Keoka Lake Water Quality – 2021

by Charlie Tarbell

April 27, 2022

This is the annual Lakes Environmental Association (LEA) report on Keoka Lake Water Quality. Bottom-line-up-front: the report appears quite favorable – and make no mistake, it clearly shows improvement stemming from all of our hard work over the last 50 years reducing point-of-source stormwater runoff and septic system pollution. Water quality ratings for 2021 (page 2) are moderate for each of the key measures (clarity, phosphorus, and chlorophyll-a). Better still, the five-year trend is favorable for clarity and phosphorus and stable for chlorophyll-a.

Even the deep-water phosphorus readings were within the acceptable range, although, as the water above the thermocline warmed through the summer, deep-water oxygen was consumed and the habitat became unsuitable for cold-water fish species (such as trout) from June through September. So, although the attached report is actually quite favorable, the poor oxygen level in Keoka’s deep water during the summer is reason for our continued concern.

Keoka’s high temperature for the year was 83.4 degrees (F). You can see from the graph on page 21 that by July the water temperature was pretty even for the first five meters (i.e., stratification). Over the course of the season, oxygen is biologically consumed by bacteria in the deep waters and the difference in water density between warm and cold waters prevents the oxygen-rich (from wave action) surface water from mixing with the nutrient-rich deep water.

Lake stratification has a benefit in that surface algae blooms are deprived of nutrients during the summer (although, unfortunately, this nutrient deficit can be offset by phosphorus-laden stormwater runoff). It also has a detriment in that cold-water fish are deprived of oxygen during this period. Surface water temperatures varied sharply throughout the season due to 1) weather variations and 2) the recent deluge nature of summer storms (see graph page 18). Storms also decrease water clarity due to sediment and tannins carried by runoff.

In conclusion, lake ecology is complicated and ever-changing. The 2021 LEA report confirms that actions we are all taking to reduce stormwater runoff are working. These actions include LakeSmart evaluations, maintaining your 100’ shorefront buffer zone, preventing driveway and road erosion, stopping stormwater from surging off of your roof, the recent improvements at the boat launch, and many other municipal and roadside efforts. If you have any questions about what you can do to manage your property and help with Keoka water quality, you can contact Charlie Tarbell at 207.671.6962 to discuss.

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