Effects of seasonal irrigation subsidies on the dietary components of estuarine detritus feeders: stable isotopic analysis focusing on nereidid polychaetes

Abstract

To assess the effects of seasonal irrigation subsidies from paddy fields, spatiotemporal changes in diets of detritus feeders including nereidid polychaetes Hediste diadroma and H. atoka (facultative suspension feeders) and Tylorrhynchus osawai (surface-deposit feeder) were examined in Idoura Lagoon, Sendai Bay, Japan, using carbon and nitrogen stable isotope ratios (δ 13 C and δ 15 N). δ 13 C ratios of Hediste species were much lower near a freshwater input (St. IB; −25.6 to −18.2‰) than in a seaward area (St. IA; −17.9 to −14.3‰), highlighting the assimilation of river subsidies in river-affected habitats. A δ 13 C-based isotopic mixing model showed that riverine phytoplankton and detrital matters became the major diet of Hediste species in August and December at St. IB (60 to 72%), which corresponded to the supply of irrigation drainage to the area. Contrastingly the contribution of microphytobenthos increased sharply in March at both stations (55 to 83%) because of benthic diatom blooms. These emphasize that the Hediste species can change their feeding modes (i.e., suspension-and deposit-feeding) depending on food availability in the habitat. In winter and/or spring, δ 13 C of H. diadroma negatively correlated with their body size, suggesting size-dependent variation in food utilization. δ 13 C ratios of the sympatric deposit-feeders and suspension-feeding bivalves were seasonally more stable, depending mainly on microphytobenthos and river derived materials, respectively. These suggest that a certain group of estuarine consumers including Hediste species act as significant links between terrestrial and estuarine food webs by incorporating riverine terrestrial subsidies to benthic secondary production in estuaries.