11/28/2001 Env Bd Packet - Laserfiche WebLink

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The 1991 TP concentrations were compared to the long -term average. Thirteen of the 19 lakes monitored had TP concentrations which were less than their respective long -term average. Overall, the 1991 TP concentrations are about 10 percent less than their respective long -term averages. The water quality in some of the lakes seems to show trends since 1972. Baldwin, Otter, and Rice Lakes seemed to have lower TP concentrations during the last few years than concentrations in the 1970s and early 1980s. Mud and Pike Lakes seem to have decreased in water quality. Secchi Transparency. In order to identify long -term changes in lake water quality, Secchi disc transparency readings were reviewed. The values shown in Table 3 -7 represent Secchi transparencies from RCWD monitoring from 1972 -91. Average Secchi transparency is also calculated. Data collected during the summer of 1991 were compared to the respective long -term averages. Of the monitored lakes, seven showed a decrease in transparency of greater than 30 percent in 1991 compared to the long -term average. Conversely, only two lakes, Otter and Sunset, showed increased water clarity of greater than 30 percent in 1991. However, during 1991 lakes were monitored only one or three times. Given the short -term fluctuations in Secchi transparencies, the 1991 data may not offer a good representation of summer water quality. Trophic Status. Using RCWD monitoring data, the average trophic state over the past 17 years was determined for 32 lakes within the RCWD. Trophic states were determined using average summer TP concentrations for the respective lakes. Values are shown in Table 3 -8. According to Heiskary (1985), lakes with trophic states of 0 -40 are oligotrophic, 41 -50 are mesotrophic, 51 -70 are eutrophic, while a trophic.. state of greater than 70 indicates a hypereutrophic lake. Using this definition, an estimate of the Minnesota lake classifications was made. The distribution is compared to the distribution of RCWD lakes in Table 3 -9. 3 -22 10/97