Spatial heterogeneity of GHG dynamics across an estuarine ecosystem

Coastal ecosystems are critical components of the global carbon cycle, exerting a disproportionate influence on the carbon budget despite their limited spatial extent. Shallow coastal ecosystems exhibit strong gradients in physical, biogeochemical, and biological processes. Yet, their effects on carbon cycling and greenhouse gas (GHG) dynamics, including carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), remain inadequately understood. This knowledge gap is compounded by substantial heterogeneity in marine biodiversity, further complicating the issue.

Surface seawater partial pressure of CO₂ (pCO₂), CH₄, and N₂O concentrations, along with seawater physical and biogeochemical properties, and air-sea gas exchange, were measured at 21 sites in southwest Finland (Baltic Sea). Sampling progressed from estuarine inner bays to the outer archipelago, covering diverse soft-sediment habitats, from sheltered to exposed areas, across a salinity gradient. Seawater pCO₂ and N₂O concentrations ranged from undersaturated (160 ppm and 9 nmol L^-1, respectively) to supersaturated (2521 ppm and 25 nmol L^-1, respectively), compared to the atmosphere, resulting in an uptake of -36 and -0.0021 mmol m^-2 d^-1, and a release up to 220 and 0.0383 mmol m^-2 d^-1, respectively. CH₄ concentrations were consistently supersaturated (19 to 469 nmol L^–1) compared to the atmosphere, resulting in a net source to the atmosphere from 0.014 to 1.39 mmol m^–2 d^–1.

Freshwater input and its mixing with seawater shaped the overall spatial patterns of GHGs. However, deviations from this salinity-driven control were seen in sheltered sites within the archipelago, where elevated pCO₂ and CH₄ concentrations likely reflected biological processes, including enhanced organic matter respiration and methanogenesis in warm, late-summer shallow waters, where limited oxidation favored CH₄ accumulation. At exposed and semi-sheltered sites, mixing processes exerted greater control, resulting in lower GHG concentrations. Our results show that both physical mixing and biological processes influence coastal GHG dynamics, with benthic ecosystems potentially playing a key but still poorly constrained role. The overall budget of air–sea GHG exchanges was dominated by CO₂ fluxes, with CH₄ consistently acting as a source, and N₂O alternating between source and sink. High environmental variability in shallow coastal systems leads to strong fluctuations in the balance between GHG production and consumption, which needs to be considered when evaluating their role in the global carbon budget.