Habitat-specific metabolism, dissolved organic carbon fluxes, and composition in a tropical seagrass meadow

Seagrass ecosystems are vital hotspots of coastal carbon cycling, yet their role in regulating the quantity, quality, and bioavailability of dissolved organic matter (DOM) and dissolved organic carbon (DOC) fluxes remains poorly understood, particularly in tropical regions. This study investigates the coupling between benthic metabolism, DOM composition and characteristics, and DOC fluxes in a tropical seagrass meadow and adjacent bare sediments using ex-situ incubations combined with chromophoric and fluorescent DOM analyses. Our results demonstrate that seagrass habitats had significantly higher metabolic activity than bare sediments. Specifically, seagrass habitats functioned as net autotrophic systems with a net community productivity (NCP) of 29 ± 1 mmol m -2 d -1 , whereas bare sediments were net heterotrophic with an NCP of -5.8 ± 1.6 mmol m -2 d -1 ). Benthic DOC fluxes were also significantly greater in seagrass habitats (626 ± 123 μmol m -2 h -1 ) than in bare sediments (63 ± 88 μmol m -2 h -1 ) ( p < 0.05). Optical revealed that seagrass habitats had higher chromophoric DOM and lower specific UV absorbance than bare sediments. Moreover, fluorescence DOM results suggested that seagrass-derived DOM was fresh, protein-like, and bioavailable, whereas bare sediments contributed more degraded, and recalcitrant DOM. These findings provide new insights into the biogeochemical significance of seagrass meadows, demonstrating that they function not only as carbon sinks but as critical sources of bioavailable organic matter. • Seagrass habitats function as net autotrophic systems and dominate ecosystem metabolism. • Seagrass habitats act as a source of DOC, representing 51% of net community production. • Seagrass-derived DOM is fresh, bioavailable and dominated by protein-like components.