Underway pCO₂ Surveys Unravel CO₂ Invasion of Lake Superior From Seasonal Variability

This study observed seasonal trends and inferred drivers of CO 2 biogeochemistry at the air‐water interface of Lake Superior. Underway carbon dioxide partial pressure ( p CO 2 ) was measured in surface water during 69 transects spanning ice free seasons of 2019‐2022. These data comprise the first multiannual p CO 2 time series in the Laurentian Great Lakes. Surface water p CO 2 was closely tied to increasing atmospheric p CO 2 over a 100‐day CO 2 equilibration timescale, while seasonal variability was controlled equally by thermal and non‐thermal drivers during the ice‐free season. Comparison to previous modeling efforts indicated that Lake Superior surface p CO 2 increased with two decades of rising atmospheric CO 2 . Spatial heterogeneity in CO 2 dynamics was highlighted by a conductivity‐based delineation of “riverine” and “pelagic” regimes, each of which was associated with net CO 2 influx over Julian days 100–300 on the order of 25 Gmol C. These findings refine previous estimates of Lake Superior C fluxes, support predictions of anthropogenic CO 2 invasion during the ice‐free season, point to new observation strategies for large lakes, and highlight an urgent need for studies of changes to lacustrine C cycling.