Can spatial heterogeneity explain the perceived imbalance in Lake Superior's carbon budget? A model study

Abstract

Lake Superior is the largest lake in the world by surface area, containing 10% of the world's surface freshwater. Yet, little is known about its role within the regional carbon budget. Observational studies on Lake Superior have been limited by harsh winters and the challenges of covering such a vast expanse. To date, carbon budgets extrapolated from observational studies are largely out of balance and suggest a large efflux of carbon dioxide to the atmosphere (∼3 TgC/yr) that cannot be supported by the estimated net inputs into the lake (<1 TgC/yr). We couple a hydrodynamic model of Lake Superior to an ecosystem model to understand the seasonal cycle of the partial pressure of carbon dioxide (pCO2) in the lake surface waters, the resulting air‐lake carbon dioxide (CO2) fluxes, and whether spatial heterogeneity can explain the previously imbalanced carbon budget. The model sufficiently simulates lake productivity, circulation, respiration, pCO2, and chlorophyll. We find that the seasonal cycle of pCO2is generally a double sinusoidal curve during the simulated period of 1996–2001. The lake acts as a sink of carbon dioxide in summer and during late winter of cold years and as a source to the atmosphere during winter and spring. We find significant spatial heterogeneity of respiration in Lake Superior, with near‐shore to offshore rates of respiration varying by two orders of magnitude. Thus, Lake Superior need not act as a significant source of carbon dioxide (∼0.5 TgC/yr) to the atmosphere in order to be consistent with in situ observations of respiration.