Permafrost also contributes to cold water runoff when it begins to thaw in June or July, and its meltwater seeps into the river. In the spring, snowmelt running into rivers reduces the water temperature to below the ambient air temperature. Water temperature fluctuates between day and night (diurnal temperature changes) and over longer time periods (e.g., seasonally). The most important source of heat for fresh water is generally the sun, although temperature can also be affected by the temperature of water inputs (such as precipitation, surface runoff, groundwater, and water from upstream tributaries), heat exchanges with the air, and heat lost or gained by evaporation or condensation. Changes in water temperature may act as a signal for aquatic insects to emerge or for fish to spawn.

Environmental cues for life-history stages.

Differences in water temperature and density between layers of water in a lake leads to stratification and seasonal turnover. Many aquatic species can survive only within a limited temperature range.

Species composition of the aquatic ecosystem.

Temperature affects metabolism, growth, and reproduction. In general, the rate of chemical reactions increases with increasing water temperature. Temperature affects the solubility and reaction rates of chemicals. The solubility of oxygen decreases as water temperature increases. Water temperature is one of the most important characteristics of an aquatic system, affecting: 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.Water Quality Indicators: Temperature and Dissolved Oxygen 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. 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.Ī typical over summer pattern of dissolved oxygen in Horseshoe Pond is shown in the diagram below. In the fall as surface water cools, it sinks below the thermocline and the lake returns to a uniform temperature. Because the deep waters are not mixing with surface waters, they tend to become oxygen depleted as the summer progresses. Renewed levels of oxygen allows fish to thrive at all depths.Īs 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. This “turnover” is an important phenomenon as it mixes oxygenated surface waters with oxygen-depleted deeper waters. In spring when ice is out winds and warming start a “mixing' process. When surface waters reach their maximum density of 39.2☏ (4☌), density is uniform throughout a lake and well-oxygenated surface water circulates freely. As shown below, winter water that’s 33.8☏ (1☌) is less dense and therefore rises above water that’s 39.2☏ (4☌) creating a thermocline which is a line of sharp demarcation in water temperature and density. Surprisingly, water is most dense when it’s at 39.2° F (4☌) not when it’s frozen at 32☏ (0☌). Lake water tends to stratify according to density with less dense water rising over more dense water. The ponds show much more depletion of oxygen in the summer due to their much smaller volume below the thermocline barrier. Historically, both Upper and Middle Bays have experienced some oxygen depletion below the thermocline (boundary of sharp temperature change).

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.

A common problem in Maine lakes is the depletion of dissolved oxygen, which occurs in the deepest waters during the summer months.

Lake turnover and water quality