Dominant role of benthic photosynthesis for the metabolism of carbonate sands in a subtropical inner shelf

Abstract Carbonate coral sands are an integral part of the carbon and nutrient cycles in subtropical and tropical coastal environments. Recent studies indicate that nearshore carbonate sands may be hotspots for organic matter production and respiration, but the processes and their controls are poorly understood due to a lack of noninvasive in situ measurements. We deployed a new triple‐sensor aquatic eddy covariance instrument to quantify seasonal O 2 ‐fluxes over a coral sand platform at ~ 10 m water depth in the Florida Keys. The noninvasive measurements revealed the strong influences of light and bottom currents on magnitude and dynamics of the benthic metabolism. Light penetration through the clear water column facilitated substantial microphytobenthos production, making the seafloor a source of oxygen during daylight hours. Daytime benthic O 2 release to the water column ranged between 0.6 mmol O 2 m −2 h −1 (winter) and 4.8 mmol O 2 m −2 h −1 (summer), while nighttime sediment O 2 respiration varied between −1.2 mmol O 2 m −2 h −1 (winter) and −3.3 mmol O 2 m −2 h −1 (summer). Bottom currents modulated the fluxes, emphasizing the role of advective pore water exchange for biogeochemical reactions in the highly permeable sediment. Similar magnitudes and dynamics of daytime sediment O 2 release and nighttime O 2 uptake revealed a tight coupling between production and degradation of highly labile photosynthetic products. We use the results to explain why O 2 respiration rates in permeable carbonate sands of oligotrophic subtropical/tropical environments can reach similar magnitudes as those reported from permeable silicate sand beds of nutrient‐rich temperate inner shelves.