Silver Lake Water Quality Report

The 2021 water quality of Silver Lake was overall the best since 2006 but still considerably worse than the period spanning 1998 2006. The 2021 summer average for SDT (2.3 meters [7.5 feet]) was considerably better than the MPCA shallow lake eutrophication standard of 1.0 meter, and there was a statistically significant trend (95% confidence) of improving water clarity for the most recent 10-year period. The 2021 summer average of chlorophyll-a (11 μg/L) was also better than the shallow lake standard 20 μg/L. The2021 summer-average total phosphorus was 69 μg/L, similar to recent years but worse than the MPCA's eutrophication standard of 60 μg/L. There were no statistically significant trends for total phosphorus or chlorophyll-a from 2012 2021.

The water quality of Silver Lake had declined rapidly from 2007 2013 before recovering somewhat over the last eight seasons of monitoring. The water quality has not returned to levels observed in the late 1990s and early 2000s, and total phosphorus concentrations remain worse than the eutrophication standard. Summer averages for total phosphorus have failed to meet the MPCA's shallow lake standard 8 of the last 10 years, and summer averages of chlorophyll-a have failed to meet the standard 4 of the past 10 years. SDT summer averages have failed to meet the MPCA standard twice in the last 10 years. Silver Lake is not currently listed as impaired, but the MPCA has proposed adding the lake to the impaired waters list for eutrophication in 2022.

Silver Lake's degraded water quality from 2007 to the present is most likely due to whole-lake herbicide treatments for Eurasian watermilfoil and curly-leaf pondweed in 2007 2008. Prior to 2007, Silver Lake had a dense community of aquatic plants. The whole-lake herbicide treatments diminished the aquatic plant community, resulting in a rapid increase in algae (i.e., phytoplankton). The elevated concentrations of algae reduced water clarity and the light reaching aquatic plants, which reduced plant growth.

Reductions in phosphorus, whether from internal (e.g., sediment) or external (e.g., runoff) sources, are expected to reduce phytoplankton concentrations and increase water clarity. This would allow more light to reach aquatic plants and the plant community to recover effectively shifting the lake back to a plant dominated system and further reducing phytoplankton concentrations during the growing season. Although summer-average total phosphorus concentrations remain worse than the water quality standard, water clarity (as measured by SDT) has been trending better recently. Aquatic plant surveys conducted in recent years have shown a recovery in the native plant community relative to years immediately after the whole-lake herbicide treatments of 2007 2008; however, water quality has not recovered to conditions observed from 1985 2006. Improvements to water quality due to recovery of the aquatic plant community may have been countered by increased phosphorus loading from the watershed. The VBWD has improved the Silver Lake Bioretention Basin and added a spent-lime facility; however, recent road reconstruction projects adjacent to and surrounding the lake and above-average precipitation (e.g., 2019 2020) may have offset those stormwater quality improvements.

The Minnesota Department of Natural Resources conducted a fish survey of Silver Lake in 2017. The survey indicated that the predominant species are northern pike, largemouth bass, and bluegill. Common carp, which can degrade water quality, were not found during the survey. The survey did record a small number of black bullhead, a bottom-feeding fish that can negatively impact water quality; however, the numbers observed in 2017 were not enough to suggest they are negatively affecting water quality. Walleye have been stocked in Silver Lake every year from 2011 2019.

Additionally, Silver Lake was stocked with many adult yellow perch in 2019 (2,205 adults, weighing 630 pounds). At the time this report was prepared, no information was available webpage for Silver lake indicating stocking had occurred in 2020 or 2021. The 2017 survey indicated that the number of bluegill in Silver Lake was within the high end of the normal range expected for a shallow lake in this region. However, the average weight was just 0.1 pound, below the normal weight for bluegill. Small bluegills consume zooplankton; heavy predation by small bluegills may reduce zooplankton that consumes phytoplankton. Larger bluegills prefer larger prey, including insects, snails, and crayfish. Maintaining a healthy population of larger predatory fish (bass, northern pike, walleye) that feed on small bluegill will reduce their numbers and increase zooplankton, which would feed on phytoplankton and could increase water quality.

Ramsey County currently owns and operates an aerator in Silver Lake that was built in 1987. The county, VBWD, and Cities of Maplewood and North St. Paul are in discussions regarding the future operation and maintenance of the aerator. While VBWD has no analytical data to show the benefit of the aerator, Ramsey County and others report that winter fish kills were common in Silver Lake prior to 1986.

The City of North St. Paul took over the operation and maintenance of the aerator in 2023.