Water Quality
Water quality monitoring has been an integral part of the management plan for Paw Paw Lake. The discussion below provides background information on lake water quality and key sampling parameters.
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 and cool-water fish such as bass and pike.
Lakes that fall between the two extremes of oligotrophic and eutrophic are called mesotrophic lakes.
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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 eutrophic or nutrient enriched.
Chlorophyll-a is a pigment that imparts a 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 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.
Paw Paw Lake Water Quality
Paw Paw Lake has many eutrophic characteristics. The lake has elevated bottom water phosphorus levels, abundant algae growth, and poor water clarity. During the summer months, the deep waters of the lake are devoid of dissolved oxygen. Historical data indicate that Paw Paw Lake is prone to periodic blooms of blue-green algae which are potentially capable of producing algal toxins. However, sampling has found algal toxins in Paw Paw Lake to be well below levels of concern to human health. Monitoring of baseline water quality conditions in Paw Paw Lake is ongoing.
2026 EGLE TMDL Workplan
Paw Paw Lake has been selected by the Michigan Department of Environment, Great Lakes, and Energy (EGLE) as a priority for creation of a nutrient total maximum daily load (TMDL) because of its listing on the Federal Water Pollution Control Act otherwise known as the Clean Water Act 303d impaired waters list. Paw Paw Lake does not currently meet designated use criteria for Coldwater Fishery due to nutrients and low dissolved oxygen. An intensive study will be conducted by EGLE staff during 2026 to determine an appropriate TMDL for Paw Paw Lake. As part of the study, the lake will be sampled approximately every two weeks (see tentative schedule) beginning in March through fall turnover (likely late October or early November). In addition, the three primary tributaries and the primary outlet will be monitored every two weeks during the same time frame to determine the relative contribution of various portions of Paw Paw Lake’s watershed to the lake’s overall water quality. This project is being funded entirely by the State of Michigan and the US Environmental Protection Agency (EPA), no local or special assessment funds (SAD) are being used to pay for the study. EGLE’s work plan provides an overview and detail of the methodology and goals of the plan.
Paw Paw Lake Anoxic Zone
Deep lakes such as Paw Paw Lake undergo annual cycles of turnover and stratification. Recent and historical water quality data indicate that during stratification periods (usually beginning in June and continuing through September) the deeper portions of Paw Paw Lake are rapidly depleted of oxygen caused by bacterial decomposition below the thermocline as depicted in the adjacent graphic.
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The internal release of phosphorus from the deep water sediments (depicted in the image above in red) in Paw Paw Lake may be a significant contributor to nuisance algae growth.
