Quantifying farmed kelp atmospheric CO₂ uptake through localized air-sea flux 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 three kelp farms across 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 control upstream of the farm. Both sensor arrays conducted hourly measurements of pH or CO₂, temperature, salinity, and oxygen during the time from seed line outplanting in winter (November to January) to spring harvest (April or May) in 2024. Nominal differences in carbonate chemistry parameters were detected between the inside and outside moorings until March, when the frequency of variability remained consistent between moorings but their respective magnitude diverged. Notably, apparent oxygen production, seawater CO₂ concentration, air-sea CO₂ flux, and strength of periodic signals varied by farm site. Integrated over the entire deployment, two farms demonstrated net negative air-sea CO₂ fluxes while one served as a net source of carbon: −84,397 mol m^-2 in Jakolof Bay, −11,115 mol m^-2 in Kalsin Bay, and 543 mol m^-2 in Windy Bay. This study highlights the nuance of farmed kelp carbon capture by demonstrating that farm site can influence overall air-sea CO₂ flux and that kelp farms are not always a net sink for atmospheric carbon.