Mirror Lake 2017 Water Quality Report

Ausable River Association and Adirondack Watershed Institute

Brendan Wiltse, Corey Laxson, Nicole Pionteck, & Elizabeth Yerger

This is the second annual report on the limnology and water quality of Mirror Lake issued by the Ausable River Association and the Adirondack Watershed Institute. Our research on Mirror Lake and threats to its water quality continues to yield 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 has emerged as the top threat to the lake, but we have much more work to do before identifying a solution that both protects the lake and provides safe surfaces for people to drive and walk on. This report highlights our most current knowledge of the impacts of road salt to the lake and outlines future work needed to effectively identify solutions.

Key findings include:

1. Measures of the lake’s trophic status (total phosphorus, nitrate, ammonium, total nitrogen, 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 concern for Mirror Lake at this time. The lake is oligotrophic (low nutrients) and has remained that way over the period of record.

2. There is a significant long-term increase in calcium, this may be the result of soil cation exchange as a result of road salt and/or the maintenance of a crushed limestone beach on 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.

3. Significant long-term trends in conductivity, sodium, and chloride remain a concern. Elevated bottom water chloride concentrations were documented and evidence exists that these concentrations are impeding the natural turnover of the lake in the spring. The disruption of this important physical process has the potential for a significant negative effect on aquatic life.

4. A prolonged period of bottom water hypoxia was documented throughout 2017. This condition is likely natural for Mirror Lake, but it is worsened by the lack of spring turnover. If fall turnover were also not to occur, a die-off of many aquatic organisms – as a result of low dissolved oxygen – would be likely.

5. Additional work is necessary to determine the reduction in road salt necessary to protect
   Mirror Lake and set it on a path of recovery. In order to accurately estimate the reduction in salt necessary to achieve this, we need the entire community within the Mirror Lake watershed to be engaged participants in the study of this problem. The more we know and understand how much salt is applied within the watershed, and where, the better we can understand how much of a reduction is necessary to protect the lake.