Effect of chromophoric dissolved organic matter on epilimnetic stratification in lakes

Abstract

Empirical studies on lake thermal regimes have shown that stratification is correlated with the concentration of dissolved organic matter (DOM). Thermal stratification has implications for photosynthesis, photoinhibition, photobleaching and carbon cycling. The objectives of this study were to quantify the role of chromophoric DOM (CDOM) on epilimnetic thermal structure and to estimate the contribution of ultraviolet (UV), visible and infrared radiation (IR) to the near-surface warming of lakes as a function of their DOM content. A simplified thermal model was applied, assuming all energy absorbed by the system is converted to heat and excluding mixing and heat losses. For the upper 0.5 m of the water column, the model generated a temperature increase of 1.5 °C after one hour of noon irradiance in a strongly humic lake, as compared to 1 °C in pure water. UV, visible light and IR contributed 6, 32 and 62% of warming in the humic lake, compared to 0, 5 and 95% in pure water. As CDOM absorption coefficient increases, UVA and especially visible light play an increasing role in stratification. Field data also support the significant role of CDOM absorption on near-surface thermocline occurrence. Near-surface stratification was a common feature during summer in a lake with a moderate DOC content (4.8 mg L−1) and color (a320 = 16.4 m−1). Based on a temperature-stability criterion of 0.2 °C m−1, this lake had a stable epilimnion 64% of the time, compared to 53% for a lake that had much lower DOC concentration (1.8 mg L−1) and color (3 m−1).