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.
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.
Lakes that fall between the two extremes of oligotrophic and eutrophic lakes are called mesotrophic lakes.
Key parameters used to classify and evaluate water quality include total phosphorous, chlorophyll-a, and Secchi transparency.
Phosphorous is the nutrient that most often stimulates excessive growth of aquatic plants and causes premature lake aging. By measuring phosphorous levels, it is possible to gauge the overall health of a lake. Lakes with a phosphorous 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 represent 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.
Volunteers from the Tri-Lakes Association collect water samples to measure phosphorus and chlorophyll-a and take trasparency readings using a Secchi disk throughout the summer as part of the Michigan Clean Water Corps Cooperative Lakes Monitoring Program (CLMP). A summary of the results from each lake can be found at https://micorps.net/lake-monitoring/individual-lake-reports/.