Concerning Information Regarding the State of Green Lake

On April 14th, Dr. Dale Robertson of the U.S. Geological Survey (USGS), presented the USGS’s annual “State of Big Green Lake” to the GLSD Board of Commissioners, the GLA Board of Directors, and the Lake Management Planning (LMP) Team. The focus of the meeting was for Dr. Robertson to provide an overview of the nutrient loading (specifically phosphorus) into the lake from the watershed as well as recap lake sampling results from 2019 and 2020. Phosphorus is the main nutrient that drives plant and algae production in Green Lake.

The USGS, with primary funding from the Green Lake Sanitary District (GLSD), has monitored Big Green Lake dating back to 2004 and its tributaries dating back to the 1980s. The USGS monitors the lake/tributaries for such things as phosphorus, nitrogen, chlorophyll-a, and depth profiles for water temperature/dissolved oxygen/pH/specific conductance. Much of the tributary monitoring is automated and occurs daily. This monitoring is undertaken to evaluate short and long-term trends which may indicate positive or negative changes in the various subwatersheds. This is also a critical tool in determining where funding for agricultural best management practices (BMPs) should be spent. The GLSD’s annual contract with the USGS comes to around $87,000 per year; twenty percent is paid for by the federal government. Approximately 70% of the tributary flow into
the lake is monitored annually.

Utilizing this data, from 2012 to 2020, the GLSD has worked closely with our Lake Management Team partners to help fund over 130 BMPs throughout the watershed. These efforts have saved approximately 5,300 lbs of phosphorus from entering Green Lake. Even with all the hard work being done by the GLSD and our partners, weather and climate are two of the main drivers of nutrient loading into the lake. The following points were made
by Dr. Robertson.
• Successive wet years from late 2018 – 2020, increased watershed runoff dramatically. Nutrient loading that had been trending downward in the mid-2010’s was shifted in the opposite direction due to these excessively wet years.
• In two years, Green Lake’s phosphorus inputs increased by 25%, from 16,700 in 2016 to 20,800 pounds in 2018.
• We have crossed an important threshold for the lake’s water quality: Green Lake’s phosphorus levels have been consistently high enough that the lake is now eligible to also be listed as impaired for high phosphorus. Deep lakes like Green Lake should have a maximum phosphorus concentration of 15 μg/L to support healthy aquatic life. Yet, based on US Geological Survey monitoring up to 2018, the lake has an average surface phosphorus concentration of 19.4 μg/L.
• The Water Clarity has lessened over these wet years as well. We determine water clarity by using the secchi disc method which, in 2020, was read to an average depth of around 9 feet. Prior to the wet years of late 2018, all of 2019, and early 2020, the average depth of a secchi disc reading was found to be around 21 feet.

The GLSD along with our Lake Management Planning (LMP) Team partners are working with the Wisconsin Department of Natural Resources (DNR) and local farm community to continually add best management practices (BMPs) to keep soil on the landscape. To this end, the GLSD was just awarded a $200,000 Lake Protection grant by the DNR to ramp up outreach into our agricultural community as well as fund critical BMPs such as our new Green Lake Watershed Soil Health Program. Participation in these practices is voluntary for the agricultural community; we cannot be successful without their support.

This is a tough question. Every BMP makes positive change for the lake. To make substantial change, we need to see continual soil coverage by high residue management, cover crops, or land use production shifting to practices such as grazing. Limiting soil or fertilizer runoff from lakeshore properties is part of the solution as well. As mentioned above, weather/climate have a big role in this equation. Intense rain events or excessively wet years work against the efforts being made. As it takes between 15 and 21 years to for water to cycle through Green Lake, change will not be seen overnight. Additionally, the advent of invasive species such as zebra mussels and their promotion of thick algae mats on the lakebed may cloud our ability to see water quality improvements as they happen. Continued monitoring of the lake and its tributaries is critical to understanding how the lake is responding to the work being done in the watershed.