Cultural eutrophication in the Choptank and Patuxent estuaries of Chesapeake Bay

The Choptank and Patuxent tributaries of Chesapeake Bay have become eutrophic over the last 50–100 years. Systematic monitoring of nutrient inputs began in ;1970, and there have been 2–5-fold increases in nitrogen (N) and phosphorus (P) inputs during 1970–2004 due to sewage discharges, fertilizer applications, atmospheric deposition, and changes in land use. Hydrochemical modeling and land-use yield coefficients suggest that current input rates are 4–20 times higher for N and P than under forested conditions existing 350 yr ago. Sewage is a major cause of increased nutrients in the Patuxent; agricultural inputs dominate in the Choptank. These loading increases have caused three major water-quality problems: (1) increased nutrients, phytoplankton, and turbidity; (2) decreased submerged grasses due to higher turbidity and epiphyton shading; and (3) bottom-water hypoxia due to respiration of excess organic matter. Oxygen in the Patuxent is consistently ,3 mg L21 in bottom waters in summer, whereas oxygen in Choptank bottom waters has been decreasing for the last 20 yr and is now approaching 3 mg L21 in wet years. The low N : P of sewage inputs to the Patuxent results in an N-limited, P-saturated system, whereas the Choptank is primarily limited by N, but with P limitation of phytoplankton during spring river flows. Insufficient action has been taken to improve the water and habitat quality of these estuaries, although reduced eutrophication in dry years suggests that both estuaries will respond to significant decreases in nutrients. The coastal zone throughout much of the world has been adversely affected by human activities (Nixon 1995). Increasing human populations have caused land-cover changes from forest to agricultural and urban areas, and these changes in turn have resulted in greater freshwater flows and increased fluxes of particulates, nitrogen (N), and phosphorus (P) from watersheds to estuaries and coastal waters (Vitousek et al. 1997; Howarth et al. 2000; Cloern 2001). These increased inputs have promoted a widespread but not universal pattern of responses. Increased loading of particles promotes turbidity and sedimentation, and increased loading