Nutrient, Environmental data and Fluxes of Gradient Sampler in Curacao

Coral reefs act as biogeochemical hotspots that regulate nutrient cycling and fluxes in oligotrophic waters, yet quantifying these fluxes in-situ remains challenging. Here we present high-resolution measurements of benthic inorganic nutrient and carbon dynamics, together with prokaryotic abundances, at two Caribbean reef sites with different coral cover. Fluxes were quantified using a gradient sampler coupled with a current sensor to characterize benthic boundary layer hydrodynamics. At the site with 20% coral cover, we observed clear diel cycles in nutrient fluxes closely aligned with reef metabolism: daytime photosynthesis and calcification transitioned to nighttime respiration, producing near-zero net fluxes that suggest efficient nutrient recycling. Mid-day exometabolomic profiles showed a high Gibbs free energy per carbon, reflecting rapid release of reduced organic molecules by benthic primary producers. In contrast, the degraded site (1% coral cover) exhibited no detectable diel cycling and a sustained net release of nutrients from the reef, indicating decoupled production and consumption, with no corresponding changes in Gibbs free energy per carbon. Fluxes at both sites were modulated by tides and current speed, with the highest rates during ebb and flood phases. These results demonstrate that hydrodynamics and benthic cover jointly shape nutrient and carbon cycling, with implications for predicting reef resilience under future change.