Analysis of Inorganic Carbon and pH in the Western Arm of Lake Superior

Abstract

Although Lake Superior is Earth’s largest lake by surface area, few studies have outlined its inorganic carbon cycle, including whether the lake is net heterotrophic (a source of CO2 to the atmosphere) or autotrophic (a sink of atmospheric CO2). Comparing the lake surface water pCO2 values to atmospheric pCO2 allows for the determination of the balance between primary production and respiration within the lake. This study expands upon previous studies by applying improved spectrophotometric pH measurement methods along with total inorganic carbon and alkalinity measurements to calculate lake surface pCO2 and overall air-water differences in pCO2 (ΔpCO2) at a greater number of sampling locations and collection times in the western arm of Lake Superior. The pH measurements in this study also provide a well-characterized timepoint for determining long term trends in lake acidity as a function of climate change. Western Lake Superior (“the western arm”) was determined to be net heterotrophic in spring and approximately neutral in summer, before again turning net heterotrophic in the fall. Surface water pCO2 values were found to be higher on average and more variable overall in the 2014 sampling season (relative to 2015) perhaps because of the harsher preceding winter. On a seasonal basis, measured pH values followed an antiphase relationship to pCO2 with the lowest values occurring in the spring and increasing throughout the summer with a higher overall average occurring after the milder 2014-15 winter. Our data indicates that Lake Superior may provide a negative feedback loop buffering changes in local climate. This is due, in part, to the strong iii dominance of heterotrophy that is observed after cold winters which delay the start of thermal stratification, ultimately outgassing more CO2.