Accumulation and transport of seagrass-derived organic matter in reef flatsediment of Green Island, Great Barrier Reef

Coral-reef sediment vegetated by seagrasses is usually enriched in organic matter as compared with unvegetated reef sediment. This study focused on co mparisons of sediment composition and mobility between vegetated and unvege tated sediments of Green Island reef (the Great Barrier Reef, Australia). S eagrass-derived macroscopic debris and amorphous detrital organic matter we re especially responsible for the organic enrichment in the seagrass-bed se diment, while carbonate-associated, acid-soluble organic matter was only sl ightly enriched in seagrass beds. A sediment-trap experiment revealed that particles trapped in the seagrass beds were richer in organic carbon than t hose collected in the unvegetated area, although the organic carbon flux de pended on wind condition rather than vegetation. The trapped organic partic les had C:N:P ratios similar to the amorphous organic fraction of sediment organic matter, being significantly enriched in N and P as compared with se agrass-derived, macroscopic organic fragments. Conservation of N and P duri ng bacterial decomposition of plant-derived organic matter is probably resp onsible for the enrichment of N and P. The characteristic time scale of the amorphous organic matter turnover by resuspension and deposition was estim ated to be tens of days or less, being comparable to or faster than mineral ization. These comparisons suggest that seagrass beds effectively enhance s edimentary storage of organic C, N and P, with the turnover of the organic pools being controlled by physical processes as well as by mineralization.