s, tiff ~ ~~iil r Circular 215 June 1975 (Iompitii for lilit Piii . $ 4, inl Iis11 1onlds AGRICULTURAL AUBURN EXPERIMENT UNIVERSITY STATION R. Dennis Rouse, Director, Auburn, Alabama CONTENTS page INTRODUCTION----------- -- -- ---- -- - -- 3 4 MATERIALS AND METHODS----------------- Effect of Light Intensity on Photosynthesis by Aquatic W eeds ------- ----- ----- ------ ------ 4 ---5 6 Competition Between Phytoplankton and Underwater W eeds--------------------Ecological Effects of DuckweedsRESULTS AND DISCUSSION-------------- ----- 6 Effects of Light Intensity on Photosynthesis by A quatic Weeds------------------------------------Competition Between Phytoplankton and U nderwater W eeds---------------------------------- 6 9 Standing Crops of Aquatic Weeds----------------------12 Ecological Effects of Duckweeds-----------------------13 S UM M A RY -- - - - - - - - - - - - - - --- - -- - -- - - - - - - - - - - - - - - - - - - - - - 18 LITERATURE CITED----------------------------------- 18 FIRST PRINTING 3M, JUNE 1975 Auburn University Is An Equal Opportunity Employer [2] COMPETITION FOR LIGHT BY AQUATIC PLANTS Claude E. Boyd* INTRODUCTION NMANAGED PONDS in most areas of Alabama often have relatively clear water and are therefore excellent habitats for growth of underwater and emergent plants. These plants cause unbalanced fish populations, interfere with fishing and fish harvest, favor mosquito production, and compete with phytoplankton for light and nutrients (5). Early studies on fertilization of fish ponds revealed that additions of inorganic nutrients increased phytoplankton production and fish yield. The resulting phytoplankton turbidity also shaded and eliminated underwater weeds from ponds (15, 16). Small, sheltered fish ponds with waters rich in organic matter are often infested with duckweeds (6,8,13). These plants grow on pond surfaces and interfere with gas exchange between water and atmosphere, inhibit photosynthesis by underwater plants, and cause low oxygen tensions in waters beneath (7). Fertilization is not effective in controlling duckweeds. The early studies of pond fertilization were primarily concerned with immediate means of increasing fish production, but through necessity, a number of practical generalizations were made on the ecology of aquatic weeds in ponds (14,15,16,18,19). Unfortunately, many of the observations on the ecology of aquatic weeds have never been substantiated with data and are open to criticism. The present report contains data on competition between aquatic plants for light and the influence of this competition on the pond environment. a Associate Professor, Department of Fisheries and Allied Aquacultures. U MATERIALS AND METHODS Effect of Light Intensity on Photosynthesis by Aquatic Weeds Nutrient solutions were prepared from well water which had been heated with stirring to reduce its dissolved oxygen content. Lowering of the dissolved oxygen content was necessary to prevent supersaturation of water with dissolved oxygen during photosynthesis experiments (10). After cooling, ammonium nitrate and potassium dihydrogen phosphate were added to raise concentrations of nitrogen and phosphorus to 1.0 and 0.25 parts per million, respectively. Available carbon in different batches was similar; pH (6.95 to 7.25), total alkalinity (41.7 to 43.8 parts per million), and carbon dioxide (5.3 to 8.4 parts per million). Dissolved oxygen concentrations in different batches ranged from 4.0 to 5.5 parts per million. An analysis of well water by atomic absorption spectrophotometry gave the following results in parts per million; calcium 4.0, magnesium 2.8, sodium 6.5, potassium 1.1, iron 3.4, manganese 0.05, zinc 0.29, and copper 0.05. Aquatic weeds were collected from depths of 1 to 3 feet in ponds on the Fisheries Research Unit of the Auburn University Agricultural Experiment Station, Auburn, Alabama. In each series of photosynthesis measurements, fresh 1.0-gram samples of plants were placed in each of 15 standard BOD bottles. The bottles were put in a dark box and transported to a nearby pond. Nutrient solution was introduced through a siphon tube which discharged at the bottom of bottles. Bottles were allowed to overflow for one complete exchange of water, stoppered, and returned to the dark box. Eleven BOD bottles were attached horizontally by universal clamps at 10-inch intervals along a metal rod. The rod was then suspended vertically so that bottles were positioned at 10-inch intervals from the surface to 8.3 feet. Two bottles (dark bottles) were wrapped in aluminum foil and suspended at depths of 1 and 4 feet. As soon as light and dark bottles were in place, nutrient solution from the two remaining bottles (initial bottles) was siphoned into 60-milliliter bottles for dissolved oxygen determinations by the Winkler method (1). Transfer of plant material was prevented by fitting a piece of bolting silk over the siphon intake. After 1-hour incubation, light and dark bottles were quickly placed in the dark. Bottles were then removed one at a time and nutrient solution siphoned to 60-milliliter bottles for dissolved oxygen analysis. Dark bottles were always siphoned last and the time of dark storage (5 to 15 [4] minutes) between the end of incubation and siphoning recorded. Rates of respiration and net photosynthesis per gram fresh plant material were calculated from the following equations. MO 2 I.B. - /MO 2 D.B. (1) Respiration (iuMO 2/hr) = 1hr. + t MO 2 (2) Net photosynthesis (ILMO 2/hr) = , MO 2 L.B. I.B. + (t) (Respiration) Where: ~MO 2/hr = micromoles of oxygen per hour, t = time light bottle, of dark storage in hours, I.B. = initial bottle, L.B. dark bottle, and 1 hour = time of incubation. and D.B. The 1-hour incubations were conducted within the periods 8:30 a.m. to 10:30 a.m. and 1:00 p.m. to 4:00 p.m. on clear, calm days during June and July, 1974. Intensity of light was measured at 10-inch depth intervals with a Tsurumi-Seiki submarine illuminance meter. Measurements were repeated every 15 minutes during each incubation period. Approximately 93 percent of the light passed through one side of the BOD bottles, so all measured light intensities were multiplied by a factor of 0.93. - Competition Between Phytoplankton and Underwater Weeds The extent of coverage by aquatic weeds of some ponds on the Fisheries Research Unit and in the vicinity was estimated during June and July, 1972. Estimates of turbidity were by Secchi disk visibility.2 Phytoplankton density was estimated from chlorophyll a analysis (20) and from primary productivity determinations by the oxygen light-dark bottle technique (1). In two ponds, measurements of Secchi disk visibility, chlorophyll a content of water, and cover of bottom by macrophytes were made at biweekly intervals from February 14 until August 31, 1973. Concentrations of soluble inorganic and total phosphorus in water samples were measured by standard techniques (1). To estimate standing crops of macrophytic algae, an openended plastic cylinder (10-inches in diameter) was pressed into the mud. Enclosed plant material was either hand-picked or removed with a dip net. All debris was removed and algae dried at 2200 Fahrenheit. Three to five, 10-inch diameter circles of algae were harvested from each pond. underwater light meter. depth at which an 8-inch diameter disk with two black and two white quadrants disappeared from view. 1 An 2The [5] Ecological Effects of Duckweeds Twelve circular pools (10 feet in diameter) were built in an unshaded area on the Fisheries Research Unit; they consisted of vinyl plastic liners attached to corrugated steel siding. The bottom of each pool was filled to 3-inch depth with a Cecil sandy loam top soil and water was maintained at depths of 20 to 24 inches by periodic additions or removals. On August 20, 1974, Spirodela polyrhiza and Wolffia columbiana were added to cover the surfaces of each of four pools, respectively. Four pools were maintained as controls. Fertilizer (3 ounces of a 20-20-5 mixture) was added to each pool on August 20, 1974, and again on September 10. Changes in water levels caused by evaporation and evapotranspiration were measured daily from August 23 to September 25, 1974, with a hook gage. A rain gage was positioned beside pools for correction of data on water level changes during days when it rained. Maximum-minimum air temperatures were obtained daily from thermometers mounted in a Weather Bureautype instrument shelter. Water temperature and dissolved oxygen concentrations were measured daily between 2:00 and 2:30 p.m. at 0, 3, 10, and 20-inch depths with a submersible thermister and a polarographic oxygen meter. Light penetration measurements were made near noon with a submersible photometer. Gross photosynthesis was measured on three dates by the oxygen lightdark bottle technique (1). Bottles were incubated at 10-inch depth. On September 20, water samples were collected from a depth of 10 inches for phytoplankton enumeration and chemical analysis (1). Standing crops of W. columbiana and S. polyrhiza were determined on two dates. Plants were collected by placing a finemesh, 8-inch diameter sieve beneath the mats of duckweeds and then lifting the sieve. Fronds were removed from the sieve and dried at 220 ° F to constant weight. RESULTS AND DISCUSSION Effects of Light Intensity on Photosynthesis by Aquatic Weeds Relationships between light intensity and net photosynthesis in eight species of macrophytic algae and five species of submersed higher aquatic plants are presented in figures 1 and 2. All regression equations were quadratic and correlations between [6] loo 1 Rhizoclonium hieroglyphicum 40 80 O/0 0, Hydrodictyon re ticula turn /O O/ 0 60 0/0/ 0' 40 / / 30 '0 0. 0 - U) Chara vulgaris I / 0 20 F 0/ 0 Pithophora kewensis 20 0 0 01 S Spiroyra sp 40 * O" Cladophora o20 crispata N 30 0 E5 0 20 o/ 0/ " to- 0 10 20 LUX X 30 103 40 50 0 to 20 LUX X 30 103 40 50 FIG. 1. Net photosynthesis by 1-gram samples of eight species of macrophytic algae which were incubated for 1 hour at different light intensities. light intensity and net photosynthesis were significant at the 0.01 level of probability. Maximum rates of net photosynthesis in algae occurred between 20,000 and 35,000 lux, .but light intensities above 35,000 lux were inhibitory to all algae except Chara vulgaris and Cladophora crispata. Photosynthesis by algae was measureable at light intensities below 2,000 lux, and 50 percent of maximum net photosynthesis was attained at light intensities between 4,000 and lower light requirements for photosynthesis than did macrophytic algae. Maximum rates of net photosynthesis in higher plants were recorded at light intensities of 10,000 to 20,000 lux. A 50 percent reduction in photosynthesis was not affected until light intensities dropped below 2,000 to 5,000 lux. These data, figures 1 and 2, suggest that macrophytic algae and submersed and that the angiosperms have rather low requirements for requirements for different species do not differ greatly. 9,000 lux. The higher plants had slightly light [7] 40 r Eleocharis ,--a E El5dea - acicularis 30 0 00 densa 20 E 00 50 40 0' I I 30 * 30 cv 20 (r~ohylmdreru demesum 20 * Najas flexilis 0 1 L0 50 140 0 1 U 0 5 FI.2E e htsnhssb -rmsmlso ieseiso umre nisemautcweswihwr nuae o Transparency of water in the pond where samples were incubated did not change appreciably during the study. Secchi disk visibilities for different days averaged 38.6 inches with a standard error of ± 0.24 inches. The maximum depth of immersion at which net photosynthesis was detectable ranged from 6.6 to 7.4 feet or roughly twice the Secchi disk visibility. Plants must produce enough extra photosynthate during daylight to supply carbohydrate for night time respiration. Therefore, the actual depth to which plants can occur will be somewhat less than twice the Secchi disk visibility. The usual recommendations hold that weed control can be affected by deepening pond edges to 2 feet and adding fertilizer at a rate sufficient to restrict underwater visibility (or Secchi disk visibility) to 12 inches (9, 19). Findings reported above concur that underwater weeds could not grow in waters deeper than 2 feet which have Secchi disk visibilities of 12 inches or less. In fact, it appears from the photosynthesis experiments that even greater Secchi disk visibilities would be permissible without encouraging weed growth. Competition Between Phytoplankton and Underwater Weeds Fish ponds which were turbid with plankton were essentially free of underwater weeds, Table 1. Secchi disk visibilities averaged 5.7 feet in ponds which were infested with weeds, but only 1.4 feet in ponds which were essentially free of weed problems. Data on gross photosynthesis and chlorophyll a concentrations, Table 1, confirm that the differences in turbidity resulted from differences in phytoplankton abundance between the two groups of ponds. Furthermore, ponds with aquatic weed problems had much lower concentrations of soluble inorganic phosphorus and total phosphorus than ponds with phytoplankton blooms. This was not surprising because most of the ponds with phytoplankton blooms had been fertilized, while most of the ponds with weed infestations had not been fertilized. More direct evidence of the influence of turbidity on macrophytic algae is illustrated, Figure 3. Spirogyra sp. and Hydrodictyon reticulatum grew well in two ponds from mid-February until early June when phytoplankton was scarce and waters transparent. Turbidity increased in early June, following a burst of phytoplankton growth. Macrophytic algae essentially disappeared from both ponds by early July and did not reappear during restriction of light penetration. [9] Recommendations on the use of fertilizers for weed control in fish ponds stress that fertilization programs be initiated only when underwater weed populations are at low abundance; usually in late winter (15,16). Otherwise, fertilizer will stimulate additional aquatic weed growth and prevent the development of a plankton turbidity sufficient to shade the weeds. Data in Table 2 illustrate that shading within dense populations of weeds will greatly restrict phytoplankton photosynthesis as compared to photosynthesis in open water areas of the same pond. Ponds with waters turbid from suspended soil particles or humic substances are often not troubled by underwater weeds. However, the turbidity of such ponds also restricts phytoplankton growth and prevents good fish production. Although fertilization is an effective means of increasing fish production and controlling weeds, excessive fertilization is wasteful, expensive, and may cause too much plankton growth. Beasley (2) demonstrated that the depth of penetration of adequate TABLE 1. LIMNOLOGICAL DATA FOR PONDS WITH PHYTOPLANKTON BLOOMS OR COMMUNITIES OF AQUATIC WEEDS. SEVEN PONDS OF EACH TYPE WERE USED FOR OTHER FOR GROSS PRIMARY PRODUCTIVITY MEASUREMENTS. DETERMINATIONS, SAMPLE SIZE WAS AND 20 AND PONDS 22 FOR PONDS WITH PHYTOPLANKTON BLOOMS WITH WEED PROBLEMS, RESPECTIVELY Measurement5% Secchi disk transparency (in) Chlorophyll a (ppb) Gross primary productivity (ppm oxygen/day) Soluble inorganic phosphorus (ppm) Total phosphorus (ppm) 1Averages ± two standard errors. TABLE 2. Ponds with less than of bottoms covered by aquatic weeds 16 ± 3 86 ± 16 6.4 ± 1.0 0.050 ± 0.031 0.151 ± 0.048 Ponds with 10 to 100% of bottoms covered by aquatic weeds 69 ± 13 13 ± 5 1.5 ± 0.3 0.011 ± 0.003 0.042 ± 0.012 GRoss PHOTOSYNTHESIS BY PHYTOPLANKTON OF OF THE SAME BENEATH MATS OF IN Pithophora kewensis DepthGross AND WITHIN STANDS Chara braunii PONDS AND OPEN WATER AREAS photosynthesis (ppm 0 2/hr) Open water pen water 0.05 0.06 (ft) 0 0.7 Beneath mat of Pithophora 0.02 0.02 Open water Open water 0.08 0.12 Within stand of Chara 0.04 0.04 1.4 2.1 2.8 3.5 4.2 4.9 0.02 0.01 0.01 0.01 0.0 .... 0.09 0.09 0.08 0.04 0.02 0.0-- 0.08 0.01 0.01 . 0.05 0.05 0.04 [10] 10 S-I I I S-II I S-13 150 S100- S-11 500 S-13 F M A M J J A FIG. 3. Upper: Relationships between chlorophyll a concentrations (dashed lines) and Secchi disk visibility (solid line) in two ponds. Lower: Abundance of macrophytic algae for comparison with data in upper portion of figure. light for photosynthesis by phytoplankton was inversely related to phytoplankton density. He also found that the depth of water containing enough dissolved oxygen for fish survival corresponded with the zone in which there was adequate light for photosynthesis. In response to low light intensity, blue-green algae form gas vacuoles and rise to areas of greater light intensity. Therefore, excessive phytoplankton growth usually results in surface scums of algae and shallow thermal and chemical stratification (4). Heavy scums of phytoplankton cause a number of problems including odors, off-flavor in fish, and direct toxicity to aquatic organisms (4). Under certain conditions, blooms of blue-green [ 111 algae suddenly die and their decomposition leads to oxygen depletion and fish mortality (4,17). Therefore, only enough fertilizer should be added to ponds to produce sufficient plankton to affect underwater weed control and the desired level of fish production. Standing Crops of Aquatic Weeds As stated previously, aquatic weeds may completely dominate plant communities of ponds and cause serious fish management problems. Weed populations may blanket the surface, cover the bottom, or completely fill the water column. These weed infestations appear to contain a large amount of plant material, but findings reported in tables 3 and 4 reveal that standing crops of weeds in Alabama fish ponds are generally below 2,500 pounds per acre dry weight. Emergent aquatic weeds which often occur TABLE 3. AVERAGES ± ONE STANDARD ERROR FOR DRY MATTER STANDING CROPS OF NINE SPECIES OF ALGAE IN PONDS Species No of samples Average standing crop (Lb/A) ± ± ± ± ± Spirogyra spp. Rhizoclonium hieroglyphicum Pithophora kewensis Chara fibrosa Chara vulgaris Chara braunii Nitella teniussima Hydrodictyon reticulatum Cladophora crispata Each sample represents the average of 3 to TABLE 4. AVERAGES 6 4 6 5 4 3 1 1 1 526 1,383 1,160 1,124 1,383 928 535 482 571 ± 80 285 196 223 214 223 5 subsamples from individual ponds. ± OF 12 SPECIES OF ONE STANDARD ERROR FOR DRY MATTER STANDING CROPS AQUATIC WEEDS IN PONDS Species No. of samples Average standing crop (Lb/A) ± Najas guadalupensis Potamogeton diversifolius Eleocharis acicularis Heteranthera dubia Myriophyllum heterophyllum M. brasilensis Ceratophyllum demersum Jussiaea repens Brasenia schreberi Herpestris sp. Wolffia columbiana Spirodela polyrhiza 1 6 6 6 1 1 1 1 6 4 3 8 4 937 1,124 2,069 1,650 731 3,559 651 3,675 1,365 1,971 598 491 ± 152 205 ± 562 ± ± ± ± ± 232 535 134 62 71 Each sample represents the average of 3 to 5 subsamples from individual ponds. [12] around the shallow edges of fish ponds produce more impressive standing crops of 8,000 to 20,000 pounds per acre (12). Some of the larger floating plants, e.g. Eichhornia crassipes of tropical and subtropical regions, may also produce standing crops greater than 10,000 pounds per acre dry weight (21). Plankton densities in fertilized ponds usually range from 20 to 40 ppm dry weight (18). In a pond with an average depth of 6 feet, the plankton crop will weigh about 360 to 720 pounds per acre. The average life of plankton organisms is brief (a few weeks or less), but there is a continuous replenishment by new growth. Aquatic weeds live longer and many of the individual plants present in the spring will survive until fall or winter. Therefore, production by plankton in fertilized ponds greatly exceeds the production of weeds in unmanaged ponds. Ecological Effects of Duckweeds Weather conditions during the study were normal for the locality and time of year. Skies were clear or partly cloudy, except for the period September 4 to 13, when a hurricane over the Gulf of Mexico caused cool, overcast conditions. Daily minimum air temperatures ranged from 52 to 68 ° F while the daily maximum values varied from 77 to 970 F. Rainfall occurred on 7 days, but at no, time did 24-hour totals exceed 1 inch. Averages - one standard error for standing crops of dry matter in pools on August 28 and September 23 were 1,052 ± 169 and 803 ± 71 pounds per acre for S. polyrhiza and 312 45 and 196 ± 9 pounds per acre for W. columbiana. Amounts of duckweed were adequate to cover surfaces of pools. Plants shaded pools and less light penetrated to a given depth in pools with duckweed than in control pools, Table 5. Larger S. polyrhiza fronds formed a more tightly massed assemblage and reflected and absorbed more light than did W. columbiana. Diurnal changes in temperature at different depths are illustrated in Figure 4. Temperatures in all pools increased from dawn until afternoon and then decreased with lowest temperatures occurring just before the next dawn. There was, however, a marked difference between temperature patterns in control and duckweed covered pools. The water column heated more uniformly in control pools during daylight hours and the temperature differential between top and bottom seldom exceeded 50 F. Duckweeds absorbed heat during daylight hours and surface waters were 5 to 7° F warmer than in control pools. Waters at - [13] Spirodela polyrhiza Wolffia columbiana Control E E E a. TIME (Hr) E E 0 o E o E - E o N TIME (Hr) TIME (Hr) FIG. 4. Diurnal changes in dissolved oxygen and temperature in plastic pools covered with one of two species of duckweed and in control pools. Each value is the average of data from four pools. 3-inch depths were at similar temperatures in all pools during daylight hours. Temperatures at 10 and 20-inch depths were considerably lower in pools covered by W. columbiana and S. polyrhiza. Temperature differentials between top and bottom in pools with duckweeds ranged from 9 to 18° F between 10:00 a.m. and 6:00 p.m. As air temperature declined, surface waters of pools covered by duckweeds lost heat and isothermal conditions existed at dawn. Temperature profiles recorded for 2:00 p.m. on September 10, Figure 4, were similar to profiles measured at 2:00 p.m. on other clear or partly cloudy days. During the period of cool, overcast weather (September 4 to 13), waters were isothermal, or nearly so at 2:00 p.m. in pools of all treatments. Total heat content per [14 1 unit surface area (calories per inch2 ) above 32 ° F was calculated for data collected on 5 days (2:00 p.m.) by multiplying weighted mean temperatures by mean depth (3). Total heat contents were usually less in pools covered by S. polyrhiza and greatest in control pools, Table 6. The average in heat content of control pools exceeded that of pools with S. polyrhiza by 9.7 percent. Therefore, duckweed cover altered vertical distribution of heat more than it altered total heat content. Water losses to evaporation in control pools exceeded losses by evapotranspiration in duckweed covered pools, Table 5. Complete surface coverage of pools by duckweed reduced the surface of water in direct contact with air and retarded evaporation even though surface waters were usually warmer than those of TABLE 5. WATER LOSSES TO ATMOSPHERE, LIGHT PENETRATION, PHYTOPLANKTON DENSITY, AND GROSS PHOTOSYNTHESIS BY PHYTOPLANKTON IN CONTROL POOLS AND IN POOLS COVERED WITH ONE OF Two SPECIES OF DUCKWEED. EACH VALUE IS THE AVERAGE OF DATA FROM FOUR REPLICATES Measurement Control Control Treatment' Wolffia columbiana Spirodela polyrhiza Total water loss by evaporation and evapotranspiration from 8/23 to 4.06 c 4.26b 4.75a 9/25 (in.) % Incident light at 1 ft 0.8 c 74.2 a 17.8 b 9/16 0.9c 32.7b 80.9 a 9/18 0.9c 73.0 a 23.2b 9/20 Phytoplankton density on 9/20 40 c 594 b 5,159 a (Individuals /ml) Gross photosynthesis by phytoplankton (ppm oxygen evolved per hour) 0.02b 0.36a 0.14b 9/16 0.12 b 0.02 b 0.88 a 9/18 0.02b 0.05b 0.94 a 9/20 Values designated by different letters were determined significantly different at the 0.05 level of probability by Duncan's New Multiple Range Test (horizontal comparisons only). TABLE 6. TOTAL HEAT CONTENTS (AT 2 P.M.) IN CALORIES PER IN THE AVERAGE 2 OF WATER SURFACE IN CONTROL POOLS AND IN POOLS COVERED WITH ONE OF EACH VALUE IS Two SPECIES OF DUCKWEED. OF DATA FROM FOUR REPLICATES Treatment 8/29 256 a 240 b 231 c Total heat content (calories/in2)1 8/31 240 a 225 b 220 c 9/5 168 a 170 b 164 a 9/13 237 a 232 b 220 c 9/20 219a 213 b 210 b Control Wolffia columbiana Spirodela polyrhiza SValues designated by different letters were determined significantly different at the 0.05 level of probability by Duncan's New Multiple Range Test (vertical comparisons only). [15] control pools. Both S. polyrhiza and W. columbiana have stomata (7), but they expose a minimum leaf area to air for evapotranspiration. Larger floating plants such as Eichhornia crassipes and Salvinia molesta normally had much higher rates of evapotranspiration (11) than smaller duckweeds, Table 5. Concentrations of dissolved oxygen increased during daylight and decreased during night in control and W. columbiana pools, Figure 4. This pattern is normal in aquatic habitats. Concentrations of dissolved oxygen throughout the 24-hour period were greater at all depths in control pools than in pools with W. columbiana. Concentrations of dissolved oxygen did not increase appreciably during daylight in pools with S. polyrhiza. Dissolved oxygen concentrations in these pools increased slightly at night probably because of continuous mixing of surface with deeper waters as Surface water cooled, Figure 4. Pools covered with S. polyrhizahad less dissolved oxygen than pools with W. columbiana. Representative data for average dissolved oxygen contents (2:00 p.m.) for pools of different treatments are summarized. Table 7. Control pools always had highest concentrations of dissolved oxygen and pools covered by S. polyrhiza had lowest concentrations. Shading by duckweeds restricted development of phytoplankton communities and reduced gross photosynthesis, Table 5. Although there was no significant difference in gross photosynthesis between pools covered with W. columbiana and those covered with S. polyrhiza, values were numerically higher in pools covered with W. columbiana. Pools with W. columbiana also had higher concentrations of dissolved oxygen in the afternoon, Table 7. W. columbiana did not form as tightly closed cover as S. polyrhiza and permitted more gas exchange with the atmosphere. Furthermore, there was an obvious increase in dissolved oxygen in pools with W. columbiana during daylight hours TABLE 7. DISSOLVED OXYGEN CONCENTRATIONS IN WATER COLUMN (AT 2 P.M.) IN CONTROL POOLS AND IN POOLS COVERED WITH ONE OF Two SPECIES OF DUCKWEED. EACH VALUE IS THE AVERAGE DATA FROM FOUR REPLICATES Average dissolved oxygen concentrations Treatment for water column (ppm) 1 8/30 Control 7.8 a 9/5 9.1 a 9/13 10.5 a 9/17 10.9 a 9/20 14.2 a 9/23 14.3 a Wolffia columbiana Spirodela polyrhiza 5.4b 3.6 b 4.2b 4.9 b 5.8b 3.3 b 7.2b 3.3 c 7.1 b 2.8 c 8.5b 3.7 c Values designated by different letters were determined significantly different at the 0.05 level of probability by Duncan's New Multiple Range Test (vertical comparisons only). S16] but not in pools covered with S. polyrhiza. These observations suggest that more photosynthesis occurred in pools with W. columbiana than in pools with S. polyrhiza. There were no significant differences between treatments with respect to concentrations of phosphate, nitrate, and ammonia and levels of total alkalinity, Table 8. Dense phytoplankton in control pools removed all free carbon dioxide while pools covered with duckweeds had high concentrations of carbon dioxide, Table 8. Lower pH in pools with duckweeds resulted from higher concentrations of carbon dioxide. Chemical oxygen demand, Table 8, was greatest in pools of control and W. columbiana treatments since these pools had greatest densities of phytoplankton, Table 5. Furthermore, in pools containing W. columbiana, decaying fronds were present in water samples and contributed to the chemical oxygen demand. The higher total solids content of control pools resulted from their greater density of phytoplankton. Tannins and lignins from duckweeds imparted a brown stain to water. Accumulation of tannins and lignins was greatest in pools covered by S. polyrhiza, Table 8. These findings reveal that surface coverage of ponds with duckweeds will greatly retard phytoplankton production and cause water quality problems. Spirodela was a more serious threat to the pool environments than was Wolffia. TABLE 8. CHEMICAL ANALYSES OF WATER FROM 1-FT. DEPTH IN CONTROL POOLS EACH COVERED BY ONE OF Two SPECIES OF DUCKWEED. VALUE IS THE AVERAGE OF DATA FROM FOUR REPLICATES AND POOLS Determination Control Treatment' Wolffia columbiana Spirodela polyrhiza 20.1 b 40.6 a 48.4 a Chemical oxygen demand (ppm) 107.6 a b 96.1 b 127.9 a Total solids (ppm) 0.65 b 0.40 c 0.14 a Tannins and lignins (ppm) 28.9 a 27.6 a 29.2 a Total alkalinity (ppm as CaCO,) 7.1 b 7.0 b 9.5 a pH 15.9 b 50.5 c 0a Carbon dioxide (ppm) 0.20 a 0.60 a 0.60 a Phosphate (ppm as P) 0.10 a 0.05 a 0.08 a Nitrate (ppm as N) 0.05 a 0.15 a 0.05 a Ammonia (ppm as N) ' Values designated by different letters were determined significantly different at the 0.05 level of probability by Duncan's New Multiple Range Test (horizontal comparisons only). [17] SUMMARY Submersed aquatic weeds had low light requirements for photosynthesis. Maximum rates of photosynthesis for 13 species occurred within the light intensity range of 10,000 and 35,000 lux and none of these species required more than 9,000 lux for 50 percent of maximum photosynthesis. Even so, phytoplankton turbidity in most fertilized ponds was more than sufficient to reduce light penetration to the bottom and eliminate aquatic weeds through shading. In unfertilized ponds, shading by aquatic weeds reduced phytoplankton growth. Surface coverage of experimental pools with duckweeds, Spirodela polyrhiza and Wolffia columbiana, restricted light penetration, reduced photosynthesis by phytoplankton, and caused serious deterioration in water quality. Surface coverage by S. polyrhiza caused more serious water quality problems than did coverage by W. columbiana. LITERATURE CITED (1) 'AMERICAN PUBLIC HEALTH ASSOCIATION. 1971. Standard methods (2) (3) (4) (5) (6) for the examination of water and wastewater. 13th ed., Amer. Pub. Health Assoc., New York. 874 pp. BEASLEY, P. G. 1965. The penetration of light and concentrations of dissolved oxygen in fertilized pond waters infested with Microcystis. Proc. Ann. Conf. SE Game and Fish Comm. 17: 222-226. BIRGE, E. A. 1915. The heat budgets of American and European lakes. Trans. Wisconsin Acad. Sci. 18: 166-213. BOYD, C. E., E. E. PRATHER, AND R. W. PARKS. 1975. Sudden mortality of a massive phytoplankton bloom. Weed Sci. 23: 61-67. DENDY, J. S. 1963. Farm ponds, p. 595-620. In: D. G. Frey (ed.) Limnology in North America. Univ. Wisconsin Press, Madison. HICKS, L. E. 1937. The Lemnaceae of Indiana. Amer. Midl. Natur. 18: 774-789. HILLMAN, W. S. 1961. (7) The Lemnaceae, or duckweeds a review of the descriptive and experimental literature. Bot. Rev. 27: 221-287. (8) JACOBS, D. L. 1947. An ecological life history of Spirodela polyrhiza (Greater duckweed) with emphasis on the turion phase. Ecol. Monogr. 17: 437-467. (9) LAWRENCE, J. M. 1949. Construction of farm fish ponds. Alabama Polytech. Inst. (Auburn Univ.), Agr. Exp. Sta., Circ. 95. 56 pp. (10) MEYER, B. S. AND A. C. HERITAGE. 1941. Effects of turbidity and depth of immersion on apparent photosynthesis in Ceratophyllum demersum. Ecology 22: 17-22. [18] (11) MITCHELL, D. S. 1974. The effects of excessive aquatic plant populations, p. 50-56. In: D. S. Mitchell (ed.) Aquatic vegetation and its use and control. United Nations Education, Scientific, and Cultural Organization. Paris. (12) POLISINI, J. M. AND C. E. BOYD. 1972. Relationships between cellwall fractions, nitrogen, and standing crop in aquatic macrophytes. Ecology 53: 484-488. (13) RAO, C. B. 1953. On the distribution of algae in a group of six small ponds. J. Ecol. 41: 62-71. (14) (15) SMITH, E. V. AND H. S. SWINGLE. 1939. The relationships between plankton production and fish production in ponds. Trans. Amer. Fish. Soc. 68: 309-315. SMITH, E. V. AND H. S. SWINGLE. 1942. The use of fertilizer for con- trolling several submerged aquatic plants in ponds. Trans. Amer. Fish. Soc. 71: 94-101. (16) SWINGLE, H. S. 1947. Experiments on pond fertilization. Alabama Polytech Inst. (Auburn Univ.), Agr. Exp. Sta., Bull. 264. 36 pp. (17) SWINGLE, H. S. 1968. Fish kills caused by phytoplankton blooms and their prevention. Proc. World Symposium on Warm-water Pond Fish Culture, FAO Fish Rep. 44: 407-411. (18) SWINGLE, H. S. AND E. V. SMITH. 1939. Fertilizers for increasing the natural food for fish in ponds. Trans. Amer. Fish. Soc. 68: 126-135. (19) SWINGLE, H. S. AND E. V. SMITH. 1947. Management of farm fish ponds. Alabama Polytech. Inst. (Auburn Univ.), Agr. Exp. Sta., Bull. 254. 32 pp. (20) VOLLENWEIDER, R. A. 1969. A manual on methods for measuring primary production in aquatic environments. IBP Handb. No. 12, Blackwell Sci. Pub., Oxford. 213 pp. (21) WESTLAKE, D. F. 1963. Comparisons of plant productivity. Biol. Rev. 38: 385-425. [19] \it I~dS tj. IC~_' Lxperi.'.lt bt) -- ,,f ii) AUBURN UNIVERSITY With an agricultural research unit in every major soil area, Auburn Univcrsitv serves the neels of field crop, livestock, forestry, and hor- tictiltitral producers in eacht region in Alalbama. Every citizen of the State has a stake in this research program, 7 since any advantage from new and more economical ways of pirolcing and handling farin products directly bInfits the consuming Q 0 6 puihlic.f Research Unit Identification 1. 2. 3 4. 5. 6. 7. 8 9. 10. 1 12. 13. 14. 15 16 17. 18. 19. 20. 21. Tennessee Valley Substation, Belle Mina Sand Mountain Substation, Crossville. North Alabama Horticulture Substation, Cullman. Upper Coastal Plain Substation, Winfield Forestry Unit, Fayette County Thorsby Foundation Seed Stocks Farm, Th rsby. Chilton Area Horticulture Substation, Clanton Forestry Unit, Coosa County Piedmont Substation, Camp Hill Plant Breeding Unit, Tallassee Forestry Unit, Autauga County Prattville Experiment Field, Prattville. Black Belt Substation, Marion Junction Tuskegee Experiment Field, Tuskegee Lower Coastal Plain Substation, Camden Forestry Unit, Barbour County. Monroeville Experiment Field, Monroeville Wiregrass Substation, Headland Brewton Experiment Field, Brewton. Ornamental Horticulture Field Station, Spring Hill Gulf Coast Substation, Fairhope