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05/06/1996 Park Board Packet
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05/06/1996 Park Board Packet
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Park Board
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Park Board Packet
Meeting Date
05/06/1996
Park Bd Meeting Type
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Section 4 - Reshanau Lake <br /> Chlorophyll--a <br /> Chlorophyll-a is a chemical pigment common to all plants and is used as a measurement of algal <br /> abundance. Chlorophyll-a concentrations in Reshanau Lake ranged from 11 to 121 µg/1 and <br /> averaged 56 µg/1. This average places Reshanau Lake in the worst 25th percentile for lakes in the <br /> Central Hardwoods Region (MPCA 1990). The peak chlorophyll-a concentration was observed <br /> within two weeks after the die off of curly-leaved pondweed. This strongly suggests that the <br /> phosphorus in the lake becomes more available for algal uptake when the seasonal curly-leaved <br /> pondweed dies off. Algae blooms are generally considered obnoxious once chlorophyll-a <br /> concentrations exceed 30 to 40 µg/1. Therefore,the 1995 chlorophyll-a concentrations indicate that <br /> Reshanau Lake has a severe algal bloom problem. This is partly caused by the extremely high TP <br /> concentrations the lake experiences after the pondweed dies off. <br /> Total Suspended Solids <br /> Total suspended solids SS is a measure of the amount of organic or inorganic articles in water <br /> Pe (TSS) g g P <br /> - and is also a measure of water quality. Total suspended solids concentrations ranged from 1 to 18 <br /> mg/1 and averaged 10 mg/l. The seasonal pattern for TSS is almost identical to the chlorophyll-a <br /> pattern for 1995 (Figures 4-3 and 4-4) and also mirrors the SDT. The sampling date on 7/11/95 <br /> recorded the highest TSS reading of 18 mg/l, although at this time the chlorophyll--a concentration <br /> was only 48 µg/l. This high TSS may be due to the increase in organic particles in the lake due to <br /> ` the massive die off of curly-leaved pondweed at this time. The relationship between TSS and <br /> chlorophyll-a suggests that most of the water clarity problems are due to algae productivity in the <br /> lake rather than inorganic sediment. <br /> Temperature <br /> Monitored temperatures for Reshanau Lake are presented in Appendix B. Reshanau Lake is a <br /> moderately shallow lake with a maximum depth of 16 feet and a mean depth of 7.9 feet. <br /> Reshanau Lake is well-mixed vertically or polymictic. This is because temperature is almost the <br /> same throughout the water column and the lake does not experience thermal stratification: Lack of <br /> thermal stratification allows the lake to mix vertically throughout the growing season. The lake did <br /> experience a brief period of thermal stratification in June. This period coincides with the peak <br /> growth and density of curly-leaved pondweed. This plant grows dense enough that it does not <br /> allow any mixing of the lakes waters by wind action. Once the plant dies off in July, the lake <br /> MONTGOMERY WATSON Page 4 3 <br />
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