Total Phosphorus and Dissolved Oxygen Trends in the Central Basin of Lake Erie, 1970–1991

Five yearly estimators of total phosphorus (TP) concentrations in the central basin of Lake Erie from 1970 to 1986 were calculated and compared to evaluate their utility in the assessment of long term trends. Data were selected from stations within a defined polygon for those years in which at least four surveys were conducted, including spring and fall isothermal conditions and at least two surveys during stratification. Included were an arithmetic average of all survey means during each year, a time-weighted average of all survey means, a spring (isothermal) average, a summer (stratified) average, and an autumn (isothermal) average. The rate of decline in TP concentrations from the spring-only estimator (−0.276 μg TP L -1 yr -1) was similar to that of the time-weighted multiple survey estimator (−0.214 μg TP L -1 yr -1). Additional spring survey data from 1987 through 1991 confirmed that the negative trend continued. The stratified season and the autumn-only estimators exhibited no significant trends, but were biased low and high, respectively, relative to the spring and multiple survey estimators. Despite the reductions in TP concentrations since 1970, the annual normalized rate of oxygen depletion in the hypolimnion of the central basin showed only a slight downward trend (-0.03 mg L -1 mo -1). In 1988 and 1989, however, the depletion rates were lower than any reported during the previous 20 years.