Differential food utilization of benthic amphipods of a tropical estuary : a stable isotopic (δ13C and δ15N) investigation

Globally, stable isotopic methods are rapidly being employed in food web studies to quantify the relative proportions of diverse primary carbon sources in an animal diet. Tropical estuarine ecosystems, one of the vital marine habitats, harbour complex food webs due to the multiplicity of habitats and resources. The complex system of inputs and exchanges of organic matter sometimes complicate identification of the carbon sources that support the higher trophic levels. To assess the primary food sources of four abundant amphipod species in the anthropogenically influenced upper Vashishti estuary, Carbon and Nitrogen stable isotopes were employed. Seasonal variations were significant for δ 13 C and δ 15 N of suspended particulate organic matter (SPOM) and also for δ 13 C of sediment organic matter (SOM), but not for the isotopic signatures of individual amphipod species. Significant variations in the δ 13 C and δ 15 N values between amphipods suggested species-specific diets. Mixing model output indicated discrepancies in seasonal variations of food preferences between species. Trophic level values indicated that all amphipod species were primary consumers. The depleted δ 15 N values of SPOM, SOM and amphipod species and the negative relationship between δ 15 N values of SPOM with SS and NO 2 - -N concentration and also between δ 15 N values of Gammaropsis megnae with increasing NO 3 - -N and NO 2 - -N concentrations were possibly due to the anthropogenic input of nitrogen in the estuary.