Executive Summary 2014

Limnological monitoring of the Rainbow Lake Chain has been carried out by the Rainbow Lake Association and the Adirondack Watershed Institute since 1997. This report presents the results of the 2014 field season and describes long term trends in the historical data of the Rainbow Lake Chain. Though the data and accompanying analysis provided in this report give insight into the water quality of the Rainbow Lakes, more detailed limnological studies may be necessary to produce management recommendations. Raw water quality data can be provided upon request. The bullets below represent the primary findings contained within this report.

v  The dissolved oxygen profiles of the study lakes are typical of most productive lakes in the Adirondacks, where dissolved oxygen is greatest in the epilimnion (surface water) and gradually decreases towards the bottom.  The bottom four meters (13 feet) of Rainbow Lake and two meters (6.5 feet) of Clear Pond were essentially anoxic during late summer. 

v  Carlson’s Trophic Status Index based on transparency, chlorophyll-a, and total phosphorus suggests a mesotrophic classification for the three study lakes. The mesotrophic classification for the lakes has been consistent since the monitoring program began.  In all of the lakes the TSI values for transparency and chlorophyll were in close agreement, however the TSI for total phosphorus tended to score the lakes in the oligotrophic range.  A disparity of this nature typically indicates that the lakes experiences periods of phosphorus limitation.

v  Total phosphorus concentration in Rainbow Lake has remained low for the last five years and has exhibited a significant downward trend at a rate of approximately 0.4 µg/L/year. We also observed substantially lower phosphorus levels in Clear Pond; however there was no statistical trend in the historical data for Clear Pond or Lake Kushaqua. Total phosphorus concentrations were 2-3 times greater in the hypolimnion of the study lakes. The increase in bottom water P was related to the hypoxic condition of the hypolimnion.

v  Chlorophyll-a concentrations in Rainbow Lake have exhibited a significant negative trend at a rate of approximately 0.15 µg/L/year (p = 0.03). No trend was detected in the historical data of the other study lakes.

v  The waters of the Rainbow Lake chain are typically circumneutral (pH 6.5-7.5) in terms of their acidity. The average pH of Rainbow Lake and Clear Pond exhibited a slight but significant downward trend at a rate of 0.03 pH units per year over the 17 years of monitoring (p = 0.01).  The average alkalinity of the lakes ranged from 10-16 mg/L, indicating that the lakes were fairly well buffered, and as a result have low sensitivity to acid deposition.

v  The apparent color values of the lakes were elevated, and historically highly variable. Elevated color is indicative of an increase in dissolved organic matter in the water and is typically due to watershed characteristics such as wetland, bogs, and coniferous forest cover. 

v  Non-impacted Adirondack Lakes have very low levels of sodium and chloride, the only substantial sources being road salt, septic output, and industrial fertilizers. For example, Adirondack lakes in watersheds without paved roads typically have sodium and chloride concentrations less than 0.55 mg/L and 0.24 mg/L respectively.  (Kelting et al 2012).   Rainbow and Kushaqua Lakes have slightly elevated concentrations of these chemicals compared to non-impacted baselines. The paved roads in the watershed, along with shoreline development, were likely responsible for the slightly elevated levels of these chemicals.  No statistical trend was detected in the historic chloride levels the study lakes.