The fate of organic carbon pulses from watersheds to lakes in northern Wisconsin

Background/Question/Methods

Landscape setting, morphometry, hydrology, and nutrient load are factors that may determine how lakes process pulses of organic carbon (OC) from the watershed. For example, small bog lakes in northern Wisconsin are sheltered from wind, tend to have poor light penetration, strong stratification, and short residence times. Whereas large, drainage lakes tend to have significant light penetration and deep surface mixed layers, as well as longer residence times. How do these lake characteristics determine the fate of OC pulses from watersheds? Answering this question is difficult because OC base loads are not easily quantified and OC pulses tend to be coincident with weather patterns that confound our measurements of lake variables. To address this question, we simulate OC loading to lakes that cover broad gradients in size and trophic status. We calibrate a one-dimensional lake physical and water quality model (DYRESM-CAEDYM) to observations from lakes in northern Wisconsin. Base loads were determined through calibration and pulses were simulated as short-term increases to base loads. Using the lake model, we studied the affects of OC pulses on lake physical, chemical and biological variables and determined the fate of the OC pulse as changes in lake pools and key ecosystem fluxes.

Results/Conclusions

We found that relatively large pulses (10x base load, sustained for 5 days) had more pronounced affect on chemical and biological variables in smaller lakes than in larger lakes. Physical variables, such as thermal stratification and light regime were affected in more subtle ways in all lakes. The manifestation of the pulse, in terms of changes in DIC and CO2 flux to the atmosphere, depended on weather patterns following the pulse in smaller lakes, as weather alters mixing and entrainment of hypolimnetic waters. In some cases, responses were delayed until weather had driven mixing events that entrained metalimnetic waters. The magnitude and timing of the response to OC pulse also depended on the delivery of the pulse and the nature of the OC. Loads delivered by cold water or loads high in particulate matter plunged into hypolimnetic waters. In dystrophic lakes, the dark, cold environment of the hypolimnion resulted in low OC mineralization rates. Overall, easily measured lake variables other than DOC responded subtly to moderate loads, suggesting that they may be difficult to detect given background noise driven by weather variability.