Seasonal lake metabolism and its consequences for long-term organic carbon cycling in lakes
Lakes and reservoirs play important roles in the landscape as transport, storage, and mineralization sites for organic carbon (OC). The fate of external, or allochthonous, loads to lakes depends on both the recalcitrant nature of the load as well as the physical-biological characteristics of the lake. Primary producers in lakes also continually fix carbon through photosynthesis, and the magnitude of this autochthonous load depends on both the external sources of limiting nutrients, such as phosphorus, and the physical-biological characteristics of the lake. Combined, these processes exhibit complex dynamics that are manifest in multiple scales of variability, in which the dominance of drivers and the balance of OC budgets changes from days to decades.
Most studies of carbon dynamics in lakes are conducted at seasonal or shorter time scales, creating a large gap between what we observe and model for process studies and our observations of lakes are long-term, cold storage sites of OC. The aim of this research is to close the gap between short-term metabolism studies and long-term lake carbon balance through simulation modeling of north temperate lakes. Specifically, we asked: how does metabolism contribute to physical-chemical characteristics that influence net ecosystem productivity and the fate of OC in lakes? Can we detect the slow and persistent accumulation of OC in lakes as part of its metabolic balance? How do lake characteristics influence our ability to connect short-term and the long-term OC cycling in lakes?
Simulations across gradients of hydrologic residence time, nutrient load, and lake trophic state demonstrate the challenges of connecting seasonal metabolism with long-term OC processing. We observed a total phosphorus (TP) threshold concentration above which summer-time net ecosystem production (NEP) was positive, indicating that lakes produced more OC than they mineralized under these conditions. At very high TP concentrations, blooms of phytoplankton may produce OC that is not accurately accounted for by process models and that may contribute to long-term carbon burial. Seasonal anoxia as a consequence of eutrophication may enhance OC storage through permanent burial. Finally, long-term OC burial rates lie within the uncertainty of seasonal NEP estimates, suggesting that seasonal metabolism may be an unreliable predictor of long-term OC burial rates.