Fatty acids as tracers of organic matter in the sediment and food web of a mangrove/intertidal flat ecosystem, Okinawa, Japan

Abstract

Surface sediments and marine invertebrates, collected from 2 intertidal flats on Okinawa Island, 1 adjacent to a mangrove system, were analysed for fatty acid composition. The detection of fatty acid markers found in mangrove leaves in the organic matter of the surface sediments, coupled with measurements of C:N ratios, showed that organic matter from the mangrove forest (in Oura Bay) is exported to the intertidal flat in both the rainy season and the dry season. This export seems to be higher in the rainy season. However, bacteria, diatoms and macroalgae were the main food source in the surface sediments, as shown by the contribution of their respective fatty acid markers. These markers were also detected in the tissues of the dominant macrozoobenthic species, fiddler crabs and gastropods. Bacteria and green macroalgae were the primary food sources ingested at both sites, irrespective of season. The organic matter derived from mangroves was also ingested by the macrozoobenthos of Oura Bay, while markers of higher plants were not found in the tissues of invertebrate species at Itoman intertidal flat, the site that was not adjacent to a mangrove system.