Water Quality Across the Belgrades, August 2025
Across the Belgrades, we continue to mostly be having a great year for water clarity. North Pond and East Pond are 7 ft. and 6 ft., respectively, clearer than last year in August. The new water quality buoy in North Pond, funded by 7 Lakes and the North Pond Association, indicates that there has been oxygen down to the bottom of the lake for the entire summer, due to the combination of weather conditions we have had that have led to mixing in the lake, i.e. enough wind and enough cool nights to mix the lake. This is key, as the North Pond Watershed Based Plan determined that half of the phosphorus in North Pond comes from the lake sediments.
Phosphorus is the key nutrient that drives algal blooms in our lakes. But this phosphorus in the lake sediments only becomes available to algae when there is no oxygen at the bottom, a condition we haven’t had yet this summer! This likely also explains why East Pond, the next shallowest lake in the system, is also seeing such great water clarity this summer. While the alum treatment conducted by 7 Lakes and the East Pond Association in 2018 helps keep phosphorus trapped in the lake sediments, we potentially see phosphorus released from shallower depths than where the alum is active during hot calm summer periods, but fortunately, this year has not been that way!
McGrath Pond and Salmon Lake are unfortunately the exceptions to this clearer water, and are 3 ft. and 2 ft. less clear, respectively, than last year in August. The most recent phosphorus data from Colby College shows Salmon Lake having the highest phosphorus in the surface layer of all the Belgrades. There is a lot of phosphorus in the water at the bottom of Salmon Lake because it loses all oxygen near the bottom in June each year. So the mixing that is keeping North and East clear might be causing this deep phosphorus in Salmon to be mixed up to the surface, where algae can use it. Salmon last had a bloom in August 2023 under similar conditions and 7 Lakes is continually monitoring the conditions on the lake.
The biggest lakes in our watershed — Great, Upper Long, Lower Long, and Messalonskee — are all clearer now than they were in August last year, by 1 ft., 4 ft., 3 ft., and 1 ft., respectively. All three of these lakes are mostly influenced by contributions of phosphorus from the watershed, so the lack of rain recently is likely why we are seeing such great conditions on the lakes.
But along with the clearer water comes a different nuisance — Gloeotrichia, the tiny green poof balls you may see off your dock. Gloeotrichia is a blue-green algae that spends part of its life cycle on the bottom of the lake. It needs light to hit the bottom of the lake in order to grow, so when the lake is clearer, the amount of Gloeotrichia often increases. This year, we have had many reports of high amounts of Gloeotrichia in Great Pond and Long Pond. Genetic testing done by Bigelow Labs indicates that the Gloeotrichia doesn’t produce the most common type of algal toxin, but some swimmers who spend a long time in the water report that it makes them itchy. But in general, Gloeotrichia is more of a nuisance algae than a cause for serious concern for those recreating in the water.
You will continue to see the Lake Science Team from 7 Lakes out on all the lakes through November!
Dr. Danielle Wain and Matt Farragher work for 7 Lakes Alliance. She is lake science director, and he is a lake scientist.
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