By Dwight Baldwin

Download or view the 2019 Chocorua Lake Water Quality Report here, and read Dwight’s comments below for analysis.

A total of 11 samples were collected and analyzed from May 3 through October 4, 2019. The results of this sampling effort are shown on the graphs and tables on the report.

This year's water-quality results are perhaps a harbinger of trends to come due to global warming. One of the forecasted effects of climate change is an increase in the number of particularly heavy storm events in the eastern United States.

One such event brought more than 3 inches of rain across the Chocorua Lake watershed on July 12, 2019. As a result, the Chocorua River flushed tea-color water heavy with naturally-occurring tannins from the wetlands into the lake. The effect of this brownish floodwater is clearly evident in Figure 3 and contributed to the decreased water clarity as depicted in the declining secchi disk readings (blue bars).

The increased inflow also brought an increased load of total phosphorus that is reflected in the increased median total-phosphorus level as shown in green in Figure 5. This in turn resulted in an increase in the concentration of algae as measured by the chlorophyll-a concentration depicted in Figure 2. Because total phosphorus is the controlling nutrient supporting algal growth, the higher the total phosphorus concentration, the higher will be the chlorophyll-a concentration. The higher algal concentrations in the water column (along with the higher dissolved color) also contributed to the lower secchi disk values seen through the middle and late summer.

In summary, the heavy rainfall event on July 12, 2019 had a significant effect on the clarity of Chocorua Lake water through the middle to late summer. But all in all, the quality of the lake remained excellent as depicted in Figure 1. The “Fair” status for Chlorophyll-a (as shown in yellow quadrant in Figure 1) resulted from the increase in dissolved phosphorus brought into the lake during the flood event. It shall be interesting to see if similar large storm events in years to come bring similar deterioration of water quality as global temperatures continue to warm.

Banner: Chocorua Lake in spring. Photo: Juno Lamb