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Long-term Changes in Water Quality and Productivity in the Patuxent River Estuary: 1985 to 2003
Citation:
TESTA, J. M., W. M. KEMP, W. R. BOYNTON, AND J. D. HAGY. Long-term Changes in Water Quality and Productivity in the Patuxent River Estuary: 1985 to 2003. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 31(6):1021-1037, (2008).
Impact/Purpose:
The purpose of this manuscript is to describe long-term changes in water quality and ecosystem processes in an estuary in which a significant decrease in point source nutrient loading occurred.
Description:
We conducted a quantitative assessment of estuarine ecosystem responses to reduced phosphorus and nitrogen loading from sewage treatment facilities and to variability in freshwater flow and non-point nutrient inputs to the Patuxent River estuary. We analyzed a 19-year data set of water quality conditions, nutrient loading, and climatic forcing for 3 estuarine regions and also computed monthly rates of net production of dissolved O2 and physical transport of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) using a salt- and water-balance model. Point-source loading of DIN and DIP to the estuary declined by 40-60% following upgrades to sewage treatment plants and correlated with parallel decreases in DIN and DIP concentrations throughout the Patuxent. Reduced point-source nutrient loading and concentration resulted in declines in phytoplankton chlorophyll-a (chl-a) and light-saturated carbon fixation, as well as in bottom layer O2 consumption for upper regions of the estuary. Despite significant reductions in seaward N transport from the middle to lower estuary, chl-a, turbidity, and surface-layer net O2 production increased in the lower estuary, especially during summer. This degradation of water quality in the lower estuary appears to be linked to a trend of increasing net inputs of DIN into the estuary from Chesapeake Bay and to above-average river flow during the mid-1990s. In addition, increased abundance of Mnemiopsis leidyi significantly reduced copepod abundance during summer from 1990 to 2002, which favored increases in chl-a and allowed a shift in total N partitioning from DIN to PON. These analyses illustrate (1) the value of long-term monitoring data, (2) the need for regional scale nutrient management that includes integrated estuarine systems, and (3) the potential water quality impacts of altered coastal food webs.