Executive Summary for the 2013 Rainbow Lake water quality report

This report presents the results of the 2013 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.

 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 are in close agreement, however the TSI for total phosphorus tends to score the lakes in the oligotrophic range.  A disparity of this nature typically indicates that the lakes experiences periods of phosphorus limitation.

 Total phosphorus concentration in Rainbow Lake has been substantially lower over the past four years. However, there is no statistical trend apparent in the 16 year data set. Within Rainbow Lake, the TP concentration was greatest at the Inlet location during all three sampling events, and was double the concentration of the other locations during the month of July. Total phosphorus concentration in Lake Kushaqua is exhibiting a significant negative trend at a rate of 2.4 µg/L/year since 2009.

Chlorophyll-a concentrations in Rainbow Lake have exhibited a significant negative trend at a rate of approximately 0.17 µg/L/year (P = 0.007).

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 are exhibiting a slight but significant downward trend at a rate of 0.03 pH units per year over the 16 years of monitoring.  The average alkalinity of the lakes ranges from 10-16 mg/L, indicating that the lakes are fairly well buffered, and as a result have low sensitivity to acid deposition.

 The apparent color values of the lakes are historically highly variable. However, despite the variability we found the color of Clear Pond to be exhibiting a slight yet significant upward trend at a rate pf 0.12 PtCo units per over the 16 years of monitoring. Increased color values are associated with an increase in dissolved organic material in the water.

 Non-impacted Adirondack Lakes have very low levels of chloride, the only substantial source being road salt, septic output, and industrial fertilizers. For example, Adirondack lakes in watersheds without paved roads typically have chloride concentrations less than 0.24 mg/L (Kelting et al 2012). The 2013 concentration in Rainbow (1.8 mg/L) and Kushaqua (0.7 mg/L) suggests that the chemistry of the lakes is being influenced by the paved roads or shoreline development within the watershed.  Clear Pond had a chloride concentration of 0.9 mg/L despite its lack of paved roads in the watershed; the slightly elevated chloride concentration may be from shoreline development. 

The dissolved oxygen profiles of the study lakes are typical of most mesotrophic lakes in the Adirondacks, where dissolved oxygen is greatest in the epilimnion (surface water) and gradually decreases towards the bottom.  The last two meters (6 feet) of Rainbow Lake are essentially anoxic from the period of mid-July until the fall turnover.  Clear Pond also experiences anoxia in August but to a lesser degree.  We believe the low dissolved oxygen in late-July in Kushaqua Lake is a field error.