Production sources and food web structure of a temperate tidal estuary: integration of dietary and stable isotope data

Food web structure and major sources of primary production consumed by metafauna of Mad Island Marsh, a coastal saltmarsh on the NW coast of the Gulf of Mexico, were compared using stable isotopes and dietary analysis. Carbon and nitrogen isotope data were entered into a mixing model containing 5 potential production sources. Results were inconclusive due to overlapping isotopic signatures of certain sources, but nonetheless indicated that most fishes and macroinvertebrates assimilated material derived mostly from variable mixtures of macrophytes and filamentous algae. Highest estimates of percentage of material assimilated directly or indirectly from C-4 marsh grasses (ranging from 30 to 82 %) were for spot Leiostomus xanthurus and Gulf killifish Fundulus grandis. Isotopic analysis could not reveal the detailed structure of predator-prey interactions at the species level; greater detail of trophic pathways was revealed by the dietary analysis. Estimates of vertical web structure (species trophic levels) by the 2 methods were largely concordant. The exceptions were 2 zooplanktivorous and detritivorous fish species and grass shrimp Palaemonetes pugio that had higher trophic levels according to nitrogen isotope ratios. For these taxa, the isotopic method more accurately indexed the number of trophic transfers than the dietary method, which depends on accurate dietary estimation for all food chain components leading to a consumer, and which assumes equal assimilation efficiencies for items found in stomach contents. The isotopic method underestimated trophic levels of several invertebrates, possibly due to inaccurate estimation of mean delta N-15 for production sources supporting these taxa and/or differential trophic fractionation. Together, stable isotope and dietary analyses provide a more accurate assessment of food web structure and dynamics of coastal marsh ecosystems than either method alone.