PHENOLOGY OF ESTUARINE RESPONSE TO ANTHROPOGENIC AND CLIMATE DRIVERS, A STUDY OF THE CHESAPEAKE BAY AND CHESTER RIVER ESTUARIES

Abstract

The effects of nutrient loading on estuaries are well-studied, given the multitude of negative water quality, ecosystem, and economic impacts that have been attributed to the presence of excess nitrogen and phosphorous. A current gap in this knowledge is the consequence of changing climate variability on the seasonal patterns of estuarine processes related to eutrophication, potentially from direct (temperature) and indirect influences (nutrient load timing) of climate warming. A coupled hydrologic-biogeochemical model (ROMS-RCA) was used to investigate the spatial and temporal changes in the phenology of hypoxia and related biogeochemical processes in the Chesapeake Bay under three different hydrologic regimes. Shifts in nutrient load timing during idealized simulations dampened the overall annual hypoxic volume, resulting from discernable, but relatively small reductions in phytoplankton biomass and both sediment and water-column respiration in three regions of the Bay. Simulated increases in water temperature caused an increase in the spring/early summer hypoxic volume associated with elevated respirations rates, but this exhaustion of organic matter in the early summer caused a decrease in late summer/fall hypoxic volume due to lowered sediment respiration. Similar simulations in nutrient load timing were conducted using a model of the Chester River estuary, a smaller, shallower sub-estuary system to the Chesapeake Bay. Nutrient load timing and magnitude effects on hypoxia were much smaller in the Chester River as compared to Chesapeake Bay, which is likely due to high concentrations of nitrogen and phosphorus within the system. Therefore, cross-system comparisons are important for understanding the sensitivity of hypoxia to alterations in nutrient load across diverse estuaries. These idealized simulations begin the process of understanding the potential impacts of future climatic changes in the seasonal timing of key biogeochemical processes associated with eutrophication.