Dynamics of benthic metabolism, O 2 , and pCO 2 in a temperate seagrass meadow

Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a dense Zostera marina meadow, we measured benthic O 2 fluxes by aquatic eddy covariance, water column concentrations of O 2 , and partial pressures of CO 2 (pCO 2 ) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O 2 flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O 2 concentration and pCO 2 , ranging from 173 to 377 μ mol L −1 and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO 2 was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO 2 vs. O 2 concentration was attributed to storage of O 2 and CO 2 in seagrass tissue, air–water exchange of O 2 and CO 2 , and CO 2 storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O 2 and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO 2 and low O 2 concentrations, even though CO 2 reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be “winners” in future oceans with elevated CO 2 concentrations and more frequent temperature extremes.