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Seasonal and Diel Oxygen and Phytoplankton Dynamics in an Estuarine Water Quality Model
Citation:
Dettmann, E., L. Charlestra, AND M. Abdelrhman. Seasonal and Diel Oxygen and Phytoplankton Dynamics in an Estuarine Water Quality Model. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-18/378, 2020.
Impact/Purpose:
This is a review of a U.S. Environmental Protection Agency model that is often used to predict trends in water quality, for instance concentrations of oxygen and algae in lakes, rivers, and coastal bays that receive nutrients from sources such as municipal wastewater treatment plants, industrial discharges, and runoff from streets and agricultural fields. In the past, these predictions have been for time periods of months, seasons, and years. This review examines the performance of the model over the daily cycle, as well as monthly and seasonal time periods. It is important to understand the behavior of the model over the daily cycle if it is to be used in water-quality management contexts for which daily variations in oxygen and algal concentrations are important.
Description:
We describe calibration and evaluation of a water quality model being implemented for Narragansett Bay to quantify the response of concentrations of phytoplankton chlorophyll a and dissolved oxygen in the Bay to loading rates of nutrients and other boundary conditions. We evaluate model capabilities as a management tool in Narragansett Bay and similar estuaries. The primary focus of this report is on variability of model predictions of near-surface phytoplankton chlorophyll a and dissolved oxygen concentrations at time scales between the daily and seasonal cycles. The model, the U.S. Environmental Protection Agency’s Water Quality Analysis Simulation Program (WASP), Version 7.52, simulates concentrations of nutrients, chlorophyll a, and dissolved oxygen in waterbodies. We use hydrodynamic transport and other parameters simulated by the Environmental Fluid Dynamics Code (EFDC), observed loads of nutrients, biochemical oxygen demand, and freshwater, as well as boundary conditions for benthic nutrient and oxygen fluxes. This implementation of WASP employs a terrain-following vertical sigma grid with 5288 segments (661 segments in each of 8 layers). Model simulations of near-surface dissolved oxygen and phytoplankton chlorophyll a are compared with data from a network of fixed-site sensors in the Bay that provide oxygen and chlorophyll a concentrations and temperature and salinity at 15-minute intervals, generally from late May through late October. WASP represents seasonal dynamics of dissolved oxygen and phytoplankton chlorophyll a well, but substantially underestimates observed diel excursions of dissolved oxygen concentrations. The model also underestimates diel excursions of fluorometrically-measured chlorophyll a, although the degree of underestimation is difficult to quantify well because the measured values are affected by nonphotochemical quenching of chlorophyll a fluorescence. Sensitivity analyses show that while near-surface chlorophyll a concentrations react as expected to modifications of incident light and nitrogen loading rates, dissolved oxygen concentrations are relatively insensitive to these manipulations, suggesting that physical processes are the dominant factor influencing modeled oxygen concentrations. We discuss factors that may contribute to the weak coupling between phytoplankton and dissolved oxygen concentrations in this model.