Key parameters used to classify lakes and to evaluate water quality include total phosphorus, chlorophyll-a, and Secchi transparency.
Phosphorus is the nutrient that most often stimulates excessive growth of aquatic plants and causes premature lake aging. By measuring phosphorus levels, it is possible to gauge the overall health of a lake. Lakes with a phosphorus concentration of 20 parts per billion or greater are considered to be eutrophic or nutrient-enriched.
Chlorophyll-a is a pigment that imparts the green color to plants and algae. A rough estimate of the quantity of algae present in the water column can be made by measuring the amount of chlorophyll-a in the water column. A chlorophyll-a concentration greater than 6 parts per billion is considered characteristic of a eutrophic condition.
A Secchi disk is a round, black and white, 8-inch disk that is used to estimate water clarity. Eutrophic lakes have a Secchi transparency of less than 7.5 feet. Generally, it has been found that plants can grow to a depth of about twice the Secchi disk transparency.
Water quality data collected from Pentwater Lake between 1988 and 2016 are summarized below.
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Pentwater Lake is considered to be meso-eutrophic since it falls between a mesotrophic lake and a eutrophic lake.
Oligotrophic lakes are generally deep and clear with little aquatic plant growth. These lakes maintain sufficient dissolved oxygen in the cool, deep bottom waters during late summer to support cold-water fish such as trout and whitefish.
Lakes that fall between the two extremes of oligotrophic and eutrophic are called mesotrophic lakes.
Eutrophic lakes have poor clarity and support abundant aquatic plant growth. In deep eutrophic lakes, the cool bottom waters usually contain little or no dissolved oxygen. Therefore, these lakes can only support warm-water fish such as bass and pike.
For many years, samples have been collected to evaluate baseline water quality conditions in Pentwater Lake. The discussion below provides background information on water quality and some key sampling parameters.
Pentwater Lake Improvement Board
Volume-weighted average total phosphorus concentration (in ppb), 1988-2016.
Chlorophyll-a concentration (in ppb), 1988-2016.
Secchi transparency (in feet), 1988-2016.