2006 Results
In 2006 Crooked Lake had slightly above-average water quality for this region of the state (NCHF Ecoregion), receiving an overall B- grade. It had earned a B letter grade the previous six years. The 2006 letter grade was pulled down by unusually high total phosphorus on August 14. The reason for that high reading is unknown, but a contaminated sample or unrepresentative sample is possible. Overall, the lake is slightly eutrophic. Water clarity in 2006 was tied with 2005 as the second-best ever recorded. The best clarity was in 2004. Annual average total phosphorus and chlorophyll-a were similar to other years since 1994. The water was green-colored most of the year, with “some algae” until mid-July and generally described as “definite algae” thereafter. ACD staffs’ subject assessment of the lake was that there was a slight swimming impairment periodically during late summer, but otherwise there were minimal problems.
Trend Analysis
Thirteen years of water quality data have been collected by the Metropolitan Council (between 1983 and 1998) and the Anoka Conservation District (between 2000 and 2006) with eight additional years of Secchi measurements by citizens. Water quality has significantly improved from 1983 to 2006 (repeated measures MANOVA with response variables TP, Cl-a, and Secchi depth, F2,10=15.47, p<0.001). Most improvements occurred between 1989 and 1994 though Secchi depth also has an improving trend since 1998.
Discussion
Current threats to Crooked Lake are nuisance growth of invasive exotic aquatic plants, poor shoreline mangagement, urban runoff, and excessive numbers of waterfowl. Aquatic plants, including an abundance of non-native curly leaf pondweed are matted to the surface in many near-shore areas, especially at the north end during spring. Because the lake is narrow and long, boat wakes are likely a significant cause of shoreline erosion and may suspend lake bottom sediments. Measures that could be considered to protect and improve water quality include correcting shoreline erosion and installing shoreline buffers of native plants, and priority street sweeping for roads that drain to the lake along with other storm water system retrofits.