Quantifying the trophic base for benthic secondary production in the Nakdong River estuary of Korea using stable C and N isotopes

The overall dependence of benthic secondary production on the main primary producers at three different habitats in the Nakdong River estuarine system, Korea, was estimated. Inventories of macrobenthic invertebrate biomass were combined with multiple-isotope-mixing models to evaluate the trophic base, comparing Scirpus triqueter-dominated and Phragmites australis-dominated marshes and bare intertidal flat. The feasible contributions of four main food sources, marsh macrophytes, the microphytobenthos, and riverine and marine suspended particulate organic matter (RPOM and MPOM), to the consumer biomasses were calculated using the isotopic mixing model. After weighting the feasible contributions of food sources to each taxon by the consumer biomass, the resultant values were summed for all the consumers at each habitat to quantify the trophic base of the benthic invertebrate community. Dual-isotope-mixing model calculations verified the varying dependence on those potential food sources among the functional feeding groups. In addition, the dependence on each source of the same functional group varied between bare intertidal and salt-marsh habitats, shifting from a dominance of benthic and pelagic microalgal sources on the former habitat to a mixed food source at the latter habitat. The biomasses of the species comprising each functional group differed among habitats and sampling dates, so that each functional group made a different contribution to the whole benthic community and its basal food source. Given the calculation of the overall dependence of macrozoobenthic community on each food source, our results indicate that the microphytobenthic source dominates (nearly half) the trophic base in all the intertidal habitats of different vegetational compositions. Marsh-macrophyte-derived organic matter and RPOM served as considerable subsidies only to salt-marsh food webs, reflecting the use of the mixed food source by salt-marsh-bed consumers. Conversely, the dominance of MPOM in the total food base was equal to that of the microphytobenthos in the bare intertidal ecosystem but increased during spring−summer in the salt-marsh systems. Our results also suggest that the river discharge concentrated during the summer monsoon does not lead to any shift in trophic base for estuarine secondary production.