Quantifying farmed kelp atmospheric CO ₂ uptake and release through localized air-sea flux measurements in the Northern Gulf of Alaska

Abstract. The rapid growth of mariculture in the United States, particularly in Alaska, has ignited interest in the co-benefit of using farmed kelp as a mitigation strategy against anthropogenic carbon dioxide (CO2) released to the atmosphere. Here, we quantified the air-sea CO2 flux in two kelp farms in the Northern Gulf of Alaska with differing oceanographic conditions and farming practices to determine the carbon sequestration potential over the growing season. Sensors were deployed on two subsurface moorings placed in proximity of one another at each farm site: one “inside” and one “outside” as a reference upstream of the farm. Both sensor arrays conducted hourly measurements of pH or partial pressure of CO2 (pCO2), temperature, salinity, and oxygen during the time from seed line outplanting in winter (November to January) to spring harvest (April or May) in 2024. In Windy Bay, nominal differences in carbonate chemistry parameters were detected between the inside and outside moorings from November until March, at which time seawaterpCO2 within the farm decreased with respect to the reference site. In Kalsin Bay, the inside mooring displayed winter seawater pCO2 values 100 µatm greater than those recorded at the outside mooring, likely due to the colonization of a biofilm on the kelp lines while young sporophytes were not yet mature; however, seawater pCO2 values at both moorings converged in spring. Integrated over the entire deployment, one farm strengthened the bay's carbon influx and one farm reversed the natural sink by releasing carbon over the deployment period: 800.1 ± 145.8 mol m−2 in Kalsin Bay and -9.23±3.6 mol m−2 in Windy Bay. This study highlights the nuance of farmed kelp carbon capture by demonstrating that a farm site can influence overall air-sea CO2 flux, whereby farming structures create artificial habitat and that kelp farms are not always a net sink for atmospheric carbon.