05/06/1996 Park Board Packet - Laserfiche WebLink

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Section 4 - Reshanau Lake Chlorophyll--a Chlorophyll-a is a chemical pigment common to all plants and is used as a measurement of algal abundance. Chlorophyll-a concentrations in Reshanau Lake ranged from 11 to 121 µg/1 and averaged 56 µg/1. This average places Reshanau Lake in the worst 25th percentile for lakes in the Central Hardwoods Region (MPCA 1990). The peak chlorophyll-a concentration was observed within two weeks after the die off of curly-leaved pondweed. This strongly suggests that the phosphorus in the lake becomes more available for algal uptake when the seasonal curly-leaved pondweed dies off. Algae blooms are generally considered obnoxious once chlorophyll-a concentrations exceed 30 to 40 µg/1. Therefore,the 1995 chlorophyll-a concentrations indicate that Reshanau Lake has a severe algal bloom problem. This is partly caused by the extremely high TP concentrations the lake experiences after the pondweed dies off. Total Suspended Solids Total suspended solids SS is a measure of the amount of organic or inorganic articles in water Pe (TSS) g g P - and is also a measure of water quality. Total suspended solids concentrations ranged from 1 to 18 mg/1 and averaged 10 mg/l. The seasonal pattern for TSS is almost identical to the chlorophyll-a pattern for 1995 (Figures 4-3 and 4-4) and also mirrors the SDT. The sampling date on 7/11/95 recorded the highest TSS reading of 18 mg/l, although at this time the chlorophyll--a concentration was only 48 µg/l. This high TSS may be due to the increase in organic particles in the lake due to ` the massive die off of curly-leaved pondweed at this time. The relationship between TSS and chlorophyll-a suggests that most of the water clarity problems are due to algae productivity in the lake rather than inorganic sediment. Temperature Monitored temperatures for Reshanau Lake are presented in Appendix B. Reshanau Lake is a moderately shallow lake with a maximum depth of 16 feet and a mean depth of 7.9 feet. Reshanau Lake is well-mixed vertically or polymictic. This is because temperature is almost the same throughout the water column and the lake does not experience thermal stratification: Lack of thermal stratification allows the lake to mix vertically throughout the growing season. The lake did experience a brief period of thermal stratification in June. This period coincides with the peak growth and density of curly-leaved pondweed. This plant grows dense enough that it does not allow any mixing of the lakes waters by wind action. Once the plant dies off in July, the lake MONTGOMERY WATSON Page 4 3