05/06/1996 Park Board Packet - Laserfiche WebLink

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-r-- Section 4 - Reshanau Lake ►- ditch. This may account for the low DO conditions since groundwater typically has low DO. The low RL-2 DO concentrations did not appear to affect the lake DO levels(Figure 4-6). Outflow conditions were monitored at site RLrO (Figure 4-1). Table 4-3 presents the outflow conditions. Total phosphorus was greater than the 100 µg11 guideline for all sampling events J except during the late October and early November events. The TSS concentrations were lower J than the 30 mg/l guideline. CONCLUSIONS Reshanau Lake is highly eutrophic and does not support swimming (MPCA 1990). The lake is small and relatively shallow with a maximum depth of 16 feet. The lake is fully exposed to the Twind because it is oriented in a north-south direction. This fetch allows the lake to continually mix throughout the summer. The only component that interrupts the mixing is the extensive growth of curly-leaved pondweed in June. This plant grows early in spring to early-mid summer and then undergoes a massive die off. This die off can elevate the TP concentrations in a lake and promote the uncontrolled growth of algae in conjunction with high TP concentrations. This was particularly the case in Reshanau Lake in 1995. The TP and chlorophyll-a concentrations were very good until - the July 11 th sampling date. This date corresponds with the curly-leaved pondweed annual die off. After this date the chlorophyll-a numbers increased dramatically along with a drastic increase in TP concentration. A majority of the pollutant load is delivered by County Ditch 25 from the east represented by sampling site RL-2 since it has the largest flow and tributary drainage area. Flows from County Ditch 25 were generally at least five times higher than flows observed at the other location. Reshanau Lake is a severely degraded eutrophic lake and is largely a reflection of the large drainage area to the lake and shallow depth. Given these attributes, water quality in Reshanau Lake would not be expected to be better than that observed in 1995 unless a different aquatic plant community was established in the lake. General recommendations include the continued application of general District water quality rules and policies throughout the Reshanau Lake watershed and a complete aquatic macrophyte survey to determine the extent of the curly-leaved pondweed growth. Surveys should be conducted twice over a growing season, once during peak curly-leaved pondweed growth in June, and once in August to determine if any native plants become established after the annual die off. MONTGOMERY WATSON Page 47