Carbon dioxide fluxes of Arctic coastal ecosystems controlled by seasonal patterns of land‐to‐ocean connectivity

Abstract

The strength of coastal Arctic Ocean CO2 uptake is vulnerable to landscape‐scale changes such as hydrological intensification, changing climate, and alterations to terrestrial and aquatic biogeochemistry. Across a period of 4 yr (2019–2023) and three distinct sampling periods, we visited five coastal ecosystems of the Beaufort Sea with varying barrier island coverage to understand drivers of Arctic coastal CO2 flux. The ice cover sampling period was characterized by the highest pCO2 saturation and dissolved O2 undersaturation. We observed a > 100 μatm difference in pCO2 over shallow depths (up to 2 m) at 73% of ice‐cover site visits. Notably, the geomorphology of barrier islands and channels controlled the flushing of colder Beaufort Sea waters across the systems and influenced the strength and appearance of both vertical thermohaline and pCO2 stratification. The ice breakup period reflected spring freshet and was a net CO2 sink during sampling, likely related to freshwater riverine dilution, CaCO3 dissolution from sea ice melt, and water column algal activity. During the open water sampling period, the interaction of marine and terrestrial contributions predicted the strength of the CO2 efflux, with freshwater inputs introducing higher temperatures and organic material, which increased remineralization. The capacity of these systems to act as CO2 sources or sinks varies throughout the year and is largely driven by geomorphic conditions. Any spatially integrative studies of CO2 flux or coastal productivity should consider the physical and biogeochemical heterogeneity of Arctic coastal ecosystems.