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

Abstract

This study observed seasonal trends and inferred drivers of CO2 biogeochemistry at the air‐water interface of Lake Superior. Underway carbon dioxide partial pressure (pCO2) was measured in surface water during 69 transects spanning ice free seasons of 2019‐2022. These data comprise the first multiannual pCO2 time series in the Laurentian Great Lakes. Surface water pCO2 was closely tied to increasing atmospheric pCO2 over a 100‐day CO2 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 pCO2 increased with two decades of rising atmospheric CO2. Spatial heterogeneity in CO2 dynamics was highlighted by a conductivity‐based delineation of “riverine” and “pelagic” regimes, each of which was associated with net CO2 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 CO2 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.