You are here:
WATERSHED AND POINT SOURCE ENRICHMENT AND LAKE TROPHIC STATE INDEX
Citation:
Neel, J. WATERSHED AND POINT SOURCE ENRICHMENT AND LAKE TROPHIC STATE INDEX. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/3-79/046.
Description:
Water in the permeable soils of the upper Pelican River watershed, Minnesota, requires slightly more than a year to move generally out of the phreatic zone into surface channels and basins. Its nutrient content seems mainly responsible for the load borne in surface waters above entrance of a wastewater effluent, and groundwater changes have been followed a year later by similar ones in surface water. In 1975 P load from non-point sources markedly exceeded that from the wastewater effluent. Nutrients in groundwater are assumed to result from soil surface application, but only quantities supplied by precipitation have been measured. The most noxious conditions in surface waters have been occasioned by heterocystous blue-green phytoplankters, but the greatest plant mass has been produced by rooted and attached vegetation. Groundwater seepage into these lakes contributed more nutrients than precipitation, but the latter supplied what may be significant amounts to watershed soils. A trophic state index based on change in Mg/Ca quotient relative to water residence time has reliably depicted relative total productivity levels in 6 lakes or ponds, and its general applicability, at least to natural lakes, now appears likely.