Lake Pocotopaug Lake and Watershed Restoration Evaluation ...

The same analysis was performed with SDT and chlorophyll a values from 1991-2001, but usingCarlsons and Frink and Norvell data (Figure 16). Scatter was worse; data did not fit eithercurve. Reducing the data set to July and August values did not increase the correlation (Figure17). Data by year was plotted. 2001 appeared to follow the trend, but values were again shifteddown and to the left (Figure 18); indicating that Pocotopaug is exhibiting lower SDT based onchlorophyll a values. One could conclude that SDT is based more on non-algal turbidity.Total phosphorus and chlorophyll a values were plotted and compared to Carlson’s and Frinkand Norvell’s data. Again, all 1991-2001 and July/August data produced much scatter (Figures19 and 20). 1991, 1993 and 2001 data were plotted separately (these are the only three yearswith chlorophyll a data). 2001 data provided the closest fit, although the slope was steeper thanCarlson or Frink and Norvell (Figure 21). The steeper upward trend indicates that there is morechlorophyll a than predicted by phosphorus concentrations. Algal density is higher thanexpected; algae are thriving in a lower phosphorus environment. However, more chlorophyll adata are needed to verify this trend.When comparing Lake Pocotopaug 2001 (TP between 10 and 20 ug/L) data to empirical modelspresented in literature, we find that average chlorophyll a in Lake Pocotopaug should bebetween 2.1 and 9.9 ug/L (Table 17). In 2001, Lake Pocotopaug average chlorophyll was 5.8ug/L, within the predicted average of the four empirical model calculations (3.1 – 7.6 ug/L).However, considering that average TP concentrations in 2001 were closer to the 10 ug/L (12and 13 ug/L), 2001 chlorophyll values are high compared to the empirical model predictions.Similarly, maximum chlorophyll values in Lake Pocotopaug were 11.7 and 14.8 ug/L at LP-1 andLP-2 respectively, within the 11.2 – 26.1 ug/L average maxima range. Indicating that LakePocotopaug resembles most lakes at their worst condition (maxima are closer to measuredLake Pocotopaug values than averages). Average 2001 SDT was 2.01 m, below the predictedaverage for both 10 and 20 ug/L. Maximum 2001 SDT was 2.99 m, below predicted maxima.Using the average and maximum predicted chlorophyll, SDT was predicted using Carlson(1977) and Frink and Norvell (1984) calculations. Carlson’s average and maximum SDT rangedfrom 1.93 – 3.57 m and 0.84 – 1.49 m, respectively. Frink and Norvell average and maximumSDT ranged from 2.98 – 4.78 and 1.18 – 2.30 m, respectively. This indicates that if LakePocotopaug behaved more like other lakes (reduce chlorophyll concentrations per unit TP),average SDT should be slightly lower than 3.57 and 4.78 m, with a worse case scenario ofslightly less than 1.49 – 2.30 m. Thus, increasing current average SDT by 1.5 m or greater,provided that SDT in Lake Pocotopaug is chl a driven.Lake Pocotopaug Restoration Evaluation 75May 2002