Inorganic carbon cycling in Lake Superior and responses to anthropogenic carbon dioxide

Abstract

The rapid increase of Earth’s atmospheric concentration of carbon dioxide drives myriad changes to marine chemistry and ecology, yet its influence in inland waters is relatively unknown. This research advances the understanding of inorganic carbon cycling in Lake Superior, the largest fresh water body on Earth by surface area, and describes how its carbon biogeochemistry responds to perturbations on seasonal to interannual scales. Improved instrumentation for measuring inorganic carbon parameters in freshwater environments was developed and deployed. These tools enabled analysis of the first multi-year underway time series describing drivers of Lake Superior’s inorganic carbon cycle, demonstrating a sustained rise in surface water carbon dioxide partial pressure (pCO2) at approximately the same rate as the atmospheric increase. Machine learning modeling produced an observation-based model of Lake Superior pCO2 and air-sea CO2 flux spanning the lake surface over 2019-2023, over which period Superior demonstrated periods of sustained net CO2 influx and efflux which nearly balanced on annual scales. This result brought Superior’s carbon budget closer to closure. Finally, aragonite saturation state was proposed as a useful variable to synthesize inorganic carbon cycling trends with calcifying species presence in inland waters in the face of the potential for lake acidification. The findings of these research projects advance the understanding of Lake Superior’s shifting carbon cycle in light of atmospheric CO2 invasion and acidification, with ecologic outcomes dependent on humanity’s choices in the coming decades.