Pathways of organic matter in an estuarine mangrove trophic network assessed by carbon and nitrogen stable isotopes

Carbon and nitrogen stable isotopes were used to describe an estuarine mangrove food web in New Caledonia, SW Pacific. Isotopic values were measured for all components of the ecosystem, from various organic matter (OM) sources to predators. Primary producers showed delta C-13 values from -32.29 parts per thousand (Bruguiera gymnorhiza) to -24.67 parts per thousand (mouth particulate organic matter), whereas delta N-15 values ranged from 1.48 parts per thousand (Avicennia marina) to 10.17 parts per thousand (Eichomia crassipes). Two potential organic matter sources, i.e. the mangrove species and E. crassipes, appeared not to be directly used by consumers although were indirectly entering the food web through particulate and sedimentary organic matter pools (POM and SOM). Overall, invertebrates showed C-depleted and N-depleted values compared with fish, highlighting their lower trophic level. The highest trophic levels in this estuarine mangrove were represented by carangids ( delta N-15 of 11.24 parts per thousand for Caranx sp. and 10.81 parts per thousand for Carangoides fulvoguttatus) and gerrids ( delta N-15 of 10.42 parts per thousand for Gerres filamentosus). Two main pathways of organic matter were identified from sources of OM to end-members, i.e. from estuarine POM and SOM toward gerrids and from marine POM towards carangids. The food chain comprised three or four trophic levels, depending on the pathway of organic matter. The position of some consumer species within the reconstructed food web might imply that an important source of organic matter was probably missing, i.e. microphytobenthos. Despite an obvious connection, the role of river inputs as potential drivers of mangrove food web dynamics appeared important only during the wet season.