Underway p CO 2 Surveys Unravel CO 2 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. Carbon dioxide gas concentrations were measured in surface waters of Lake Superior for 4 years, forming the first multi‐year data set of direct observations of carbon dioxide gas concentration in the Laurentian Great Lakes. Lake Superior's surface carbon dioxide concentration was closely tied during the ice‐free season to that of the atmosphere on time scales longer than 1 year. Seasonal variations in carbon dioxide concentration were driven by water temperature, biological activity, river influence, and gas exchange with the atmosphere. Lake Superior released and absorbed carbon dioxide cyclically at different times during the year, absorbing more than it released from April to November. The carbon dioxide gas concentration in mixing surface waters was observed to have increased over a multi‐decade period with that of the atmosphere. This work improves the scientific understanding of carbon cycling in Lake Superior and advances techniques for carbon cycle observation and modeling of other lakes in a time of large‐scale changes to surface water CO 2 cycling. Underway p CO 2 was measured in Lake Superior from 2019 to 2022 to form the first multi‐year p CO 2 time series in the Laurentian Great Lakes The seasonal p CO 2 cycle showed competition of