Dissolved Oxygen
A common problem in Maine lakes is the depletion of dissolved oxygen, which occurs in the deepest waters during the summer months. Dissolved oxygen is essential to aquatic life, both directly for respiration, and indirectly as depleted levels can spur the release of sediment-bound phosphorous, which stimulates the growth of algae.
Historically, both Upper and Middle Bays have experienced some oxygen depletion below the thermocline (boundary of sharp temperature change). The ponds show much more depletion of oxygen in the summer due to their much smaller volume below the thermocline barrier.
Spring and Fall Turnover
Lake water tends to stratify according to density with less dense water rising over more dense water. Surprisingly, water is most dense when it’s at 39.2° F (4°C) not when it’s frozen at 32°F (0°C). This is why ice floats. As shown below, winter water that’s 33.8°F (1°C) is less dense and therefore rises above water that’s 39.2°F (4°C) creating a thermocline which is a line of sharp demarcation in water temperature and density.
In spring when ice is out winds and warming start a “mixing" process. When surface waters reach their maximum density of 39.2°F (4°C), density is uniform throughout a lake and well-oxygenated surface water circulates freely. This “turnover” is an important phenomenon as it mixes oxygenated surface waters with oxygen-depleted deeper waters. Renewed levels of oxygen allows fish to thrive at all depths.
As the surface waters warm during the summer a thermocline appears again, this time with warmer waters circulating above it and cooler, denser waters remaining below. Because the deep waters are not mixing with surface waters, they tend to become oxygen depleted as the summer progresses.
In the fall as surface water cools, it sinks below the thermocline and the lake returns to a uniform temperature. Wind and cold can extend the length of the overturn to several months. Finally, as temperatures drop, a reverse of the summer condition occurs: the densest water remains at the bottom while the cooler water and ice float above.
Dissolved Oxygen in Horseshoe Pond
A typical over summer pattern of dissolved oxygen in Horseshoe Pond is shown in the diagram below. It is very similar to many lakes where the water volume below the thermocline is not large. Kezar lake is the only lake in the watershed with enough volume such that this steep depletion does not occur to an appreciable degree. The May profile shows nearly uniform distribution of dissolved oxygen due to the spring turnover. As the water stratifies in the summer, the oxygen gets depleted at depths below the thermocline (at about six meters depth). By late August there is almost no oxygen below about nine meters. When the fall turnover happens in October, the oxygen becomes uniform again as the thermocline disappears and the water mixes.
Because of this lack of oxygen at cool depths in late summer, Horseshoe Pond cannot support fish species that need cool water and enough oxygen to survive. Land Locked Salmon and Lake Trout, or Togue, are two such species that cannot live in lakes like Horseshoe Pond.
The interesting bump up of dissolved oxygen at the thermocline is the result of algae that tends to populate the thermocline area and generates oxygen, super-saturating the water at that level as a result of sunshine. The readings were taken right around noon on cloudless days with maximum illumination thus maximum photosynthesis.
Dissolved Oxygen in Kezar Lake
Buoy with suspended oxygen loggers that record data over a period of several months.
The concentration of dissolved oxygen in our lake is a critical factor in sustaining aquatic life, from fish to zooplankton and bacteria. The oxygen requirements of cold water fish vary from species to species, with trout requiring seven ppm, just a bit more than salmon, while pickerel require just two ppm.
KLWA monitors dissolved oxygen, and the 2019 data for both temperature and oxygen can be seen below. The abrupt shifts in temperature at the thermoclines can be seen in Upper and Middle Bays, while the shallowness of Lower Bay prevents such abrupt stratification. Dissolved oxygen levels can be seen as reaching their lowest levels in August, in Lower and Middle Bay, and September in Upper Bay, as it is the deepest basin of the three.
Fortunately in 2019, the dissolved oxygen levels did not dip below the critical threshold of five ppm, although this has occurred historically. KLWA continues to track these parameters as we build a historic profile of data, which will help to identify and gain perspective on any future changes due to human disturbance and/or a changing climate.