Chesapeake Bay has undergone water quality degradation over the previous 60 years. Evidence includes increasing N and P concentrations resulting in higher phytoplankton biomass in surface waters, decreasing submerged grass coverage in shallow waters, and low or depleted oxygen in bottom waters. These symptoms of eutrophication resulted from increased human populations, sewage discharges, atmospheric N deposition, and fertilizer applications on lawns and agricultural fields.
The Choptank watershed and estuary are components of a coastal plain tributary draining into Chesapeake Bay. The eutrophication of the Choptank estuary is a microcosm of the same processes in Chesapeake Bay, with similar major land uses, population densities, and histories of degradation. In recent years, we have observed evidence of improvements in Chopank estuarine water quality, and we explored whether reductions in one or more of the three major nutrient sources (atmospheric deposition, agricultural diffuse sources, and wastewater inflows) was responsible for the observed improvements in estuarine water quality. We integrated long-term data from NADP, USGS, NPDES (CAST 2017d), and CIMS with our own measurements of watershed N and P export to compare trends in estuarine water quality with trends in atmospheric deposition, agricultural diffuse sources, and wastewater discharges.
Results/Conclusions
There is a 34-year history of changes in water quality in the Choptank estuary. During 1984-2005, annual average chlorophyll a increased in surface waters as oxygen declined in summer bottom waters. However, after 2005 there were decreases in chlorophyll a in surface waters with more oxygen in summer bottom waters. These improvements were not due to decreasing atmospheric deposition, as there were no significant decreases over this time period. In addition, data from multiple agriculturally dominated watersheds representing 33% of land draining to the estuary showed only small decreases in stream baseflow TN concentrations and no changes in stream baseflow TP, with high concentrations persisting over the last decade. In the 1980s wastewater plants in the Choptank watershed had secondary treatment of combined storm and wastewater lines. Installation of separate storm drains in the 1990s and tertiary treatment in the 2000s resulted in greatly reduced wastewater discharges of N (76%) and P (91%) from 2000 to 2010. These reductions in N and P preceded improvements in estuarine water quality by 1-2 years. These results indicate that federal and local investments in storm/waste line separation and tertiary treatment are responsible for current improvements in Choptank estuarine water quality.