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King County monitors the water quality of its highly-valued lakes to ensure their continued health and the health of swimmers, boaters, fishermen, lakeside property owners and other people who utilize their many resources. One way to characterize the health of lakes is by the numerical Trophic State Index (TSI).
The graphs above depict the trophic state of Lakes Washington, Union, and Sammamish during the summer months.
With the TSI, lakes can be rated and compared according to the level of biological activity (e.g., level of nutrients, algal growth, and so forth). This index provides a standard measure to compare lake quality on a scale of 0 to 100. Each major division (10, 20, 30, and so forth) represents a doubling of algal biomass and is related to nutrients and transparency (water clarity). The summer mean values of the three most common lake parameters-Secchi depth transparency, total phosphorus, and chlorophyll-a concentrations-are used to develop the TSI. The calculated TSI values provide three ranges of lake classification-oligotrophic, mesotrophic, and eutrophic-as shown in the table below.
Average Summer (June-September) Trophic State Index Values
| TSI Value |
Classification |
Characterization
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| < 40 |
Oligotrophic |
Low biological productivity resulting in high water clarity, low algal levels. and low phosphorus concentrations |
| 40-50 |
Mesotrophic |
Moderate levels of plant and animal activity, resulting in moderate water clarity, moderate, algal levels, and low phosphorus concentrations |
| > 50 |
Eutrophic |
High biological productivity resulting in low water clarity, high algal levels, and high phosphorus concentrations |
Secchi Depth Transparency
Secchi depth transparency is a measure of water clarity or transparency as measured by viewing a Secchi disk—an 8-inch disk for fresh water or a 12-inch disk for marine water, with alternating black and white quadrants. The disk is lowered into the water until the observer can no longer see it. This depth of disappearance, called the Secchi depth, is a measure of the water’s transparency. Algae, soil particles, and other materials suspended in the water affect transparency. The Secchi depth will decrease as these factors increase. In King County, clarity tends to be lower during periods of high algal growth (spring and summer) and during periods of high stormwater flows (winter).
Phosphorus
A certain amount of nutrients such as nitrogen, phosphorus, and silica are necessary for plant and animal growth. An excessive amount of nutrients, however, can increase the growth of aquatic plants, which subsequently decay and deplete oxygen to levels incapable of sustaining aquatic organisms. Phosphorus is the primary nutrient of concern in freshwater systems because, if present in excess amounts, it can cause nuisance algal blooms or, on occasion, toxic algal blooms. Phosphorus enters water bodies via discharge of detergents, runoff containing fertilizers, or seepage from failing septic systems. Sediment can also be a source of phosphorus, as phosphorus readily binds to soil particles and is washed into the lakes. Phosphorus is later released into the water column when DO concentrations fall below 0.2 mg/L.
Chlorophyll-a
Chlorophyll is the green pigment in plants that allows them to create energy from light (photosynthesis). Chlorophyll serves as an indirect measure of the amount of plants/algae in the water column. Chlorophyll-a is a measure of the portion of the pigment that is still actively photosynthesizing at the time of sampling.
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