Greenhouse Gas Dynamics in Coastal Ecosystems: Insights from the Baltic Sea and Auckland, New Zealand

Coastal ecosystems play a significant role in the cycling of greenhouse gases (GHGs), yet they remain understudied compared to open oceans and terrestrial systems. Here, we present measurements of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) concentrations from shallow coastal environments along the Swedish Baltic Sea coast and Auckland, New Zealand, highlighting the variability and drivers of GHG dynamics across diverse habitats.

In the Baltic Sea, we conducted measurements in April and September 2024, utilizing cavity ring-down spectroscopy coupled with a water equilibration system. Our focus was on shallow coastal bays in the wider Stockholm archipelago, including eutrophic and habitat-altered bays. These environments exhibited exceptionally high CH₄ concentrations in the surface water reaching up to 580 nmol L^-1, suggesting the potential for significant CH₄ emissions. Notably, CH₄ concentrations below 200 nmol L^-1 showed a negative correlation with N₂O, while CH₄ levels above 200 nmol L^-1 revealed a distinct shift to a positive correlation with N₂O. We hypothesize that this transition reflects a change in oxygen availability, where hypoxic conditions (0.2< O₂ < 2 mL L^-1) favor CH₄ production and reoxygenation of euxinic sediments contributes to an additional late-summer N₂O peak. Furthermore, GHG concentrations in the surface seawater were associated with environmental parameters such as water retention time, vegetation coverage, total organic carbon content, turbidity, chlorophyll-a concentration, pH, and total phosphorus levels.

Expanding our investigation to coastal systems in the suburban regions of Auckland, New Zealand, in January 2025 we conducted a spatial survey across a range of coastal habitats, including tidal flats, mangroves and river estuaries. By linking the findings from the Baltic Sea with emerging insights from New Zealand’s coastal systems, we aim to better understand the influence of habitat type, redox conditions, and nutrient dynamics on GHG emissions in coastal zones globally.

Our comparative study underscores the need for integrated approaches to better understand GHG emissions in coastal zones, which are often subject to compounded anthropogenic pressures, such as excessive nutrient inputs and habitat alteration. These findings contribute to the broader understanding of coastal zones as dynamic interfaces in the global carbon and nitrogen cycles and the development of evidence-based policies.