Historical Changes in Water Quality at German Branch in the Choptank River Basin

Many management strategies to improve the health of Chesapeake Bay focus on reducing losses of sediments and nutrients from agricultural land. Plot-scale studies have suggested that Best Management Practices (BMPs) reduce these losses, and natural resource managers have since supported implementation of a variety of BMPs on farms in the Chesapeake Bay watershed over the last two decades. As a test of the efficiency of these BMPs at the watershed scale, all farms within German Branch watershed had BMPs implemented in the early 1990s. Using water quality from two past monitoring programs (i.e., in 1986 and 1991-1995) and current water quality monitoring (i.e., collected 2003-2006), we detected a 28% decrease in baseflow P concentrations a decade after BMP implementation. There were no significant changes in nitrate or total nitrogen concentrations between BMP implementation and the most recent sampling. However, the significant rate of increase (similar to 0.08 mg N L(-1) year(-1)) from 1986 to the 1990s did not continue to 2003-2006 baseflow conditions, which suggests that BMPs may have suppressed the rate of increase in nitrogen observed earlier in German. These data suggest that other management practices that increase agricultural N losses and natural processes that attenuate N losses at the watershed scale may obscure significant N reductions by current BMPs in the watershed.