The source and fate of organic matter and the significance of detrital pathways in a tropical coastal ecosystem

Stable isotope analysis (SIA) and fatty acid profiling were used to elucidate the supply and fate of energy in a tropical coastal ecosystem in Hong Kong, southern China. To investigate seasonal changes in source supply on the diets of different trophic guilds, measurements were taken in three rocky bays before and after the onset of the summer monsoon, when supply of both marine macroalgal and streamborne terrestrial detritus increases. Particulate organic matter, comprising a mixture of marine and terrestrial sources, was the ultimate food source (.60%) for suspension feeders, which were the major prey items of a predatory gastropod (Thais). Increased levels of bacterial fatty acid biomarkers (BaFA) were recorded after the onset of the summer monsoon, indicating amplified dependence on detrital materials in both primary and secondary consumers. The considerable increase in the detritus fraction of sedimentary organic matter at the onset of the summer monsoon was reflected by enhanced levels of BaFA, possibly due to the degeneration of macroalgae. Significant contributions of this marine algal detritus to deposit feeders (Holothuria, 36%) and of terrestrial detritus to an echinoid grazer (Salmacis, 14%) were revealed by SIA mixing models as well as elevated BaFA concentrations. These results indicate a higher dependence on heterotrophic food chains based on decomposing marine algae and terrestrial detritus after the onset of the summer monsoon. Such seasonal variation in the importance of detrital energy sources is, therefore, likely to be important to coastal ecosystem functioning in the monsoonal tropics. Most primary production is not consumed directly, and dead autotrophic material becomes an important energy and nutrient source for organisms via detrital pathways (Mann 1988; Moore et al. 2004). Dual stable isotope ratios and fatty acid signatures have been used as tracers to show how detritus from a variety of origins can serve as an energy source for marine communities (e.g., riverine discharges—Paterson and Whitfield 1997; seagrass beds— Boschker et al. 2000; macroalgae—Norderhaug et al. 2003; phytoplankton and mangroves—Bouillon et al. 2004; Mfilinge et al. 2005). These studies show that the relative importance of energy from terrestrialand marine-derived detritus vs. energy from phytoplankton and benthic algae is highly variable within and among coastal marine ecosystems. High spatial variation in detrital availability and composition has made it difficult to generalize about the importance of detrital pathways, and complexity is added in instances where the relative contributions of different energy sources are influenced by tidal exchange and seasonal changes (Guest et al. 2004; Doi et al. 2005). The relative importance of these food sources to secondary production in a particular system is, therefore, predicted to be case specific. On seasonal, tropical shores in Hong Kong, monsoons have an important effect on the supply of detritus (Dudgeon 1982; Kaehler and Williams 1996). During the cool, dry northeastern monsoon (December–March), extensive beds of erect macroalgae (especially Sargassum spp.) develop along rocky coasts, but these die back when water temperatures rise in May to June as the warmer, southwestern monsoon begins to dominate (Hodgkiss 1984; Kaehler and Williams 1996). As a result, large quantities of detached and decomposing macroalgae are exported by the tides to other intertidal and subtidal habitats (Kaehler and Williams 1996), creating a potential trophic link between these different habitats. During the hot, wet southwestern monsoon (June–September), rainfall increases significantly and contributes to ,70% of the average annual rainfall 1 Corresponding author ([email protected]).