Quantifying the effects of nutrient loading on dissolved O2 cycling and hypoxia in Chesapeake Bay using a coupled hydrodynamic–biogeochemical model

Abstract

The Regional Ocean Modeling System (ROMS) was coupled to a biogeochemical model (RCA) to understand the controls on dissolved oxygen (O2) depletion in Chesapeake Bay. The model was calibrated to observational data in the year 2000 and subsequent simulations were performed for a 10-year period, where water-column state variables were validated against observations using multiple error metrics and model-simulated rate processes were compared to available measurements. ROMS–RCA captured observed seasonal and regional dynamics of water-column chlorophyll-a, dissolved O2, and nutrient concentrations, as well as sediment–water nutrient and oxygen fluxes and community respiration rates, but for the year 2000, the model over-predicted surface-water chlorophyll-a and bottom-water O2 in some regions. A series of model experiments were made using the physical regime for the year 2000 to understand ecosystem responses to altered loads of nitrogen and phosphorus and to quantify the spatial and temporal response of Chesapeake Bay to altered nutrient loading. Nutrient loading experiments revealed a non-linear response of hypoxia to nitrogen load, where hypoxic-volume-days maximized at nitrogen loads twice of that observed in the year 2000. O2 levels were more sensitive to nitrogen loads than phosphorus loads, consistent with the preponderance of nitrogen limitation in Chesapeake Bay in late spring and summer months. Expanded hypoxic volumes under higher nitrogen loads were associated with increases in water-column production and respiration in seaward regions of Chesapeake Bay during summer (June to August) months. Analysis of the 10-year model run with realistic hydrodynamics and nutrient loading revealed a similar pattern, emphasizing phytoplankton growth during summer in more nitrogen-limited, lower-Bay regions as a mechanism supporting elevated summer hypoxic volumes. This analysis (1) presents ROMS–RCA as a tool for investigating linked biogeochemical processes in coastal ecosystems, (2) identifies phytoplankton growth in seaward Bay regions as a key link between nitrogen loading and hypoxic volume, and (3) suggests that given similar climatic conditions, nutrient load reductions will lead to reduced hypoxic volumes.