus of the ecosystem as a
whole.
Selecting key indicators in a study requires
adequate knowledge of the biology of the indica-
tor(s), in addition to knowing the ecological role
of the species in the community. Certain "key-
stone predators" are often used as examples of
strong interactors and their importance is obvi-
ous in many communities (Power et al. 1988).
However, predators should not receive dispro-
portionate attention just because they are big-
ger, easier to identify or more easily manipulat-
ed than less conspicuous species in the system.
Several studies have used sedentary aquatic
insects (e.g., case-dwelling hydropsychids) as
subjects that can be followed through density
manipulations in streams (Hart 1985, 1986;
19
McAuliffe 1983, 1984). These types 
of studies
set the stage for more thorough investigations 
of
both direct and indirect effects that can impact
communities, or food webs, as species-specific
densities change in response to external (e.g.,
rapidly fluctuating water levels) or internal (e.g.,
altered predation pressure) events. The major
use of key indicators, aside from their use in
pollution studies, seems to have been in evaluat-
ing suitable habitat for instream flow assess-
ments. Bovee (1986) described procedures for
selecting key indicators for use in evaluating
habitat requirements, although he emphasized
fish and not benthic macroinvertebrates. Bovee
recognized that it can be impractical to conduct
a separate habitat suitability study on each
"target" species so he suggested grouping species
that behave similarly into guilds. Bovee (1982)
defined a guild as "a group of species having
similar habitat requirements and exhibiting
similar responses to changes in streamflow," and
suggested that habitat suitability criteria (e.g.,
depth, current velocity, substrate) be developed
for macroinvertebrates on the basis of factors
such as biomass, density, or secondary produc-
tion. Recently, Orth (1987) also recommended
inclusion of secondary production estimates in
instream flow assessments. Bovee further sug-
gested a method of circumventing the assump-
tion of equal availability of macroinvertebrates
as fish food by dividing the macroinvertebrate
community into accessible versus nonaccessible
species. Drift samples could be used to deter-
mine which species are most common in the
drift. Then, criteria would be based on the
density or production of only these species.
This approach would be valid, of course, only if
the fish of interest fed mainly on drift. Bovee
also suggested the use of functional feeding
groups as a more traditional method of assign-
ing macroinvertebrates to guilds.
Functional feeding groups may not be always
the ecologically meaningful ones (Minshall
1988). Minshall referred to a study by Hawkins
et al. (1982) that did not find the expected shift
from shredders to grazers in unlogged versus
logged watersheds. Minshall felt that more
information could have been obtained if the 8 to
10 species comprising most of the biomass were
evaluated separately rather than combining the
entire community into 3 to 6 composite catego-
ries (e.g., grazers, collectors, and shredders).
Minshall also pointed out that, because one or
two taxa often determine the outcome for a
particular feeding group, this taxonomic identity
should be maintained.
Gore (1977, 1978) described the use of
benthic macroinvertebrates as "indicators" of
optimum flow conditions in riffle habitats along
the Tongue River in Montana. He based the
indicator concept on the knowledge that certain
organisms have a narrower tolerance than others
for changes in the frequency and intensity of
discharges. Gore modeled species responses
using habitat suitability criteria for the variables
of depth, current velocity, and substrate. Gore
and Judy (1981) proposed new predictive models
for determining optimum conditions for stream
macroinvertebrates with respect to maintaining
suitable lotic habitat. These authors concluded
that the concept of indicator invertebrate spe-
cies, on a site-specific basis, can be adequately
utilized to predict necessary stream flows. Orth
and Maughan (1983) also concluded that the
derivation of habitat preference criteria should
be on a species-specific basis and that a complex
of variables should be considered. In recent
reviews, Gore addressed many of the criticisms
that have been directed toward the use of habi-
tat suitability criteria in assessing instream flow
requirements, particularly their use with benthic
invertebrates (Gore 1987; Gore and Nestler
1988; Gore 1989).
Future Needs. Except for the work by Gore
(1977, 1978) and Gore and Judy (1981), stream
ecology studies have not utilized "indicator
species" in assessing microhabitat requirements
for benthic macroinvertebrates. However, the
use of indicator organisms is not new in ecology.
Lenhard and Witter (1977) reviewed the use of
aquatic insects as indicators of environmental
pollution. Other studies have suggested use of
communities, rather than species or species
complexes (Gaufin and Tarzwell 1956; Harris et
al., 1984); habitat guilds (Bovee 1986) including
production estimates (Orth 1987); or species
diversity (Gore et al. 1992) as "indicators" of
flow needs. In the Southeast, macroinverteb-
rate key indicators and/or habitat guilds, have
not been identified in streams and studies are
needed to evaluate these approaches for deter-
mining optimum habitat for stream inverte-
brates.
20
TROPHIC RELATIONSHIPS BETWEEN
FISH AND MACROINVERTEBRATES
Understanding the interactions between fish
and macroinvertebrate communities in stream
ecosystems is fundamental both to the dynamics
of natural systems and to the management of
aquatic resources for food production and
recreation. In lentic waters, there is clear
evidence that fish predation can directly alter
the composition of macroinvertebrate communi-
ties (Brooks and Dodson 1965; Hall et al. 1970).
Yet, in lotic waters, trophic linkages to fishes
are not well understood. In fact, in a review of
predator-prey relationships in streams, Allan
(1983) concluded that fish do not commonly
play a major role in structuring invertebrate
communities.
Several studies in high gradient streams have
shown that, in general, invertebrate densities
and community structure are unaffected by fish
predation (Allan 1982; Reice 1983; Flecker and
Allan 1984; Culp 1986; Reice and Edwards
1986). These experiments were conducted in
streams having coarse substrates with average to
moderate flows. In addition, salmonids were
typically the only major vertebrate predator in
each case except in studies by Reice (1983) and
Flecker and Allan (1984). Differing results were
reported, however, by Hemphill and Cooper
(1984), in which trout eliminated conspicuous
taxa such as amphibian larvae, notonectids, and
lestids that live on substrates or in the water
column, but taxa that burrowed into the sub-
strate, or were less conspicuous, were generally
unaffected. Hemphill and Cooper demonstrated
the importance of refugia in determining the
effects of predators on specific prey taxa.
In other studies, Flecker (1984) and Koetsier
(1989) tested the impact of predation by benthic
fish (sculpins) on invertebrates. Koetsier
(1989), working in Idaho, found that mottled
sculpins significantly reduced the density and
biomass of several invertebrate species; and
Flecker (1984), working in West Virginia, no-
ticed that a vertebrate predator guild, including
sculpins and blacknose dace, significantly de-
pressed the abundance of chironomids and the
stonefly Leuctra. Because chironomids com-
prised about 85 percent of the benthic fauna in
this study, fish predation played an important
role in structuring the macroinvertebrate
community.
Schlosser and Ebel (1989) stocked four
species of cyprinids in an experimental stream.
The cyprinids significantly reduced invertebrate
abundance, however, the effect of predation was
habitat-related because abundance decreased
most in "structurally complex" pools, but showed
little response to predation in shallow riffles and
raceway habitats. For example, numbers of
pool-dwelling chironomids and crustaceans
declined more in the presence of fish than riffle-
dwelling hydropsychids and simuliids. Schlosser
and Ebel concluded that cyprinids probably
restrict their habitat to deep stream pools with
cover to minimize their exposure to terrestrial
wading predators.
Several investigators have suggested that
invertebrates are more susceptible to fish preda-
tion in streams containing silt-sand substrates
than in streams with coarse materials
(Angermeier 1985; Gilliam et al. 1989). Juve-
nile creek chubs (Cyprinidae) in a silty-bottom
stream reduced total invertebrate volume and
density by 79-90 percent and 55-61 percent,
respectively (Gilliam et al. 1989). Chironomids
showed no fish effects, but the two dominant
taxa, Oligochaeta and Isopoda, were heavily
grazed by the chubs thus causing a shift to
smaller size classes for both groups. The main
differences in the stream used by Gilliam et al.
and streams in other studies were soft sedi-
ments, higher density of fish, low drift rates, and
lower flow rates.
The work by Gilliam et al. (1989) strongly
supports the view that stream fish can signifi-
cantly influence benthic invertebrate communi-
ties in warmwater streams. This conclusion
contrasts with published work on salmonid fish
in streams (Allan 1982; Culp 1986; Reice and
Edwards 1986). In fact, in the study by Flecker
(1984) in which fish predation played an impor-
tant role in structuring the macroinvertebrate
community, he deemphasized the effect by
concluding that strong predation effects (i.e., by
fish) are likely only with macroinvertebrates that
are not replenished rapidly by drift (e.g., the
chironomids and Leuctra), or that occur in slow
moving waters.
Drift is the major mechanism for dispersal of
aquatic stages of stream invertebrates (Williams
and Hynes 1976), and several studies have
suggested that many fishes obtain a portion of
21
their diet from the drift (Henry 1979; Mancini et
al. 1979; Benke et al. 1985, 1986). Benke et al.
(1985) found that, although most of the larger
fish species consumed invertebrates originating
from snag habitats, several smaller species (e.g.,
minnows and darters) tended to ingest either
sand-dwelling midges or terrestrial insects from
the water surface. Their work suggested two
food chains in this Coastal Plain stream: a sand
fauna -> small fish -> piscivore food chain; and
a snag fauna -> sunfish food chain. Benke et
al. (1985) did not include any analysis of chang-
es in community structure because of fish preda-
tion, however they suggested that because of the
importance of snag habitat as a source of macro-
invertebrates for fish, extensive removal of snags
in low-gradient streams could be devastating to
the fish community, especially sunfishes.
Future Needs. There is strong evidence that
in warmwater streams of the Southeast, higher
water temperatures enhance the ability to detect
predation effects of fish on macroinvertebrates
because the fish have higher metabolic and
consumption rates (Flecker 1984). However, to
adequately address trophic relationships between
fish and macroinvertebrates, it may be necessary
to conduct experiments in artificial streams 
that
are large enough to accommodate appropriate
predator-prey populations, but in which environ-
mental variables can be controlled.
Field studies also are needed to determine,
in a greater range of streams, the importance 
of
snag habitat as a source of food for fishes in
low-gradient streams. Would removal of snag
habitat actually cause a shift in the fish commu-
nity from sunfishes to one dominated by suckers
and the small fishes that feed on benthic fauna?
Would addition of snag material to barren
streams enhance the diversity, abundance, and
production of both the invertebrates and the
fishes that depend on them for food? Because
the role of fish predation in streams has consid-
erable practical as well as theoretical interest, a
critical need exists to develop improved ap-
proaches to answer these questions.
BENTHIC MACROINVERTEBRATES
IN REGULATED RIVERS
The mechanisms that macroinvertebrates
have evolved to cope with water level fluctu-
ations in unregulated streams may be nonfunc-
tional in regulated streams, especially below
hydropower dams where there are rapid changes
in flow. In fact, little information is available on
functional responses of macroinvertebrates to
peaking hydroelectric flows (Gore 1989; Gore et
al. 1989; Gore et al. 1990; Troelstrup 
and
Hergenrader 1990). The large and rapid (within
minutes) changes in discharge at hydroelectric
dams result in corresponding changes in flows in
tailwaters. In addition, associated with the
change in flow are changes in other variables
(e.g., depth, width, velocity, water temperature
and quality). The potential impacts of these
short-term, recurring disturbances downstream
from dams are important considerations for the
biota in tailwaters (see review by Armitage
1984).
Observed Effects. Fluctuations in flow
resulting from hydroelectric peaking operations
have been associated with reductions in river
productivity in terms of both the tailwater
fishery (Trotzky and Gregory 1974) and benthic
macroinvertebrates on which the fish popula-
tions depend (Fisher and LaVoy 1972; Trotzky
and Gregory 1974; Nestler et al. 1986; Curtis et
al. 1987; Nestler et al. 1988).
A number of variables have been used by
stream ecologists to document the reduced
productivity or carrying capacity of tailwaters
affected by rapidly fluctuating flows. These
include reduced macroinvertebrate diversity,
density, and biomass (Fisher and LaVoy 1972;
Kroger 1973; Trotzky and Gregory 1974;
Gislason 1985). Studies conducted in the
Southeast have found high macroinvertebrate
density, but low diversity just downstream from
both flood-control and hydropower dams
(Webber 1979; Ney and Mauney 1981; Herlong
and Mallin 1985; Jackson 1985; Hudson and
Nichols 1986; Reed 1989). However, studies in
which change in flow was gradual over a period
of several days revealed negligible effects on
macroinvertebrate communities (Williams and
Winget 1979; Gersich and Brusven 1981).
Underlying Mechanisms. As flow is altered
below dams, several variables are affected
including velocity, depth, width, and wetted
perimeter (the distance along the stream bottom
from one shoreline to the other). Cross-section-
al geometry is the primary determinant of the
relationships among these variables, thus, the
empirical relationship between discharge and
22
velocity is site-specific. For example, Williams
and Winget (1979) reported that, when flow was
reduced, mean velocity was reduced more than
width or depth.
One direct consequence of increased variabil-
ity in discharge is that the daily range between
minimum and maximum flow, velocity, depth,
width, or other hydrologic variables may in-
crease over the corresponding unregulated
range, therefore, over short periods of time, the
range of physical habitat conditions to which the
biota are exposed can be greater in regulated
than in unregulated streams (Cushman 1985).
This situation could pose a threat to macro-
invertebrates, such as various filtering collectors
(e.g., net-spinning caddisflies or simuliids) that
require specific velocities for food capture
(Gordon and Wallace 1975; Cudney and Wallace
1980). The lack of a hydraulic equilibrium in
tailwaters also violates a major assumption of
the Instream Flow Incremental Methodology
developed by the U.S. Fish and Wildlife Service
(Bovee 1982) to assess instream flow require-
ments of aquatic biota; this could confuse the
use of such methodologies in assessing instream
flows necessary for protecting tailwater fisheries
(Cushman 1985). However, Gore et al. (1989)
demonstrated that dynamic flow models can
correct the problem of lack of equilibrium flows.
Another consequence of dams involves the
elimination of high seasonal discharges in down-
stream reaches. This change promotes sedimen-
tation, which reduces habitat heterogeneity,
including the availability of the hyporheic zone
(Williams and Winget 1979). Williams and
Winget also found enhanced algal growth as a
result of this change in discharge. Although the
number of macroinvertebrate species changed
little in their study, Williams and Winget found
marked shifts in community structure.
High flows may still occur downstream from
dams, if only during hydroelectric operations.
These high flows have often caused extensive
armoring (i.e., flushing of fines leaving coarse
bed materials) of the stream bed (Matter et al.
1983a; Gislason 1985) and created substrate
instability that negatively impacts populations of
shelter-building macroinvertebrates and mol-
lusks (Gashignard and Berly 1987).
Rapid flow variations below dams can also
be accompanied by rapid changes in water
quality and temperature, especially when there
is a hypolimnial discharge (Gore 1980). Conse-
quently, spates of discharged water can be quite
different from downstream water with respect to
temperature, dissolved oxygen (DO), hydrogen
sulfide, ammonia, iron, manganese, and other
chemicals (Brooker 1981). Temperature chang-
es downstream from reservoirs can be dramatic,
changing by several degrees Celsius (Matter et
al. 1983a; Walburg et al. 1983); thus causing
shifts in community structure (Pfitzer 1954;
Gore 1977; Hauer and Stanford 1982) and
altering emergence patterns of aquatic insects
requiring specific thermal stimuli (Hauer and
Stanford 1982; Walberg et al. 1983; Yemelina
1988; review by Ward and Stanford 1979).
Low DO levels in hypolimnetic discharges
reduced macroinvertebrate density and diversity
in several studies downstream from Tennessee
Valley Authority (TVA) reservoirs (Isom 1971;
Hill 1980; Yeager et al. 1987) and below Jordan
Dam in Alabama (Harris et al. 1989). In gener-
al, macroinvertebrate communities in tailwaters
with seasonally low DO were dominated by
chironomids. Reductions in macroinvertebrate
density downstream from reservoirs has also
been attributed to poor water quality other than
low DO (Petts 1984). For example, oxidation of
iron and manganese compounds are known to
cause benthic deposits and coatings on sub-
strates that may indirectly influence distribution
and structure of benthic communities (Krenkel
et al. 1979).
Changes in discharge or water level have
been found to stimulate invertebrate drift.
Increases in drift rate have been associated with
initial discharge surges (Matter et al. 1983b;
Irvine 1985), although other investigators
(Minshall and Winger 1968; Armitage 1977;
Corrarino and Brusven 1983) reported increased
drift rates following reductions in stream dis-
charge. Increased drift, particularly if it occurs
during daylight hours, could increase feeding
activity by fish (Minshall and Winger 1968;
Corrarino and Brusven 1983), although if in-
creased drift continued for a long time, the
benthos obviously could be depleted (Minshall
and Winger 1968; Gore 1977), causing lower fish
productivity. Drift losses during peaking opera-
tions have been calculated as almost 14 percent
of the macroinvertebrate standing crop in 1
month in a 12-km tailwater reach (Matter et al.
23
1983a). However, it is also clear that inputs of
seston (zooplankton and Chaoborus) from the
reservoir supplements the tailwater food base
through drift (Novotny and Faler 1982; Matter
et al. 1983a; Jackson 1985). In addition to the
changes in discharge, changes in depth, width,
and velocity have also been implicated in the
stimulation of drift (Gore 1977; Ciborowski et
al. 1977).
Invertebrate drift changes, both in quantity
and quality, with distance downstream from an
impoundment. Zooplankton populations that
usually comprise the main component of the
drift immediately below the dam decline rapidly
with distance downstream (Keefer 1977;
Novotny and Faler 1982; Jackson 1985), primari-
ly because of their removal from the water
column by filter-feeding macroinvertebrates
(Herlong and Mallin 1985). With increasing
distance downstream, the drift composition
begins to resemble that of an unregulated
stream (Keefer 1977; Novotny and Faler 1982).
As zooplankton moves downstream, there is a
differential removal of organisms with the larger
microcrustaceans disappearing from the drift
before the smaller rotifers (Herlong and Mallin
1985). They attributed this finding to the larger
mesh nets of filter-feeders in the immediate
tailwaters and selective removal of the larger
microcrustaceans.
Macroinvertebrate Communities. Southeast-
ern tailwaters are usually dominated by small-
size collectors, both filterers and gatherers, with
few shredders, predators, or grazers (Krenkel et
al. 1979; Walburg et al. 1983; Herlong and
Mallin 1985; Novotny 1985; Hudson and Nichols
1986; Yeager et al. 1987). Several kilometers
downstream from a dam, Herlong and Mallin
(1985) and Walburg et al. (1983) found the
fauna to resemble that predicted by the RCC for
unregulated streams with filtering collectors still
dominant, but gathering collectors, predators,
and grazers were well represented.
Food is a critical factor determining the
composition of macroinvertebrate communities
of tailwaters, along with the abiotic variables
already described. In regulated streams, the
reservoir is the main source of food for down-
stream macroinvertebrates, although tributaries
to tailwaters may contribute significant quanti-
ties of food. Reservoirs serve as particle traps
and retain much of the POM washed in from
the watershed (Matter et al. 1983b) and because
reservoirs release POM in the form of limnetic
plankton (Webster et al. 1979), tailwaters gener-
ally do not receive the same type of allochthon-
ous POM recharge as that found in natural
headwater streams.
Higher quality seston (i.e., greater quantities
of plankton) is more common from surface-
release dams than that found in hypolimnial
releases, mainly because the water originates
from the epilimnion of the reservoir where
plankton occurs in greater densities than in the
hypolimnion. Gore (1977, 1980) reported low
densities of aquatic insects below a reservoir
that he attributed, in part, to the lack of seston
in the hypolimnial discharge. However, several
tailwater reaches in the Southeast, fed by hypoli-
mnial releases, have high macroinvertebrate
densities, apparently supported by entrained
plankton (Novotny and Faler 1982).
Species richness reported in Southeastern
tailwaters is often low, relative to that above the
reservoir (Hill 1980; Novotny 1985). However,
Hudson and Nichols (1986) found over 200
invertebrate taxa below Hartwell Dam on the
Savannah River and concluded that the high
diversity may have been previously overlooked
because the fauna consisted mostly of small
oligochaetes and chironomids, often not identi-
fied beyond the class or family level. Macroin-
vertebrate diversity commonly increases further
downstream as environmental conditions begin
to resemble unregulated reaches.
Future Needs. Considerable data exists
regarding various responses of benthic macroin-
vertebrates to stream regulation (Ward and
Stanford 1987), although interpretation is often
limited by a lack of basic knowledge (e.g., life
history data, food habits, microhabitat require-
ments) for many lotic macroinvertebrates. A
critical need exists for studies (i.e., biomass and
production) that address how the life cycles of
macroinvertebrates in tailwater reaches have
adapted to altered thermal patterns, flow re-
gimes, and food supply (i.e., quantity and quali-
ty).
The short-term flow fluctuations downstream
from hydroelectric dams strand macroinverte-
brates along shorelines and induce drift
(Brusven 1984). Because drift dynamics in
tailwaters differ greatly from that found in
unregulated streams, several questions need to
24
be addressed.
(1) What are the recolonization mechanisms
in tailwater reaches?
(2) Does catastrophic drift characterize
habitats below dams with frequent disturbance
(peaking flows), and will recovery by macroinve-
rtebrate communities be faster in these fre-
quently disturbed streams than in rarely dis-
turbed streams because the component species
have adapted to the disturbance events?
(3) What type of drift periodicity is exhibited
by the principal macroinvertebrates colonizing
substrates in tailwaters?
(4) What are the ecological mechanisms that
determine behavioral drift in regulated stream?
There is also need for better understanding
of how macroinvertebrate communities have
adjusted their microhabitat preferences in
tailwaters to utilize the seston originating from
the reservoir. It is clear that tailwater communi-
ties are dominated by collectors that depend
heavily on reservoir seston. Finally, there is a
strong need for more information on the trophic
relationships between the macroinvertebrate
communities in tailwaters and the resident
fishery. We found one study on this relation-
ship in regulated streams of the Southeast
(Odenkirk 1987).
If changes in structural and functional vari-
ables occur along the river continuum, then the
impact of stream regulation on these variables
should be influenced by dam location (Ward and
Stanford 1987). The serial discontinuity concept
(SDC) was developed in the last decade, to
provide a broad conceptual framework with
which to address the disruptions (discontinu-
ities) in the river continuum brought on by
stream regulation (Ward and Stanford 1983).
These authors listed four objectives of the SDC:
(1) to recognize the importance of dam position;
(2) to propose discontinuity distance as a mea-
sure of the upstream or downstream shifts (e.g.,
in heterotrophy versus autotrophy) that result
from impoundment; (3) to promote a watershed
approach for research in regulated streams; and
(4) to offer a predictive conceptual model to be
tested. With the SDC model, Ward and Stan-
ford predicted downstream shifts of thermal
regimes and POM dynamics. Stated differently,
streams influenced by impoundments tend to
mimic lower order areas upstream.
The SDC has had only limited testing to
date. Stanford and Ward (1984) examined the
limnology of a series of impoundments along
the Gunnison River in Colorado and reported
results consistent with their predictions of serial
discontinuity; that is, shifts in distribution were
attributed to resets in hydrologic patterns,
temperature changes, and nutrient availability.
Gore and Bryant (1986) examined impacts of
multiple impoundments on the Arkansas River.
Implications of the SDC include a displacement
of benthic macroinvertebrate functional groups
and fish assemblages downstream, such that mid-
order reaches should resemble low-order
streams in comparable natural stream systems.
In general, Gore and Bryant found SDC predic-
tions valid. However, their work indicated that
substrate was the primary factor determining
macroinvertebrate distributions; and impacted
areas exhibited communities that contained low-
order functional groups. However, fish commu-
nities did not resemble a low-order community
in the RCC; thus the impoundment did not
function as a reset mechanism for fish, but as a
distributional barrier.
ACKNOWLEDGMENTS
This work was sponsored, in part, by the U.S.
Fish and Wildlife Service, National Ecology
Research Center, under Contract Number
1416000989962 with Auburn University. Con-
structive criticisms, new insights and helpful
suggestions were made by J.H. Crance, L.S.
Ishinger, C.B. Stalnaker, W.L. Fisher, D.B.
Rouse, J.H. Grover, and G.W. Folkerts. Inde-
pendent, critical reviews were provided by W.T.
Mason and J. Gore and their suggestions were
invaluable in making final revisions.
25
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