Eutrophication Trends in the Water Quality of the Rhode River (1971–1978)

Abstract

Five to eight-year data sets on turbidity and a series of nutrient parameters have been taken in the Rhode River, a small tidal river tributary to Chesapeake Bay. Trends of change at headwater stations indicate changes due primarily to local watershed runoff, while changes at stations near the river’s mouth are indicative of changes in Chesapeake Bay.

The data have been analyzed first by summarizing as monthly and seasonal means and variances, then by looking for year-to-year trends. Thus, for example, linear regressions of summer and fall turbidity versus time for 1971–1978 had very low slopes and low coefficients of determination. An interesting finding was the pattern for total phosphorus in surface waters. At the mouth of Rhode River, seasonal mean concentrations increased steadily for each season each year from the fall of 1971 to the fall of 1976. The most dramatic increases (four-fold) were observed in the summer and fall. A much smaller increase occurred in winter and spring values. In contrast, year-to-year concentrations of total phosphorus in surface waters in upstream stations over the same time period showed less clear-cut trends. Also, phosphorus loadings from local watershed runoff fluctuated widely, but had no steady rapid rise with time. Although total phosphorus increased dramatically at the mouth of Rhode River, dissolved orthophosphate, nitrate, and dissolved ammonia remained essentially constant, especially in the summer and fall. These data could indicate an increasing impact of summertime anoxic bottom waters on the phosphorus dynamics of the upper western shore of the Bay.