A Seasonal to Interannual View of Inorganic and Organic Carbon and pH in Western Lake Superior

To investigate the carbon cycle of Lake Superior, Earth's largest freshwater lake by surface area, we performed total organic carbon, dissolved organic carbon, chlorophyll, spectrophotometric pH, total inorganic carbon, and alkalinity measurements on seasonal samples from the western lake. The last three parameters, along with in situ temperature, were used to calculate the partial pressure of carbon dioxide in surface water (pCO_2(w)) using the CO2SYS algorithm. There was a strong positive correlation between pH and water temperature and a weaker but significant positive correlation between pH and chlorophyll concentration. Total organic carbon exhibited higher nearshore concentrations (as determined by a negative correlation with total water column depth); such a spatial relationship did not appear in the inorganic carbon parameters (total inorganic carbon, pH, or pCO_2(w)). Western Lake Superior exhibited net outgassing in spring, little net gas transfer in summer, and some outgassing in the fall. The pCO_2(w) values were negatively correlated with both water temperature and chlorophyll concentration. Seasonal differences in pCO_2(w) in Lake Superior appeared more strongly driven by biology and terrestrial inputs as compared to direct effects of temperature on CO₂ solubility. Interannual data from the long-term Great Lakes Environmental Database data set indicated that lake alkalinity has been increasing over the past 20 years, and lake surface water pH appeared relatively stable. Modeling pH change over the same time frame in CO2SYS shows that increases in alkalinity and lake surface water temperature counteract the increase in atmospheric carbon dioxide concentration, leading to a relatively constant pH, consistent with observational data.