Light and tidal inundation and exposure regulate the sensitivity of estuarine benthic greenhouse gas fluxes to warming and ocean acidification

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Abstract Coastal sediments are globally significant sources and sinks of greenhouse gases (GHGs), yet their contributions to climate feedbacks of warming and ocean acidification remain uncertain, in part due to limited understanding of short‐term variability. Here, we use a fully factorial laboratory experiment to disentangle how diel light–dark and tidal inundation and exposure interact with warming and elevated p CO 2 to regulate benthic fluxes of CO 2 , CH 4 , and N 2 O in estuarine sediments, alongside concurrent changes in benthic oxygen exchange. While warming and p CO 2 exerted strong independent effects, their influence was shaped by diel and tidal fluctuations in redox conditions and oxygen availability, reflecting shifts in metabolic balance between primary production and respiration. Light consistently limited CO 2 , CH 4 , and N 2 O emissions through enhanced autotrophic uptake and oxygenation, while dark promoted anaerobic production pathways. N 2 O showed the greatest sensitivity to the combined effects of climate forcing and redox dynamics. Despite warming‐driven stimulation of benthic heterotrophy and the production of all GHGs, CO 2 remained the dominant greenhouse gas, with minimal CH 4 and N 2 O fluxes due to the limited organic matter availability within the sediment. This reflects the strong redox controls on CH 4 and N 2 O production, which relies on both oxygen depletion and organic substrate supply. Our findings emphasize that fine‐scale temporal variability can significantly shape both the magnitude and climate sensitivity of benthic GHG emissions. Capturing these fine‐scale controls is essential for accurately modeling the contributions of estuarine sediments to global GHG budgets and their feedbacks.

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