USGS Presents State of the Lake

PHOSPHORUS LOADING: Over the past 20 years, the lake has seen a decrease in annual phosphorus loading by approximately 25% (from approx. 18,000+ lbs. in 1997-98 to 16,000+ lbs. in 2013-16). This is significant because phosphorus is the primary problem nutrient which causes degradation in lakes including Big Green Lake. Furthermore, phosphorus concentrations coming from our main tributary, Silver Creek, have decreased by approximately 50%. Since almost one-half of Big Green Lake’s tributary water flow comes from Silver Creek, this decrease is significant.

WATER QUALITY: The water quality of Big Green Lake, based on accepted water quality standards (phosphorus and algal populations), has remained fairly stable over the past 20 years.

WATER CLARITY: The water clarity of Big Green Lake has become clearer since the mid-1980s. Mostly likely, this water clarity improvement from 14 feet to 21 feet can be tied to Zebra Mussels which were introduced to our lake 10+ years ago. Reminder, “clearer” doesn’t mean “cleaner”. AIS (Aquatic Invasive Species) and climate change remain major concerns to Big Green Lake moving forward.

The USGS, in cooperation with 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 evaluates Big Green Lake’s tributaries to the lake to quantify nutrient loading to the lake. The USGS uses very consistent and rigorous sampling protocols that enable the water quality of the lake and its tributaries to be regularly evaluated and examined for short and long-term trends. GLSD’s annual contract with the USGS comes to just under $100,000 per year, of which twenty five percent is paid for by the federal government. Tributary monitoring identifies total phosphorus and suspended sediment concentrations entering the lake; while lake monitoring includes measurements for total phosphorus, dissolved nitrate plus nitrate-nitrogen, dissolved ammonia nitrogen, total Kjeldahl nitrogen, chlorophyll a, secchi (water clarity) and depth profiles for water temperature/dissolved oxygen/pH/specific conductance.

In addition to the GLSD’s annual contract with the USGS which is paid for by your GLSD Tax Levy (refer to the article on back cover titled “Green Lake Sanitary District Taxes”), our LMP team is also conducting other important water quality research, more will be coming on this in the near future.