Trends in Nutrient and Suspended Sediment Concentrations in Lake Erie Tributaries, 1975–1990

Abstract

During the last twenty years, intensive efforts have been aimed at reducing the eutrophication of Lake Erie. Although point source inputs have been greatly reduced, models indicated that non-point source inputs would need reduction as well, to meet phosphorus management goals for Lake Erie. Since non-point inputs enter the lake via tributary inflow, it is important to examine tributary records for evidence of trends in nutrient concentrations which might reflect success in reducing non-point inputs. Data series for the Maumee, Sandusky, and Cuyahoga Rivers, and for Honey Creek, spanning 9 to 16 years and up to 6,500 observations, were examined for trends in nutrients and suspended solids. Mean daily flows and two forms of weighted mean concentrations were compiled at monthly intervals, and studied using parametric and non-parametric techniques of trend detection. Total and soluble reactive phosphorus, suspended sediment, and nitrate-plus-nitrite were examined. Flow and suspended sediment generally showed statistically non-significant minor trends. Total and soluble phosphorus both showed downward trends, statistically significant for most data series, of 5 to 40 μg/L per year. Nitrate-plus-nitrite showed usually statistically significant increases of 10 to 140 μg/L per year, except for the Cuyahoga data, which showed a statistically significant downward trend of about 70 μg/L per year. These results are important both because they reflect important progress in the remediation of Lake Erie, and because they demonstrate the possibility of detecting trends in tributaries, given sufficient data and appropriate statistical approaches.