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Scientific Investigations Report 2008–5168

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

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Implications of Limnological Studies

A number of implications can be gleaned from the results of the numerous water-quality monitoring studies in Coeur d’Alene Lake. Coeur d’Alene River is a source of dissolved and particulate phase metals to the lake, and existing lakebed sediments serve as a sink for particulate phase metals and a source of dissolved metals in the lake. Even though efforts are continuing in the Coeur d’Alene River basin to control metals contamination as part of the ROD, conditions in the lake with regard to metals cannot be expected to improve significantly if the river source and bed sediment source are not both addressed. In addition, wind-driven circulation patterns and riverine inflow plumes contribute to the spread of contaminants throughout the lake. Therefore, no part of the lake is immune to contamination.

Although the trophic state of the lake has been classified as oligotrophic or mesotrophic (depending on the variable used for classification) in both the 1991–92 and 2004–06 studies, phosphorus concentrations and lake productivity have increased over time, and nitrogen-to-phosphorus ratios (when detected) appear to have decreased over time. Increased loads of nutrients to the lake (primarily nitrogen and phosphorus) can increase the growth of algae and aquatic plants. When this organic material decomposes, oxygen dissolved in the water is consumed. Depletion of dissolved oxygen concentrations in the bottom waters of the lake could lead to geochemical processes that increase the rate of release of mining associated metals contaminants (such as dissolved arsenic, cadmium, mercury, lead, and zinc) from lakebed sediments, as well as stored nutrients that would stimulate additional plant and algae growth in a self-perpetuating cycle that may be difficult or impossible to interrupt. These processes could increase risks to human health and adversely affect the environment and the economic and social welfare of the community.

Phosphorus limitation is currently keeping biomass low, and results of the ELCOM-CAEDYM simulation model show that diatoms, the current dominant phytoplankton in the lake, are kept in control by toxic levels of zinc (Hipsey and others, 2006; Kuwabara and others, 2006). Zinc loads to the lake would have to decrease more than 1–2 orders of magnitude before any appreciable reduction in zinc toxicity would be noted. However, if nutrient loads continue to increase as zinc loads decrease, phytoplankton dominance in the lake may shift from diatoms (Bacillariophyceae) to a more diverse assemblage of green algae (Chlorophyceae) and nuisance or bloom-forming species of blue-greens (Cyanobacteria) which are more zinc-intolerant (Hipsey and others, 2006). Nutrient load management should be addressed as a component of a Lake Management Plan.

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