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background information on bacteria in water and the Coliscan test procedure.

Secchi Depth

Secchi depth is a parameter used to determine the clarity of surface waters. The measurement is made with a “Secchi” disk, a black and white disk that is lowered into the water and the depth is recorded at which it is no longer visible. A Secchi depth recording of 5 ft indicates that the device was last visible at 5 ft below the surface. High Secchi depth readings indicate clearer water that allows sunlight to penetrate to greater depths. Low readings indicate turbid water which can reduce the passage of sunlight to bottom depths. Limited light penetration can be a factor in diminished aquatic plant growth below the surface, thus reducing biological reaeration at lower depths.

Secchi disk readings vary seasonally with changes in photosynthesis and, therefore, algal growth. In most lakes, Secchi disk readings begin to decrease in the spring, with warmer temperature and increased growth, and continue decreasing until algal growth peaks in the summer. As cooler weather sets in and growth decreases, Secchi disk readings increase again.

In lakes that thermally stratify, Secchi disk readings may decrease again with fall turnover. As the surface water cools, the thermal stratification created in summer weakens and the lake mixes. The nutrients thus released from the bottom layer of water may cause a fall algae bloom and the resultant decrease in Secchi disk reading.

Rainstorms also may affect readings. Erosion from rainfall, runoff, and high stream velocities may result in higher concentrations of suspended particles in inflowing streams and therefore decreases in Secchi disk readings.

Secchi disk depth is usually reported in feet to the nearest tenth of a foot, or meters to the nearest tenth of a meter.


Temperature affects water quality by imposing a heat burden on aquatic life, and by limiting the level of dissolved gases in water.

The project will determine if there is a relationship between temperature and bacteria and temperature and Secchi depth.

Dissolved Oxygen

Dissolved Oxygen (DO) is one of the most important determinants of habitat suitability for aerobic organisms – those that need oxygen to live. In streams and lakes, the DO concentration is altered by photosynthesis, respiration, nutrient input, re-aeration, and temperature, all of which have seasonal cycles.

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