Agenda - Environmental Policy Board - 10/17/2016

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Water Quality Report — 2009 Rogers Lake Cities of Oak Grove,Ramsey,and Nowthen,LAKE ID#03-0104 Background Rogers Lake is in west-central Anoka County, and lies partially within the jurisdictional areas of both the Lower and Upper Rum River Watershed Management Organizations. It has a surface area of 40 acres and a maximum depth of 6 feet. The shoreline is about 1/3 developed,primarily on the western shore. There are no streams of any consequence entering or leaving this lake; it is an isolated basin with a small watershed. There is no public access. Rogers Lake is designated as"impaired" for excess nutrients by the MPCA. Water Quality Results In 2009 Rogers Lake received an overall B letter grade for water quality. The lake's condition has changed significantly within the last 2-4 years. The water became clearer and plant growth exploded between 2006 and 2008. This condition continued in 2009. In 2006 total phosphorus was high(averaged 110 ug/L),the water was brown and turbid(average 12 FNRU), and algae levels were relatively high(average chlorophyll-a 38.5 mg/L). Plants were limited by the turbid water, and ACD staff estimated 40%of the lake had plants growing to the surface. Floating-leaved plant species were most abundant,probably because light levels were low below the surface. Other monitored years before 2006 had better water quality,but similar aquatic plant growth. In 2008 and 2009 water quality was notably better and plant growth dramatically increased. In 2008 average phosphorus was 32 ug/L,better than the state water quality standard of 40 ug/L. In 2009 average phosphorus was 50 ug/L,but this was driven by a single high reading of 170 ug/L(contaminated sample?). Excluding that high reading the average phosphorus in 2009 was 37 ug/L. Chlorophyll-a was low in 2008 (12.3 ug/L) and even lower in 2009(7.1 ug/L). The water was clear in both years(average turbidity 3 FNRU both years). Plants grew densely and to the surface across 95%of the lake. The entire water column was filled with plants. Species included curly-leaf pondweed,large-leaf pondweed,floating-leaf pondweed,water shield, and lilies. Large-leaf pondweed was most abundant. Curly-leaf pondweed was least abundant. The plant abundance is benefiting some aspects of water quality but negatively affecting recreation and the fishery. Abundant plants are consuming phosphorus,out-competing algae, and minimizing sediment disturbance so the water is clearer. However the abundance of plants eliminates almost all boating, swimming and fishing. Decomposition of the abundant plants consumes oxygen,depleting it below levels needed by most fish. The layer of plants at the surface reduces wind mixing that would oxygenate water. By early June dissolved oxygen levels dropped below 4 mg/L. Dissolved oxygen levels decreased further later in summer,remaining below 2 mg/L for over three months. No dead fish were seen,but a resident said similar conditions occurred in 2007,likely killing most fish at that time. Schools of I"bullheads and tadpoles were the only aquatic animals seen in 2009. The water quality in 2008-09 was not unusual for this lake but the abundance of plants was unusual. Water quality records from 1998,2000, and 2003 are similar to 2008 and 2009. But a review of aerial photos shows that before 2007 there was much less plant growth on the lake (see photos below). In 2000,2003, and 2006 aerial photos plants grew to the surface on<40%of the lake. Similar or less plant growth is seen in 1938, 1953, 1964, and 1970 aerial photos. In 2008-09 plants covered 95%of the lake almost the entire open water season. Trend Analysis Six years of water quality monitoring have been conducted by the Anoka Conservation District and Secchi depths were taken by citizens one other year. This is not enough data to perform a trend analysis. Discussion In recent years Rogers Lake has changed dramatically. In 2006 and earlier the lake had high phosphorus, algae, and turbity. In more recent years water has been clear,but aquatic plants have increased many-fold. This has created recreational and low dissolved oxygen problems. Generally, a rich aquatic plant community is desirable and healthy in a shallow lake,but here it has arguably become excessive and problematic.