Como Lake TMDL
1C. Pollutant of Concern Role of Phosphorus in Shallow Lakes
Como Lake is classified by the MPCA as a shallow lake. The MPCA defines a lake as shallow if its maximum depth is less than 15 ft, or if the littoral zone covers at least 80% of the lake’s surface area. Total phosphorus is often the limiting factor controlling primary production in freshwater lakes. It is the nutrient of focus for this TMDL, and is sometimes referred to as the causal factor. As phosphorus concentrations increase, primary production also increases, as measured by higher chlorophyll-a concentrations. Higher concentrations of chlorophyll lead to lower water transparency. Both chlorophyll-a and Secchi transparency are referred to as response factors, since they indicate the ecological response of a lake to excessive phosphorus input. There is often a positive relationship between TP and chlorophyll-a, and a negative relationship between TP and Secchi depth, as is the case with Como Lake (Figure 5 and Figure 6). Similarly, a negative relationship is apparent between chlorophyll-a and Secchi depth (Figure 7).
140
Chlorophyll-a (µg/L)
120
100
80
60
40
20
0 0
100
200
300
400
500
600
700
Total Phosphorus (µg/L) Figure 5. Relationship of Chlorophyll-a to TP in Como Lake, 1993-2007.
Emmons & Olivier Resources, Inc.
11