https://aurora.auburn.edu/handle/11200/44140 Sun, 05 Apr 2026 16:08:10 GMT 2026-04-05T16:08:10Z https://aurora.auburn.edu/handle/11200/50759 Riverine Carbon Cycling Over the Past Century in the Mid-Atlantic Region of the United States The lateral transport and degassing of carbon in riverine ecosystems is difficult to quantify on large spatial and long temporal scales due to the relatively poor representation of carbon processes in many models. Here, we coupled a scale-adaptive hydrological model with the Dynamic Land Ecosystem Model to simulate key riverine carbon processes across the Chesapeake and Delaware Bay Watersheds from 1900 to 2015. Our results suggest that throughout this time period riverine CO2 degassing and lateral dissolved inorganic carbon fluxes to the coastal ocean contribute nearly equally to the total riverine carbon outputs (mean +/- standard deviation: 886 +/- 177 Gg C center dot yr(-1) and 883 +/- 268 Gg C center dot yr(-1), respectively). Following in order of decreasing importance are the lateral dissolved organic carbon flux to the coastal ocean (293 +/- 81 Gg C center dot yr(-1)), carbon burial (118 +/- 32 Gg C center dot yr(-1)), and lateral particulate organic carbon flux (105 +/- 35 Gg C center dot yr(-1)). In the early 2000s, carbon export to the coastal ocean from both the Chesapeake and Delaware Bay watersheds was only 15%-20% higher than it was in the early 1900s (decade), but it showed a twofold increase in standard deviation. Climate variability (changes in temperature and precipitation) explains most (225 Gg C center dot yr(-1)) of the increase since 1900, followed by changes in atmospheric CO2 (82 Gg C center dot yr(-1)), atmospheric nitrogen deposition (44 Gg C center dot yr(-1)), and applications of nitrogen fertilizer and manure (27 Gg C center dot yr(-1)); in contrast, land conversion has resulted in a 188 Gg C center dot yr(-1) decrease in carbon export. https://aurora.auburn.edu/handle/11200/50759 https://aurora.auburn.edu/handle/11200/50758 Anthropogenic and climatic influences on carbon fluxes from eastern North America to the Atlantic Ocean: A process-based modeling study The magnitude, spatiotemporal patterns, and controls of carbon flux from land to the ocean remain uncertain. Here we applied a process-based land model with explicit representation of carbon processes in streams and rivers to examine how changes in climate, land conversion, management practices, atmospheric CO2, and nitrogen deposition affected carbon fluxes from eastern North America to the Atlantic Ocean, specifically the Gulf of Maine (GOM), Middle Atlantic Bight (MAB), and South Atlantic Bight (SAB). Our simulation results indicate that the mean annual fluxes (1 standard deviation) of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the past three decades (1980-2008) were 2.370.60, 1.060.20, and 3.570.72TgCyr(-1), respectively. Carbon export demonstrated substantial spatial and temporal variability. For the region as a whole, the model simulates a significant decrease in riverine DIC fluxes from 1901 to 2008, whereas there were no significant trends in DOC or POC fluxes. In the SAB, however, there were significant declines in the fluxes of all three forms of carbon, and in the MAB subregion, DIC and POC fluxes declined significantly. The only significant trend in the GOM subregion was an increase in DIC flux. Climate variability was the primary cause of interannual variability in carbon export. Land conversion from cropland to forest was the primary factor contributing to decreases in all forms of C export, while nitrogen deposition and fertilizer use, as well as atmospheric CO2 increases, tended to increase DOC, POC, and DIC fluxes. https://aurora.auburn.edu/handle/11200/50758 https://aurora.auburn.edu/handle/11200/50757 Chesapeake Bay nitrogen fluxes derived from a land-estuarine ocean biogeochemical modeling system: Model description, evaluation, and nitrogen budgets The Chesapeake Bay plays an important role in transforming riverine nutrients before they are exported to the adjacent continental shelf. Although the mean nitrogen budget of the Chesapeake Bay has been previously estimated from observations, uncertainties associated with interannually varying hydrological conditions remain. In this study, a land-estuarine-ocean biogeochemical modeling system is developed to quantify Chesapeake riverine nitrogen inputs, within-estuary nitrogen transformation processes and the ultimate export of nitrogen to the coastal ocean. Model skill was evaluated using extensive in situ and satellite-derived data, and a simulation using environmental conditions for 2001-2005 was conducted to quantify the Chesapeake Bay nitrogen budget. The 5 year simulation was characterized by large riverine inputs of nitrogen (154 x 10(9) g N yr(-1)) split roughly 60: 40 between inorganic: organic components. Much of this was denitrified (34 x 10(9) g N yr(-1)) and buried (46 x 10(9) g N yr(-1)) within the estuarine system. A positive net annual ecosystem production for the bay further contributed to a large advective export of organic nitrogen to the shelf (91 x 10(9) g N yr(-1)) and negligible inorganic nitrogen export. Interannual variability was strong, particularly for the riverine nitrogen fluxes. In years with higher than average riverine nitrogen inputs, most of this excess nitrogen (50-60%) was exported from the bay as organic nitrogen, with the remaining split between burial, denitrification, and inorganic export to the coastal ocean. In comparison to previous simulations using generic shelf biogeochemical model formulations inside the estuary, the estuarine biogeochemical model described here produced more realistic and significantly greater exports of organic nitrogen and lower exports of inorganic nitrogen to the shelf. https://aurora.auburn.edu/handle/11200/50757 https://aurora.auburn.edu/handle/11200/50756 Impacts of Multiple Environmental Changes on Long-Term Nitrogen Loading From the Chesapeake Bay Watershed Excessive nutrient inputs from land, particularly nitrogen (N), have been found to increase the occurrence of hypoxia and harmful algal blooms in coastal ecosystems. To identify the main contributors of increased N loading and evaluate the efficacy of water pollution control policies, it is essential to quantify and attribute the long-term changes in riverine N export. Here, we use a state-of-the-art terrestrial-aquatic interface model to examine how multiple environmental factors may have affected N export from the Chesapeake Bay watershed since 1900. These factors include changes in climate, carbon dioxide, land use, and N inputs (i.e., atmospheric N deposition, animal manure, synthetic N fertilizer use, and wastewater discharge). Our results estimated that ammonium (NH4+) and nitrate (NO3-) export increased substantially (66% for NH4+ and 123% for NO3-) from the 1900s to the 1990s and then declined (32% for NH4+ and 14% for NO3-) since 2000. The temporal trend of dissolved organic nitrogen (DON) export paralleled that of dissolved inorganic N, while particulate organic nitrogen export was relatively constant during 1900-2015. Precipitation was the primary driver of interannual variability in N export to the Bay. Wastewater discharge explained most of the long-term change in riverine NH4+ and DON fluxes from 1900 to 2015. The changes in atmospheric deposition, wastewater, and synthetic fertilizer were responsible for the trend of riverine NO3-. In light of our model-based attribution analysis, terrestrial non-point source nutrient management will play an important role in achieving water quality goals. Plain Language Summary Excessive nitrogen can enter estuarine and coastal areas from land, disturbing coastal ecosystems and causing serious environmental problems. The Chesapeake Bay is one of the regions that have experienced hypoxia and harmful algal blooms in recent decades. This study estimated nitrogen export from the Chesapeake Bay watershed (CBW) to the estuary from 1900 to 2015 by applying a state-of-the-art numerical model. Nitrogen loading from the CBW continually increased from the 1900s to the 1990s and has declined since then. The key contributors to nitrogen export have shifted from atmospheric nitrogen deposition (before the 1960s) to synthetic nitrogen fertilizer (after the 1980s). Antipollution policies and implementation measures have played critical roles in the decrease of nitrogen export since the 1980s, and further reduction in riverine nitrogen export will likely require regulation on the application of nitrogen fertilizer. https://aurora.auburn.edu/handle/11200/50756