On April 1st, Dr. Dale Robertson of the U.S. Geological Survey (USGS), presented the annual “State of the Lake” to the GLSD Board of Commissioners, several members of the GLA staff and Board of Directors, as well as the Lake Management Planning (LMP) Team and members of the public. The focus of this annual presentation was for Dr. Robertson to provide an overview of water quality in Green Lake 2025 compared to past years, and the nutrient loading (specifically phosphorus) into the lake from the watershed. Though 2025 produced a very dry summer and autumn, there was above normal rainfall last spring. As Dr. Robertson outlined, we’re learning more about the importance and subsequent impact of rainfall on the nutrient loading to and cycling within the lake.
Unfortunately, the increased runoff last spring, along with natural in-lake nutrient cycling, contributed to an uptick in the lake’s phosphorus concentrations after steadily declining over the past several years. During the 2025 season, the average total phosphorus concentration on the west side of the lake was 23.4 mg/L and on the east side was 28.7 mg/L, which was quite a bit higher than the prior three-year average of 12.8 mg/L and 13.1 mg/L, respectively. Dr. Robertson noted that the September samples were taken during the unusual surface water enrichment that happened during the lake-wide bloom event and that the timing of the sampling partly contributed to the sharp increase in the summer average.
The GLSD and our Lake Management Team Partners continue to work together throughout the watershed to ramp up efforts to reduce nutrient loading into the lake through a myriad of projects including agricultural best management practices and the long-term restoration of the County K Marsh. By restoring the County K Marsh, we will create a healthy ecosystem with rooted aquatic plants to stabilize its phosphorus-laden substrate, stop shoreline erosion, and facilitate clear water flowing into Green Lake. This restoration should provide lasting phosphorus reduction and create more resiliency within the lake itself.
We are heading into the busy summer months, and the aquatic plant harvesters will soon be seen in action around the lake. This is a good opportunity to remind landowners that the GLSD harvesting program must follow the DNR-issued permit for its operations. The DNR permits us to harvest aquatic plants to maintain navigational lanes around the lake; not to clear out near-shore recreation areas. It’s important to remember that aquatic plants only grow in shallow areas in depths up to 15 to 20’. On our lake, that shallow riparian area constitutes a very small fraction of the lake area. While often thought of as a nuisance, these important plants uptake available nutrients which limit algae growth, provide habitat for zooplankton, fish, and other critters, and help hold lake sediments in place which limit their resuspension by large boat wakes. Please help us to protect this important part of our lake ecosystem by limiting aquatic plant removal from your nearshore area.
Permanent piers and aerators are a topic about which we have received many calls this past winter. Many residents are not aware that if you have a pier that you do not remove from the lake in fall but use aeration to try to protect from ice heaving, you must follow State of WI requirements in how impactful your aeration may be. Aerators, used in summer or winter, are not allowed to disturb the lake bottom. Focusing aeration at lake sediments to dislodge them to increase depth along a shoreline is not permitted. This creates available phosphorus in the water column for increased algae growth. Additionally, and most concerning from a lake access standpoint during winter, aeration in front of a parcel is not allowed to cause an “obstruction to navigation” for their neighboring parcels. This means an aerator absolutely may not degrade ice conditions in front of adjacent parcels. This past winter we saw many instances where aerators reduced ice thickness or removed ice completely in front of their neighbor’s parcels. The use of these aerators prohibited ice/lake access entirely from those parcels. This is not allowed and provides a significant liability risk to the owner of the aerator that caused the degraded ice conditions. For more information, please see this DNR Fact Sheet: