The data in graphs below was collected by 7 Lakes Alliance, Colby College, Pete Kallin, McGrath Pond and the Salmon Lake Association.
For more information about lake research at the 7 Lakes Alliance, or to get involved in volunteer lake monitoring, please contact the Lake Science Director, Dr. Danielle Wain, at firstname.lastname@example.org.
Because Messalonskee is deep, it stratified early and remained stratified until fall turnover in later October. The water clarity was good throughout the summer at both sampling locations. At the shallow site (right panels) at the north end of the lake, anoxia developed early in the summer and remained until the water column started mixing in September. At the deepest hole (left panels), anoxia began later, but persisted longer. In the middle of the summer, a metalimnetic oxygen minimum formed in the thermocline. This typically occurs because warm water is less dense than cold water, so there can be a sharp density gradient at the thermocline. When organic matter (like dead algae) settles in the water column, it can be trapped on thermocline, where it is degraded by microbes, a process which consumes oxygen.
Great Pond is also deep, so it stratified early and stayed stratified into October. Anoxia started almost immediately after the onset of stratification. Because the deep holes (where the sampling locations are) in Great Pond are funnel shaped, the portion of the water column that is anoxic grows quickly. But the water clarity remained good throughout the summer, indicating that, despite this, there were no blooms.
Like the other deep lakes, Long Pond stratified early and stayed stratified until the end of October. The shallower upper basin (left panels) also goes anoxic below the thermocline for much of the summer. The deeper lower basin (right panels) goes anoxic, but in much less of the water column. These oxygen graphs show how interconnected the lakes are. Water comes into Long Pond from Great Pond and organic matter is deposited in the deep hole in the upper basin. As this organic matter decomposes, it uses up oxygen near the bottom. In the lower basin, while we have some anoxia near the bottom, more striking is the metalimnetic oxygen minimum. As in Messalonskee, this is due to degradation of organic matter in the thermocline, but in this case the organic matter likely comes from the upper basin of Long Pond. The water clarity was good through the summer on both basins of Long Pond.
North Pond is the shallowest of the Belgrades and had a bloom for much of the summer, for the first time in many years. The water clarity in North Pond was low from the beginning of the summer. Generally, North Pond remains well mixed through the summer. But because of the lack of clarity, sunlight could not penetrate to the bottom and was absorbed in the upper portion of the water column, leading to stratification, particularly in August when it was hot and calm. This led to anoxia near the bottom, and consequent release of phosphorus, which was then mixed into the water column, fueling the bloom even more.
East Pond has long suffered algal blooms. Last summer, an alum treatment was conducted on the lake to reduce phosphorus in the water column and bind phosphorus in the sediments. The first phase of the treatment was in June. After the treatment, East Pond remained clearer than ever for the entire summer. Because the water was so clear and sunlight penetrated to the bottom, the lake did not stratify and remained well mixed and oxygenated through the summer.
McGrath is another one of the shallow Belgrade lakes. The lake only weakly stratified when the weather was hot and calm in August, but the water clarity and oxygen remained at good levels throughout the summer.McGrath is another one of the shallow Belgrade lakes. The lake only weakly stratified when the weather was hot and calm in August, but the water clarity and oxygen remained at good levels throughout the summer.
While not a large lake, Salmon Lake is still relatively deep. The lake stratified in June and remained stratified until October and was anoxic below the thermocline during this time. The phosphorus released from the sediments during this time remained trapped deep where algae could not use it, thus the water clarity remained good until September. Wind mixing in the late summer and early autumn can erode the thermocline, mixing deep water and its nutrients up to the surface where it can be used by algae. Residents reported a bloom on the lake in October after fall turnover, as the rest of the nutrients in the deep water were mixed up to the surface.