2005 Results:
In 2005 Sullivan Lake had poor water quality compared to other lakes in this region (NCHF Ecoregion), receiving an overall D grade. This was similar to the previous eight years. The lake is highly eutrophic, and phosphorus levels are two to three times the threshold for an “impaired” designation by the Minnesota Pollution Control Agency. The lake is unsuitable for swimming during the entire growing season. ACD staff’s subjective observations were that algae levels were “high” or “severe” and the lake was unsuitable for swimming during the entire period from May through September. Measurements of dissolved oxygen and temperature with depth in August 2004 and June 2005 showed that dissolved oxygen is too low for most fish (<4 mg/L) below four feet, and is too low for most aquatic life (<1 mg/L) near the bottom. This is likely due to oxygen consumption by decomposition of expired algae.
Trend Analysis:
Eleven years of water quality data have been collected by the Metropolitan Council (1993-2003) and Anoka Conservation District (2004-05). Water quality is showing a significant, downward trend (repeated measures MANOVA with response variables TP, Cl-a, and Secchi depth, F 2,9=5.99, p=0.02). The trend is slow, but significant. The trend is p rob ably due to increases in total phosphorus and secchi depth, but is certainly due to increases in algae. This is likely this is occurring because algae are increasingly out-competing the larger plants for nutrients and light. Increasing levels of nutrients may not be to blame for the downward water quality trend, but may be contributing.
Discussion:
Sullivan Lake likely has poor water quality the entirety of the growing season because polluted storm water inputs are large and dwarf biological factors that cause seasonal fluctuations in water quality in other water bodies. Improvements to the storm water system that could benefit Sullivan Lake should be explored.