Controls of lake water color on stratification, mixing, and seasonal water temperatures in small temperate lakes

Background/Question/Methods

Small temperate lakes (< 10 ha) are ubiquitous and are typically rich in dissolved organic carbon (DOC). DOC attenuation of penetrating radiation influences the vertical distribution of water temperatures, mixed layer depths, and heat budgets of small lakes. Future climate predictions indicate likely changes in air temperatures and the timing and magnitude of precipitation, while many globally distributed lakes have shown long-term increases in DOC. These current and predicted trends all have the potential to influence the strength of vertical stratification, but our understanding of the additive physical effects of these three physical drivers is currently limited. We used a mechanistic physical model to examine the impact of variability in DOC concentrations on the physical properties of small temperate lakes. Themodel was validated on eight small (0.06-3.8 ha) lakes in Wisconsin and Michigan.

Results/Conclusions

Attenuation of photosynthetically active radiation (400-700 nm) in the water column was regulated by DOC concentrations, and was important in the vertical structuring of water temperatures. Epilimnetic depths were a function of attenuation, and the vertical partitioning of heat and nocturnal convection were the primary controls on the epilimnion depth. Heat exchange below the surface mixed layer was near the molecular rate, increasing the importance of water clarity as a control on the heat content of deeper waters. We applied scenarios of a 50% increase and 50% decrease to historical measurements of DOC concentrations for one lake (Trout Bog, Wisconsin), and found average seasonal water temperatures to vary in response, with the increased DOC scenario being > 2 °C colder than the reduced scenario. We modeled a range of DOC concentrations (2 mg L-1 to 30 mg L-1), and found clearer (lower DOC) simulations to be more sensitive to climate variability, as the model showed that the range of inter-seasonal water temperatures was greater in decreased DOC simulations during the same simulation periods (1989-2010). Small lakes are globally important regulators of biogeochemical cycles and are structurally different from larger lakes. Important feedbacks to physical processes must be accounted for when understanding the effects of changing DOC and climate on small lakes.