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

Abstract

Coral-reef sediment vegetated by seagrasses is usually enriched in organic matter as compared with unvegetated reef sediment. This study focused on comparisons of sediment composition and mobility between vegetated and unvegetated sediments of Green Island reef (the Great Barrier Reef, Australia). Seagrass-derived macroscopic debris and amorphous detrital organic matter were especially responsible for the organic enrichment in the seagrass-bed sediment, while carbonate-associated, acid-soluble organic matter was only slightly enriched in seagrass beds. A sediment-trap experiment revealed that particles trapped in the seagrass beds were richer in organic carbon than those collected in the unvegetated area, although the organic carbon flux depended on wind condition rather than vegetation. The trapped organic particles 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 seagrass-derived, macroscopic organic fragments. Conservation of N and P during bacterial decomposition of plant-derived organic matter is probably responsible for the enrichment of N and P. The characteristic time scale of the amorphous organic matter turnover by resuspension and deposition was estimated to be tens of days or less, being comparable to or faster than mineralization. These comparisons suggest that seagrass beds effectively enhance sedimentary storage of organic C N and P with the turnover of the organic pools being controlled by physical processes as well as by mineralization.