The Lake Erie agroecosystem program: water quality assessments

Abstract

In contrast with the watersheds draining into Lakes Superior, Michigan, Huron, and Ontario, where forestry is the dominant land use, the dominant land use in Lake Erie's watershed is row crop agriculture. Consequently, the tributaries draining into Lake Erie carry, on average, much larger loads of sediments, nutrients, and pesticides than do the tributaries entering the other Great Lakes. To support the development, operation, and assessment of agricultural non-point pollution control programs in the Lake Erie Basin, the major tributary watersheds are analyzed as large-scale agroecosystems, using mass balance approaches. Material export from the watersheds is based on detailed tributary loading studies which were initiated in the mid-1970s. The monitoring programs have now been in operation for up to 18 years, producing data that serve: (1) to assess the effectiveness of non-point pollution control efforts, (2) to guide future non-point pollution control programs, and (3) to illustrate many of the regional water quality impacts of agricultural land use. The water quality data illustrate the large day-to-day, season-to-season, and year-to-year variability in both pollutant concentrations and loads, which is characteristic of non-point pollution. The data also illustrate systematic shifts in pollutant concentration and loading patterns that occur in relation to watershed size. Although gross erosion rates in northwestern Ohio tributaries are relatively low, the phosphorus and nitrate export rates are high in comparison with other US streams and rivers. Analysis of the water quality data reveal significant downward trends in time for total and soluble phosphorus and significant upward trends in nitrate. The reductions in phosphorus export apparently reflect the effectiveness of agricultural pollution abatement programs that combine more careful fertilizer management with increasing use of conservation tillage. The increasing nitrate concentrations may reflect a trade-off associated with the adoption of conservation tillage.