Alkalinity, pH, and pCO₂ in the Laurentian Great Lakes: An initial view of seasonal and inter-annual trends

Ongoing human perturbations to the global inorganic carbon cycle can cause various changes in the pH and alkalinity of aquatic systems. Here seasonal and inter-annual trends in these variables were investigated in the five Laurentian Great Lakes using data from the U.S. EPA GLENDA database. These observations, along with temperature, were also used to predict the partial pressure of carbon dioxide in surface water (pCO₂). There are strong seasonal differences in pH in all five lakes, with higher pH levels in summer than in spring. All lakes show significantly higher pCO₂ values in spring than in summer. Michigan and Ontario show higher alkalinity values in spring; Huron shows lower spring values. Inter-annually, open-lake pH shows the highest values in all lakes around 2010, the time frame of lowest lake water levels, though only lakes Superior and Erie show statistically significant inflection points at that time. Inter-annual alkalinity trends differ considerably from those of pH. Superior's alkalinity increases until ∼2008 and then begins dropping; Ontario's alkalinity decreases until ∼2004 and then begins increasing, with the decrease coinciding with the introduction and establishment of Dreissenid mussels. The other lakes show much less clear inter-annual alkalinity trends. For pCO₂, inter-annual trends in each lake show either increases from 1992 to 2019 (for Superior, Michigan, and Huron) or shifts from slightly decreasing values to increasing values for the other lakes. The timing of this shift is from 2008 to 2010.