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algal blooms and it appears that the algicide was more effective in Lake Olson than in Lake DeMontreville. A possible explanation is that in addition to applying algicide, more residents on Lake Olson treated weeds than on Lake DeMontreville. Since the weed treatment also contains copper sulfate, it will act as an algicide. As shown in Figures 4-8 and 4-11, the water transparency of Lake Olson was better than the water transparency of Lake DeMontreville during July and August of 1991 and 1992. The additional improvement in water quality noted in Lake Olson during 1991 and 1992 made the lake more enjoyable for swimming during the late summer period. Survey results from the MPCA indicate most people would associate transparencies of less than 1.5 meters as either "swimming impaired" or "no swimming". Based on the results of its surveys, the MPCA recommends that the Secchi disc transparency of a lake remain greater than 2 meters a majority of the time to avoid the perception of impaired swimming. During 1991 and 1992, .application of an algicide improved the lake's water transparency from around 1.5 meters to around 2.5 meters. Although improved water transparency was noted throughout the late summer period, following application of the algicide, a decline was noted as algal growth replaced the loss caused by the algicide. Water transparency was greater than one and one half meters during the 1991 and 1992 late summer periods, except for September of 1991 when water transparency declined to less than one meter. Data from Lake Olson during 1990 indicate Aphanizomenon flos-aquae was one of two dominant blue-green algae present during the late summer period. Nuisance algae (particularly Aphanizomenon flos-aquae) appear to become resistant to copper sulfate treatments over a period of time. Any algicide treatment program for Lake Olson is likely to decrease in effectiveness over the growing season, possibly requiring the need for more frequent sulfate later in the season to achieve the same results. applications of copper Historical total phosphorus, chlorophyll a, and Secchi disc transparency growinseason averages from Lakes Olson and DeMontreville are very similar to one anther.. Long term average growing season total phosphorus concentrations for Lakes Olson and DeMontreville are 0.038 mg/L and 0,044 mg/L, respectively, Lon term average chlorophyll a concentrations are 29.4 mg/ma and 33.3 mg/m3 for Lake Olson anda DeMontreville, respectively. Likewise, long term average growing season Secchi disc transparencies for Lakes Olson and DeMontreville are 1.9 meters for each lake. Results of a trend analysis of Lake Olson water quality data showed no significant changes in total phosphorus, chlorophyll a or transparency (Secchi disc) over the 1971 - 1990 period. A significant change in all three indicators is needed to indicate a significant change in water quality. Lakes Olson and DeMontreville were chemically treated to control weeds during 1991 through 1993. The maximum allowable area (10% of the littoral area) was treated in both lakes. The specific dimensions of the treatment areas differed somewhat, however. In Lake Olson, an area that included 3,224 feet of shore and extending approximately 100 feet from shore was chemically treated. In Lake DeMontreville, the cherically treated area included 2,959 feet of shore and extended approximately 100 feet from shore. 4.2.8.5 Public Use and Related Water Quality Concerns The City of Lake Elmo and residents around Lake Olson expressed concern about the effect of the additional water from the Olson Lake Estates pond on Lake Olson water quality. In response to the concerns, VBWD directed its engineer to determine the water quality impacts of the Olson Lake Estates pond discharge on Lake Olson. The results of the engineer's water 23\82\045\SEC4.RPT\KMH 27 DRAFT: June 23, 1994