Lake Monitoring

Pope County has many valuable water resources. They provide the basis for our tourist and recreational businesses. Although their economic impact was not evaluated in this study, it must be large. High quality water is vital to maintaining these businesses and their associated tax base. Therefore, it is essential for all parties, responsible for land management decisions, to initiate programs to safeguard these valuable resources.

The Pope County COLA monitoring project, which started in 1994, has continued to date. Thus, sixteen lakes, with seventeen sample sites, are covered in this report. Due to State budget deficits, funds were not available to the County, in 2003, from the Board of Water and Soil Resource. Consequently chlorophyll a was not sampled in 2003. However, total phosphorus is generally the nutrient most closely related to chlorophyll a production and weed growth, when nutrients are the limiting factor. Thus, these two parameters are expected to vary in unison. Likewise, the amount of chlorophyll a should be directly related to the depth of light penetration, which is measured by Secchi disc. Therefore, the Secchi disc and chlorophyll a data should show similar trends.

This is not the case in some of our lakes. Secchi disc readings can be misleading, because suspended sediments, highly colored water, and very clear water affect transparency. Unfortunately, other unsampled factors are frequently related to seasonal and year-to-year variation. Spring (May) and early summer (June) chlorophyll a densities, high total phosphorus concentrations, and low Secchi disc readings are usually related to the amount of spring run-off and/or inflow. Many additional factors could be sampled in the future, if funds become available.

Algal blooms and reduced transparency can change the character and recreational use of our lakes. Such changes are related to excess nutrient and sediment loading due to human activities in the watershed. The Minnesota Pollution Control Agency (MPCA) uses a lake’s summer average total phosphorus (TP) to determine whether lakes meet swimmable use. Our lakes are classified as:

(1) full support: TP = under 30 parts per billion (ppb),
(2) support threatened: TP = 30 – 40 ppb,
(3) partial support: TP = 40 – 60 ppb,
(4) non-support: TP = over 60 ppb.

Nuisance algal blooms are associated with total phosphorus concentrations greater than 30 ppb, in our region. Remember, 30 ppb equals one teaspoon ofTP in 48,880 gallons of water or a pool 10 by 20 by 30 feet. In addition, one pound of TP will produce 500 pounds of weeds. Swimmable use during the summer season is related to the frequency and duration of these conditions.

The first table ranks our lakes using to the above criteria for each year: 1994-2004 and for the average of all eleven years. The eleven-year average is used to place each lake in one of the above support classes as shown in the next four graphs. Please note the large year-to-year variation for some of the lakes, not all lakes peak in the same year. Thus, one cannot generalize because such variation strongly influences the interpretation.

Each lake has its own distinctive pattern and trend among total phosphorus and Secchi disc values. In addition to the year-to-year variation, there is variation within years. Thus, for the specific details, refer to the tables and charts for each lake.

This report provides the basic data for our lakes by updating the total phosphorus, chlorophyll a, and Secchi disc tables and charts through 2004. No attempt made to explain these trends and variation. Future reports will try to provide such information.