Effects of different feeding habits of three bivalve species on sediment characteristics and benthic diatom abundance

To reveal the feeding habits of 3 bivalve species and the effects on the sediment and benthic diatoms, a field experiment was conducted in the estuarine Gamo lagoon, Japan, using enclosures stocked with bivalves. The surface-deposit feeder Macoma contabulata reduced silt-clay, total nitrogen, and total organic carbon content and the delta C-13 value of the sediment, while the surface-deposit feeder Nuttallia olivacea and the suspension feeder Ruditapes philippinarum did not. The surface-deposit feeders reduced phytobenthic biomass (chlorophyll a, ch1 a) and diatom density at the sediment surface and increased the C/N ratios of the sediment. In contrast, R. philippinarum increased ch1 a although it decreased benthic diatoms, suggesting the promotion of microalgal growth other than diatoms. All the bivalve species reduced the densities of the dominant diatom species differently, and M. contabulata decreased the species richness. Multidimensional scaling and 1-way analysis of similarity revealed that the bivalves specifically modified the abundances and composition of the diatoms in relation to their feeding habits, while the relative diatom species abundances hardly changed. These data suggest that the bivalves show little selectivity when they ingest benthic diatoms. A 2-source mixing model using delta C-13 values revealed that benthic diatoms were the more important carbon source (> 69%) for the deposit-feeding bivalves, both in natural habitats and in enclosures. In contrast, R. philippinarum obtained 61% carbon from suspended solids (SS) in the natural habitat, whereas in the enclosures it obtained 62% carbon from benthic diatoms. The dietary change of this species might be related to the supply of SS and resuspended sediment.