Mirror Lake 2019 Water Quality Report
Ausable River Association and Adirondack Watershed Institute
Brendan Wiltse, Corey Laxson, & Elizabeth Yerger
This is the fourth annual report on the water quality of Mirror Lake issued by the Ausable River Association (AsRA) and Paul Smith's College Adirondack Watershed Institute (AWI). Our ongoing work to study Mirror Lake, and threats to its water quality, continually yields new insights about the lake. Our goal is to provide stakeholders with the data and science necessary to make informed and effective decisions about how best to protect Mirror Lake. Road salt remains the top threat to the lake but, despite steps toward salt reduction over the past year, there has not been a documented improvement in the water quality of Mirror Lake. Beginning in 2019, however, we started to see active engagement and collaboration between the state, town, village, local businesses, and residents to achieve reductions in salt loading to protect Mirror Lake. This is something we have encouraged for several years and anticipate will yield meaningful reductions in the salt load entering Mirror Lake if continued. Report highlights include:
Measures of the lake’s trophic status (total phosphorus, nitrate, chlorophyll-a, transparency, and trophic state index) continue to show no significant long-term trends. While many lakes across the state and country are facing threats related to eutrophication, this is not a immediate concern for Mirror Lake at this time. The lake is oligotrophic (low nutrients) and has remained that way over the period of record. However, road salt pollution does put the lake at higher risk of algal blooms both through food web interactions and elevated internal phosphorus loading.
There is a significant long-term increase in calcium, this may be the product of soil cation exchange as a result of road salt and/or the maintenance of a crushed limestone beach on a portion of the lake. Increased calcium poses no specific threat to the water quality of the lake, other than an increased likelihood that zebra mussels could become established in the lake if they were introduced.
This was the first year that a significant increase in pH was detected. The historical record never showed the lake as acidic, but there has been a 0.56 increase in pH over the past 47 years. This is likely a combination of recovery from acid deposition due to pollution control measures implemented at the federal level, as well as the increased buffering capacity provided by the crushed limestone beach maintained on the lake.
Significant long-term upward trends in conductivity, sodium, and chloride remain. Elevated bottom water chloride concentrations were documented and evidence exists that this is impeding the natural turnover of the lake in the spring. The disruption of this important physical process has the potential for the greatest negative effect on aquatic life. The highest chloride concentrations ever reported for the lake (129 mg/L) were observed in February and March.
A prolonged period of bottom water anoxia was documented throughout 2019. This condition is likely natural for Mirror Lake, but is significantly worsened by the lack of spring turnover. If fall turnover were to also not occur, a significant die off of many aquatic organisms as a result of low dissolved oxygen would be likely. There is additional cause for concern related to climate change extending the length of the summer stratified period and delaying the replenishing of oxygen that occurs during fall turnover.
