Export and dispersal of coastal macrophyte-derived organic matter to deep offshore sediment around the Tokara and Yaeyama Islands, southwest Japan: Evaluation using quantitative DNA probing techniques

Abstract

Vegetated coastal habitats such as seagrass meadows, macroalgal beds, and mangroves export large amounts of organic carbon (OC) to offshore regions. The exported OC may be consumed as a food source by various pelagic and benthic organisms, enhancing secondary production, or may settle and be buried in offshore sediment, contributing to carbon sequestration. Hence, OC export is an important ecosystem service of coastal wetlands supporting connectivity from coastal to offshore habitats. We studied the dispersal of detrital organic matter derived from coastal macrophytes to offshore sediment around the Tokara and Yaeyama Islands (subtropical western North Pacific) using DNA probing techniques and compared the results with the bulk OC concentration and granulometric properties of the sediment. The results showed that dispersal of macrophyte detritus was constrained hydrographically by the strong Kuroshio current flowing near the study areas. Mangrove-and seagrass-derived organic matter exported from coastal habitats of the Yaeyama Islands was detected around the Yaeyama Islands and accumulated in deep-basin sediments (>1000 m) of the southern Okinawa Trough; however, its dispersal seemed to be confined by the Kuroshio. By contrast, sediments around the Tokara Islands often contained macroalgal materials that were rarely found in those around Yaeyama Islands. Most of the macroalgal organic matter found in the sediments of the Tokara area likely originated in coastal habitats of continental China and was transported by the Kuroshio across the shelf of the East China Sea to the northern Okinawa Trough, where it was trapped within sediment. The bulk OC concentrations in the sediment of both areas were constrained by the granulometric properties of the sediment, such as specific surface area. However, the abundance of macrophyte-derived organic matter did not correlate with the concentration or stable isotope ratio of the bulk-sediment OC, implying that macrophyte OC represents a minor fraction of the bulk OC stored in the sediment of the study areas.