Ausable River Association and Adirondack Watershed Institute

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

Mirror Lake has been enrolled in a variety of water quality monitoring programs over the past 45 years. These range from citizen volunteer water quality monitoring programs to studies conducted by a variety of contractors and researchers. The purpose of this report is to summarize all the available water quality data on Mirror Lake to develop a comprehensive understanding of the current state of the lake. This report will serve as the basis for future decisions on how best to continue to monitor the health of Mirror Lake, and how to develop effective programs to reduce water quality impairments. Report highlights include:

1. There are challenges with comparing historical data to current monitoring efforts due to inconsistency in sampling methodology, frequency, and location over time. Developing a rigorous, methodologically sound, monitoring program is important to understanding future and ongoing changes in the health of Mirror Lake.

2. Measures of the lake’s trophic status (total phosphorus, nitrate, chlorophyll-a, transparency, and trophic state index) do not show significant long-term trends. Earlier reports of increasing total phosphorus may be the result of inconsistency in sampling location.

3. The lake experiences seasonal anoxia (no oxygen) in the bottom waters during the summer stratified period. There is not enough long-term data to assess whether this a natural condition of the lake or the result of urban development in the watershed. This condition, coupled with potentially longer periods of stratification, may pose a long-term threat to the lake trout population.

4. Calcium concentrations are higher today than measurements made in 1971. Current concentrations are within the reported ranged need to support a viable zebra mussel population.

5. There are significant long-term trends of increasing sodium and chloride in the lake. Concentrations are 9- and 11-times higher than the early 1970s, and 52- and 239-times higher than Adirondack lakes not impacted by road salt, respectively. Chloride builds up in the bottom waters of the lake during winter and spring.

6. Stormwater directly entering the lake through outfalls contributes high concentrations of total suspended solids, chloride, and total phosphorus to the lake. High concentrations of all three parameters are found at locations that drain state maintained roads and areas that drain village and town maintained roads and sidewalks.