Trophic State Index (TSI)
There are 3 measurements that are tracked regarding lake health; phosphorus (nutrients), chlorophyll A (algae concentration) and Secchi depth (transparency). They are related to each other. As phosphorus increases, there is more food available for algae, resulting in increased algal concentrations. When algal concentrations increase, the water becomes less transparent and the Secchi depth decreases. The results from these three measurements cover different units and ranges and thus cannot be directly compared to each other or averaged. In order to standardize these three measurements, we convert them to a trophic state index (TSI).
The mean TSI for Clearwater Lake falls in the mesotrophic range. Mesotrophic lakes (TSI 40 ‐ 50) are characterized by moderately clear water most of the summer. "Meso" means middle or mid; therefore, mesotrophic means a medium amount of productivity. Mesotrophic lakes are commonly found in central Minnesota and have clear water with algal blooms in late summer.
Clearwater Lake is phosphorus limited, which means that algae and aquatic plant growth is dependent upon available phosphorus. Total phosphorus was evaluated in Clearwater Lake in 2007 ‐ 2008. The data does not indicate much seasonal variability. The majority of the data points fall into the mesotrophic range.
Chlorophyll A is the pigment that makes plants and algae green. Chlorophyll a is tested in lakes to determine the algae concentration or how "green" the water is. Chlorophyll a was evaluated in Clearwater Lake in 2007 ‐ 2008. Chlorophyll a concentrations for all dates except one remained below 10 ug/L, indicating clear water most of the summer.
Transparency (Secchi Depth)
Transparency is how easily light can pass through a substance. In lakes, it is how deep sunlight penetrates through the water. Plants and algae need sunlight to grow, so they are only able to grow in areas of lakes where the sun penetrates. Water transparency depends on the amount of particles in the water. An increase in particulates results in a decrease in transparency. The mean transparency for Clearwater Lake ranges from 10.2 to 15.4 feet. Overall, Clearwater Lake shows no detectable trend in transparency, meaning that transparency is stable