Spatial and Tidal Controls on Greenhouse Gas Dynamics in Temperate Coastal Bays of Northern New Zealand

Coastal bays are dynamic environments where both natural processes and human activities influence greenhouse gas (GHG) cycling. We investigated spatial and temporal variability of surface water CO₂, CH₄, and N₂O across coastal systems in northern New Zealand, spanning a gradient of ecological condition. Spatial surveys in Mahurangi Harbour, the Firth of Thames, the Hauraki Gulf and Auckland Harbour revealed pronounced heterogeneity in GHG distributions. Elevated CO₂ and CH₄ concentrations were consistently observed in upper bay reaches, particularly near mangrove-dominated areas, underscoring the role of tidal wetlands in coastal carbon dynamics. Distinct local hotspots of CH₄ and N₂O were detected in the Firth of Thames, associated with mussel aquaculture, suggesting aquaculture operations may enhance localized emissions. Complementary tidal investigations in Mahurangi and Whangateau Harbours highlighted higher CO₂ and CH₄ concentrations during low tide, linked to mangrove export, tidal pumping, and water-column processing. Notably, the persistence of elevated CO₂ at low tide under fully marine conditions highlights the strong influence of tidal wetlands and benthic processes, even in the absence of a salinity gradient. These measurements also demonstrated significant export of dissolved inorganic carbon (DIC) and alkalinity under fully marine conditions, indicating strong coupling between carbon cycling and exchange with the coastal ocean. To quantify these dynamics, a box-model approach incorporating DIC, total alkalinity, air–sea exchange, and export fluxes was applied to estimate carbon production and transformation. Together, these findings demonstrate how natural habitats and aquaculture activities jointly shape GHG fluxes and provide new insights into the spatial and tidal controls governing emissions in temperate coastal environments.