Agenda - Public Works Committee - 09/16/2014

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mean" values) and compared to minimally impacted reference lakes in the NCHF ecoregion (Table 2). References lakes included in the last column in Table 2 include those selected to be typical of the ecoregion and minimally impacted, and allow for comparison to Grass Lake. Table 2. Grass Lake 2012-2013 as compared to typical range for NCHF ecoregion reference lakes' Parameter Grass Lake Grass Lake Grass Lake 2-year summer 2012 2013 mean values Typical range for minimally impacted lakes in NCHF Number of reference lakes 43 Total phosphorus(µg/L) 13.8* 12.3* 13.0* 23-50 Chlorophyll mean (µg/L) 6.7 4.9 5.8 5-22 Transparency (feet) (meters) 1.2 1.3 1.3 1.5-3.2 Total Kjeldahl Nitrogen (mg/L) 0.91 0.82 0.87 < 0.60-1.2 Alkalinity (mg/L) 104 107 105.7 75-150 Chloride (mg/L) 35 30 33 4-10 Total Suspended Solids (mg/L) 2.8 2.7 2.76 2-6 Total Suspended Inorganic Solids (mg/L) 0.24 0.24 0.24 1-2 TN:TP ratio 70:1 71:1 71:1 25:1-35:1 3.9 4.3 4.3 4.9-10.5 1 Concentrations shown are the mean of samples taken May -September (i.e., summer means) ug/L = micrograms per liter; mg/L = milligrams per liter * One TP reading in 2012 and two in 2013 were below detectable lab limits. For the purposes of summarizing data, both non -detect samples were rounded up to the lowest detectable limit for phosphorus, which is 10 ug/L. Total phosphorus (TP) is often considered the nutrient that "limits" algal growth in lakes. This is because it is essential to algal growth and it is typically in the shortest supply. Grass Lake's summer -mean TP is much better than the typical range for NCHF lakes (Table 2), notable since it is located in such a highly developed area. For the purposes of summarizing the data for Grass Lake, three non -detect samples were rounded up to the lowest detectable limit for total phosphorus of 10 ug/L. The majority of the remaining samples hovered near the lowest detectable limit level, with only July 2012 and 2013 readings reaching 17 and 18 ug/L, indicating that even during the hottest and driest time of the year, Grass Lake's water quality is excellent2. Nitrogen, while also an essential nutrient for algal growth, is typically not the "limiting nutrient" in most Minnesota lakes. Total Kjeldahl nitrogen is a measure of organic nitrogen (i.e., nitrogen found in algae) and ammonia- nitrogen. When combined with inorganic nitrogen, this represents total nitrogen (TN). Since inorganic nitrogen is often at or below detection in lakes, we often use total Kjeldahl alone to represent TN. The ratio of TN to total phosphorus (TP) is used as a simple basis for discerning which nutrient, TN or TP, is the limiting nutrient. Lakes are often considered "nitrogen -limited" when the TN: TP ration falls below about 10:1. In the case of Grass Lake, its 71:1 ratio is very high, over the typical range for NCHF lakes, indicating that phosphorus is the nutrient controlling algal growth in this lake. The addition of phosphorus to the lake could increase the production of algae and aquatic plants. 2 The phosphorus detection method used by the Minnesota Department of Health changed in 2012 and 2013, and phosphorus taken up by chlorophyll -a was not being included in final lab results, resulting in a low bias. The phosphorus values should be considered estimates. Grass Lake (02-0113) • August 2014 Minnesota Pollution Control Agency 3