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

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Future Monitoring and Research Needs

Water-quality conditions in the lake have a significant effect on lake biota, tourism, recreation, and quality of life for basin residents, including the Coeur d’Alene Indian Tribe. A number of phenomena recorded during the course of USGS and Tribe limnological studies discussed in this report merit further investigation to provide a more comprehensive understanding of the myriad of processes that affect water-quality conditions in Coeur d’Alene Lake.

The ELCOM-CAEDYM model could serve as a valuable tool for Coeur d’Alene Lake managers for simulating the response of the lake to various remediation strategies in the basin. A long-term, comprehensive monitoring plan would allow lake managers to monitor for future changes in water-quality conditions; to track the frequency, duration and extent of anoxia in the lake’s hypolimnion; to fill data gaps that could improve ELCOM-CAEDYM; and to measure the lake’s response to future remediation measures. Generally, previous monitoring programs have been designed to answer specific questions and have differed in scope, constituents sampled, analytical methods, and timing of sampling. If possible, any future monitoring program would benefit from consistency in terms of space, time, constituents sampled, and analytical methods used.

A possible monitoring program could include monthly sampling of four stations within the pelagic zone (stations 1, 4, 5, and 6, or a new station between stations 3 and 4 in place of 4), with samples collected from within the euphotic zone and hypolimnion. In addition, the program could include monthly or more frequent sampling at major inflows and the lake’s primary outflow (USGS gaging station 12415140, St. Joe River nr Chatcolet, Idaho; USGS gaging station 12413860, Coeur d’Alene River nr Harrison, Idaho; USGS gaging station 12417598, Spokane River at Lake Outlet at Coeur d’Alene, Idaho) to track changes in loads and in the lake’s mass balance of selected constituents. Sampled constituents in the lake, at major inflows, and at the primary outflow could include algal biomass (cell counts), chlorophyll-a, total nitrogen, total phosphorus, total organic plus ammonia nitrogen, nitrite plus nitrate, dissolved ammonia, dissolved orthophosphate, major ions, trace metals including zinc, lead, and iron, total and dissolved organic carbon, suspended-sediment concentration, dissolved oxygen, water temperature, pH, conductivity, and turbidity. Continuous data collection through installation of a long-term system for real-time measurements of wind speed and direction, chlorophyll-a, and depth profiles of temperature, pH, conductivity, turbidity, dissolved oxygen, and zinc concentration in the lake would substantially enhance understanding of lake conditions and would allow better tracking and modeling of inflow plumes.

Data from both the 1991–92 and the 2004–06 USGS limnological studies have shown that littoral areas are statistically similar in water quality to pelagic areas and therefore do not need more extensive characterization. The inclusion of one or two representative littoral stations in a future monitoring program likely would be sufficient to track long-term changes in this area, unless land-use practices are suspected to cause near-shore alterations in water quality.

Some areas of the lake are prone to remobilization of dissolved metals from the bed sediments into the water column due to low dissolved oxygen concentrations during certain times of the year. This phenomenon could be tracked over time throughout the lake. In addition, the occurrence of nutrient sequestration by metals could be further investigated. For example, some phosphorus is bound to iron, resulting in decreased dissolved orthophosphate concentrations. A decrease in iron in the lake could result in increased dissolved orthophosphate. Sedimentation and burial rates of metals could be quantified to better understand the flux of particulate associated metals to sediments in various areas of the lake and to improve overall knowledge of metal transport, fate, and mass balance.

Although the St. Joe and Coeur d’Alene Rivers are major sources for nutrient loading to the lake, additional nutrient inputs from increased development along the shoreline and from ungaged tributaries have not been fully assessed over time. To effectively monitor and manage future nutrient inputs, this influence could be quantified using the ELCOM-CAEDYM model and empirical variables and verified through point samples collected near suspected source locations and inflows.

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