Hydrologic Regulation of Lake Carbon Cycling in Both Time and Space

Lakes are areas of intense biogeochemical processing. There is approximately an order of magnitude more organic carbon stored in lake sediments than in the remainder of the terrestrial biosphere (e.g. land plants, soils), and current global estimates of lake CO₂ emissions offsets nearly 20% of carbon sequestered by terrestrial land plants on an annual basis. Clearly, lakes play an integral role in global carbon cycling; however, quantification of regional and global process rates is based upon crude scaling of site-specific rates to regional and global lake area coverage. These types of scaling processes disregard the complex interactions of hydrologic setting, climate, land cover, and geomorphology and are not suitable for making projections in an altered climate. My dissertation research addressed the role of hydrologic behavior in regulating lake carbon cycling both within lakes through time and across lakes on a landscape.

My dissertation demonstrated the important role of lake hydrologic characteristics play in determining the fraction of carbon received from the watershed that is processed within the lake. Using cross-system surveys, I found that seepage lakes (evaporation dominates hydrologic export) process a greater fraction of carbon received from the watershed than drainage lakes (fluvial transport dominates hydrologic export) for a given hydrologic residence time. I used a whole-ecosystem manipulation experiment to switch a seepage lake to a drainage lake and confirmed spatial patterns of carbon processing between seepage and drainage lakes. I also demonstrated that lake carbon cycling responds strongly to natural perturbations of hydrology as extreme precipitation events induced high carbon loading to lakes, enhanced lake heterotrophy, and elevated the mineralization rate of dissolved organic carbon (DOC). I used data assimilation techniques to demonstrate that variability in terrestrial DOC reactivity coupled with high rates of DOC discharge from the watershed leads to elevated mineralization rates of DOC following precipitation events. Finally, I incorporated these important hydrologic characteristics into a large-scale lake carbon model and demonstrated that the fraction of evaporation as hydrologic export explains a majority of the variability in lake carbon fluxes across thousands of lakes on a landscape.