Nutrient and trace-element enrichment of Coeur d'Alene Lake, Idaho

This study of Coeur d'Alene Lake was undertaken because of concerns over the potential for release of previously deposited nutrients and trace elements from the lakebed if an anoxic hypolimnionwere to develop as a consequence of eutrophication. The primary purpose of the study was to determine the lake's assimilative capacityfor nutrients. The scope included characterization of water quality in the limnetic and littoral zonesof the lake, quantification of hydrologic and nutrient budgets, development of a nutrient load/lake response model, and characterization of trace-element enrichment in surficial and subsurface lakebed sediments. Coeur d'Alene Lake was classified as oligotrophic during 1991-92, on the basis of annual geometric mean concentrations, in micrograms per liter, of total phosphorus (4.1), total nitrogen (247), and chlorophyll-a (0.54). On the basis of nitrogen-to-phosphorus ratios, phosphorus was the nutrient most likely to limit phytoplankton growth. Despite its oligotrophy, Coeur d'Alene Lake developed a substantial hypolimnetic dissolved-oxygen deficit in both years during the late summer. In the deep, northern half of the lake, hypolimnetic dissolved oxygen was reduced to 6.4 milligrams per liter (58 percent saturation). In the lake's shallow, southern end, anoxic conditions developed in the lower water column. A review of historical studies of the lake revealed that substantial hypolimnetic dissolved-oxygen deficits were measured in 1911 and 1975. Historical loads of nutrients and oxygen-demanding substances were estimated to have been much larger than those measured during this study and, thus, capable of producing the earlier deficits. The lake's current (1992) oligotrophic classification differs from the mesotrophicclassification it received in 1975 during the National Eutrophication Survey. The shift in trophic state is consistent with nutrient load reductions in the lake's 9,690- square-kilometer drainage basinsince the early 1970's. During 1991, loads of nitrogen and phosphorus to Coeur d'Alene Lake were about half those in 1975. Most of the nutrient load reduction has occurred in the Coeur d'Alene River drainage basinwhere phosphorus-export coefficients, in kilogramsper square kilometer, have declined from 26 in 1975 to 4.5 in 1991. Approximately 85 percent of the lakebed's surface area was highly enriched in antimony, arsenic, cadmium, copper, lead, mercury, silver, and zinc. Median concentrations of total cadmium, lead, and zinc in the enriched lakebed sediments were 56, 1,800, and 3,500 milligrams per kilogram, respectively. In contrast, median concentrations of cadmium, lead, and zinc in unenriched lakebed sediments in the lake's southern end were 2.8, 24, and 110 milligrams per kilogram, respectively. Most of the trace elements in the surficial and subsurface sediments were associated with ferric oxides, not sulfides, as previously postulated. Under reducing conditions, such as within an anoxic hypolimnion, the ferric oxides would be readily soluble and the trace elements would be released into the overlying water column. The trace-element enrichment of the lake was largely restricted to the lakebed sediments; however, the median concentration of total recoverable zinc in the water column was 98.6 micrograms per liter. This concentration exceeded the criterion of 32.4 micrograms per liter for protection of freshwater biota in Coeur d'Alene Lake.