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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5168

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Summary

Insights into the limnological functioning of Coeur d’Alene Lake were based on information developed from two large-scale limnological studies of the lake conducted during calendar years 1991–92 and water years 2004–06. The fate and transport of water and associated constituents following their delivery into a lake are determined by the interactions of a myriad of physical, chemical, and biological processes operating in the lake over a wide range of spatial and temporal scales.

Differences in hydrologic conditions were largely responsible for interannual differences in loads delivered into and discharged from Coeur d’Alene Lake because streamflow is a key factor in the calculation of constituent load. Overall, the lake is a sink for total nitrogen, total phosphorus, total cadmium, total lead, and total zinc loads. Total nitrogen inflow and outflow loads decreased in-between the 1991–92 and 2004–06 studies, primarily due to improvements in wastewater treatment processes and resulting decreases in total nitrogen concentrations in treatment plant effluent in major inflows to the lake. Total phosphorus inflow and outflow loads fluctuated with streamflow in both study periods, but loads relative to streamflow were higher in the 2004–06 study than the 1991–92 study because total phosphorus concentrations in gaged inflows were statistically higher in the latter study. Inflow trace-metal loads generally decreased between the 1991–92 and 2004–06 studies, but differences in sampling design and analytical reporting limits between the studies prevented a robust and accurate comparison of loads.

Comparison of water temperatures and water column transparency among the 4 years and five pelagic stations indicated no significant changes in water column heating and convective circulation. However, thermocline depths throughout the lake were highly variable during the years studied.

No significant changes in dissolved oxygen concentrations between study periods were noted at any stations. Concentrations were significantly lower in the hypolimnion than in the euphotic zone at all stations. The highest concentrations at each station were measured in winter in association with minimum water temperatures. Minimum concentrations were measured in the hypolimnion of each station during late summer or autumn as prolonged thermal stratification restricted mixing of the oxygenated upper water column with the hypolimnion.

Increased chlorophyll-a and total phosphorus concentrations were measured throughout the lake in the 2004–06 study compared with the 1991–92 study. No significant change was detected in hypolimnetic dissolved inorganic nitrogen concentrations between study periods. Total zinc, total cadmium, and total lead concentrations decreased from the 1991–92 study to the 2004–06 study everywhere except at pelagic station 6, where frequency of detection was low.

Large differences in median concentrations of dissolved inorganic nitrogen were measured in the euphotic zone and hypolimnion of the three deep stations (1, 3, and 4). These differences were attributable to several limnological processes, including seasonal inflow plume routing, isolation from wind-driven circulation and associated hypolimnetic enrichment, phytoplanktonic assimilation during summer months, and benthic flux.

Median detected nitrogen-to-phosphorus ratios decreased from the 1991–92 study to the 2004–06 study. Whereas the lake was clearly phosphorus-limited in 1991–92, in 2004–06 the lake may have been much closer to the boundary value of 7.2 that separates nitrogen from phosphorus limitation. However, due to changes in analytical reporting limits over the two studies, the data are insufficiently certain to draw reliable conclusions with regard to limiting nutrients.

For both studies, the trophic state of the lake was classified as oligotrophic, or less productive, with regard to total phosphorus and chlorophyll-a concentrations and as mesotrophic, or moderately productive, with regard to Secchi-disc transparency depth.

The significant concentration gradients measured across the lake were:

• Dissolved inorganic nitrogen–gradient north and south radiating from central station 4 (high);
• Total phosphorus–gradient from south end (high) to north end (low);
• Total lead–gradient north and south radiating from central station 4 (high);
• Total zinc–gradient north and south radiating from central station 4 (high);
• Dissolved oxygen–gradient from north end (high) to south end (low); and
• Water column transparency–gradient from north end (high) to south end (low).

The southern end of the lake is a highly sensitive area that is subject to periodic anoxia, higher phosphorus concentrations, and higher turbidity. Although metals contamination is relatively low, the southern end of the lake is not completely isolated from metal-laden inflows from the Coeur d’Alene River or from wind-driven circulation of contaminated bed sediments from other parts of the lake. Higher lake productivity and hypolimnetic anoxia in this area will tend to remobilize any metals present in bed sediments.

Internal circulation from wind-generated waves and from changes in the thermocline is an important process for distribution of water-quality constituents throughout Coeur d’Alene Lake. Surficial distribution of trace metals throughout most of the lake, including the bays, is relatively uniform. Even south of the Coeur d’Alene River mouth, lakebed sediments are contaminated with trace metals. Generally, no significant difference was detected in water quality between the pelagic euphotic zone and littoral zone (when comparing pelagic stations to closest littoral stations), indicating a relatively uniform surficial distribution of constituents across the lake.

The fate and transport of constituents such as nutrients, trace metals, and sediment in and occasionally through Coeur d’Alene Lake are highly dependent on inflow-plume routing of the two primary inflow sources, the Coeur d’Alene and St. Joe Rivers. Most riverine inflows move through the lake as overflow during summer, interflow during spring and autumn, and underflow during winter. However, shallow depths in the southern part of the lake may have allowed full-depth, convective circulation and muted the median-concentration difference between the euphotic zone and lower hypolimnion for most analytes.

Benthic flux experiments showed that movements of dissolved metals, nutrients, and dissolved organic carbon out of lakebed sediments in the deep part of the lake, likely are due to molecular diffusion. This outward benthic flux of dissolved materials is in contrast to the sedimentation of the same materials to lake sediments when associated with particulate material, meaning that the sediments may be a source for dissolved metals and nutrients while still being a sink for the same constituents in particulate form. When anoxic conditions are not already present at the sediment-water interface, the transition from oxic to anoxic conditions occurred in the top 1–5 cm of bed sediments. In the transition zone, the reduction of manganese and ferric oxyhydroxides can result in the release of trace metals in the dissolved phase. When anoxic conditions are present at the sediment-water interface (such as in the southern area of the lake), no transition zone is present, and trace metals may be released right at the interface and would be available for transport by physical limnological processes.

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