ABSTRACT

Silver Lake is typically an oligotrophic-to-mesotrophic, soft-water, terminal lake in northwestern Wisconsin. A terminal lake is a closed-basin lake with surface-water inflows but no surface-water outflows to other water bodies. After several years with above-normal precipitation, very high water levels caused flooding of several buildings near the lake and erosion of soil around much of the shoreline, which has been associated with a degradation in water quality (increased phosphorus and chlorophyll a concentrations and decreased water clarity). To gain a better understanding of what caused the very high water levels and degradation in water quality and collect information to better understand the lake and protect it from future degradation, the U.S. Geological Survey did a detailed study from 2004 to 2008. This report describes results of the study; specifically, lake-water quality, historical changes in water level, water and phosphorus budgets for the two years monitored in the study, results of model simulations that demonstrate how changes in phosphorus inputs affect lake-water quality, and the relative importance of changes in hydrology and changes in the watershed to the water quality of the lake.

From 1987 to about 1996, water quality in Silver Lake was relatively stable. Since 1996, however, summer average total phosphorus concentrations increased from about 0.008 milligrams per liter (mg/L) to 0.018 mg/L in 2003, before decreasing to 0.011 mg/L in 2008. From 1996 to 2003, Secchi depths decreased from about 14 to 7.4 feet, before increasing to about 19 feet in 2008. Therefore, Silver Lake is typically classified as oligotrophic to mesotrophic; however, during 2002–4, the lake was classified as mesotrophic to eutrophic.

Because productivity in Silver Lake is limited by phosphorus, phosphorus budgets for the lake were constructed for monitoring years 2005 and 2006. The average annual input of phosphorus was 216 pounds: 78 percent from tributary and nearshore runoff and 22 percent from atmospheric deposition. Because Silver Lake is hydraulically mounded above the local groundwater system, little or no input of phosphorus to the lake is from groundwater and septic systems. Silver Lake had previously been incorrectly described as a groundwater flowthrough lake. Phosphorus budgets were constructed for a series of dry years (low water levels) and a series of wet years (high water levels). About 6 times more phosphorus was input to the lake during wet years with high water levels than during the dry years. Phosphorus from erosion represented 13–20 percent of the phosphorus input during years with very high water levels.

Results from the Canfield and Bachman eutrophication model and Carlson trophic state index equations demonstrated that water quality in Silver Lake directly responds to changes in external phosphorus input, with the percent change in chlorophyll a being about 80 percent of the percent change in total phosphorus input and the change in Secchi depth and total phosphorus concentrations being about 40 and 50 percent of the percent change in input, respectively. Therefore, changes in phosphorus input should impact water quality. Specific scenarios were simulated with the models to describe the effects of natural (climate-driven) and anthropogenic (human-induced) changes. Results of these scenarios demonstrated that several years of above-normal precipitation cause sustained high water levels and a degradation in water quality, part of which is due to erosion of the shoreline. Results also demonstrated that 1) changes in tributary and nearshore runoff have a dramatic effect on lake-water quality, 2) diverting water into the lake to increase the water level is expected to degrade the water quality, and 3) removal of water to decrease the water level of the lake is expected to have little effect on water quality.

Fluctuations in water levels since 1967, when records began for the lake, are representative of what has occurred since 1900; fluctuations of about 4 to 10 feet occurred about every 15 years. During periods of sustained low water levels caused by a series of dry years and low runoff, the lake probably was oligotrophic to mesotrophic, whereas during sustained high water levels caused by a series of wet years and high runoff, water quality of the lake degraded and the lake probably was mesotrophic to eutrophic. Therefore, the recent degradation in water quality is consistent with possible past occurrences associated with high water levels. During the 1940s and 1950s, because of high phosphorus inputs from agriculture in the watershed, the lake may have been typically classified as mesotrophic to eutrophic and possibly classified as moderately eutrophic during years with high water levels, especially around 1945 and 1954 when water levels were likely to have been very high.

As of 2008, Silver Lake is typically oligotrophic to mesotrophic because of the relatively small contributions of phosphorus from its watershed, although its water quality changes dramatically because of natural changes in water and phosphorus inputs during years of high runoff. Because of the limited phosphorus that is presently input into the lake, small increases in phosphorus input could have a very large effect on lake-water quality. Therefore, management actions to minimize future phosphorus input to this lake are likely to have great effect on maintaining the quality of its water.

First posted June 19, 2009