Polk County Land & Water presents undeveloped-lake study: Margaret Lake extremely clear; Omer, Pogo show elevated phosphorus

Polk County Land and Water staff presented results of a two-year study of seven “undeveloped” lakes at the Environmental Services Committee meeting, reporting wide variation in water clarity and nutrient levels across sites.

“Margaret Lake was an outlier,” said Colton Sorensen, summarizing laboratory results. Sorensen reported average Secchi (water-clarity) readings and laboratory values for total phosphorus and chlorophyll a for the seven lakes. He said Margaret Lake had an average Secchi depth of about 17 feet, an average total phosphorus of roughly 4 micrograms per liter and chlorophyll around 2 micrograms per liter.

Caitlin Anderson, who led the presentation with Sorensen, explained site selection and methods. The team excluded small, shallow or highly vegetated water bodies, then selected seven lakes with limited development to compare water quality across the county. They sampled in July–September 2023–24 under DNR protocols and collected Secchi depth, surface phosphorus and chlorophyll a, and profiles of temperature and dissolved oxygen where appropriate.

Results showed most undeveloped lakes were shallower and smaller than the county average. Two lakes in the study — Pogo and Omer — had elevated total phosphorus relative to the undeveloped-lake group and to many county lakes. Sorensen reported average phosphorus of about 38 micrograms per liter at Pogo and about 64 micrograms per liter at Omer, with chlorophyll values that were elevated at Omer but not as high as phosphorus might predict. The presenters noted lake depth, watershed size and connectivity, and historical land use as key drivers of differences among lakes.

Anderson said the county has extensive water-quality records in the DNR database (SWIMS) and used comparable recent data for about 68 Polk County lakes to place the undeveloped-lake results in context. She said most of the county’s lakes with comparable recent data are classified as mesotrophic or eutrophic and that the undeveloped-lake group fell across trophic categories: Margaret was oligotrophic, several lakes were mesotrophic, and some were at or just over the eutrophic threshold.

During public comment, longtime resident and committee member David Olson (public comment) praised the study and urged continued monitoring and public involvement. Olson said of Margaret and Ginger lakes, “I’m not at all surprised to see those results.”

Land and Water staff recommended using the study as a baseline and continuing monitoring and outreach to lake associations and landowners. The committee did not vote on policy changes during the presentation; members suggested scheduling additional outreach and an informational session for the public on proposed ordinance changes that could affect shorelines and lake health.