Stable isotope addition reveals dietary importance of phytoplankton and microphytobenthos to saltmarsh infauna

Despite the paradigm that Spartina spp.detritus is the basis for estuarine food webs, other primary producers may contribute to the diets of saltmarsh consumers.To determine the dietary contribution of primary producers to benthic infauna in the Plum Island Estuary, Massachusetts, USA, we examined natural abundance stable isotopes in 4 intertidal saltmarsh habitats and conducted an 15 N enrichment experiment in 2 habitats.Natural abundance isotope data suggested that Spartina spp.detritus was of limited dietary importance to infauna in all habitats (including Spartina spp.understory) and instead benthic algae and phytoplankton were the dominant food sources.15 N enrichment was used to improve dietary resolution of benthic algae and phytoplankton sources that had similar natural abundance values.To label only benthic algae, 15 N-enriched Na 15 NO 3 was applied daily for 14 d to sediment in mudflat and creek-wall habitats.Food-web incorporation of 15 N-labeled benthic algae was found in most species.However, label uptake in the polychaetes Manayunkia aestuarina, Fabricia sabella and Streblospio benedicti indicated that phytoplankton was the most important food source for these consumers.Label uptake in the polychaete Nereis diversicolor differed between habitats, suggesting a large dietary contribution of microphytobenthos (MPB) in mudflat and phytoplankton in creek wall.The oligochaete Paranais litoralis consumed both MPB and phytoplankton regardless of habitat.The harpacticoid copepod Heterolaophonte sp.consumed primarily epiphytic diatoms.Overall, infauna in this system relied on phytoplankton and benthic algae as dominant food resources, and dietary contributions from primary producers varied among species and habitats.