POTENTIAL IMPACT OF MIXING AND STRATIFICATION PATTERNS ON CARBON BURIAL IN MACROPHYTE- AND PHYTOPLANKTON-DOMINATED SHALLOW LAKES

Oxygen exposure can have strong effects on carbon burial in lakes and the oceans, and stratification restricts oxygen ventilation in all of these systems. Small, shallow lakes are generally considered to be polymictic, with a relatively homogenous distribution of nutrients, oxygen, and temperature throughout the water column in comparison to stratified lakes. Because the carbon burial rate of shallow lakes is highly variable, the mixing regime and trophic state of shallow lakes may help explain this variability. We present the mixing and stratification patterns of 12 prairie shallow lakes, and contrast these patterns between clear-water, macrophyte-dominated lakes, and turbid-water, phytoplankton dominated lakes. We found the type of dominating primary producer and maximum depth of the lake was important in predicting the stratification patterns. Turbid, “deep” lakes (>2.5 m z_max) generally experienced continual stratification, as the surface waters averaged 1°C warmer temperatures and 58% higher dissolved oxygen concentrations than the water above the sediment. In contrast, shallow (~1 m z_max) turbid lakes were more regularly mixed throughout the summer. Stratification patterns in the clear lakes were less influenced by depth; for example lakes dominated by Chara sp were stratified even in lakes only 1 m deep, and had on average 1°C warmer temperatures and 64% higher dissolved oxygen concentrations in the surface waters than the water above the sediment. The stratified lakes with lower oxygen and somewhat cooler temperatures near the sediment may facilitate higher carbon preservation in the sediment. The constraining stratification patterns among macrophyte- and phytoplankton-dominated lake regimes likely contributes to differences in organic carbon burial in these shallow lakes.