The effect of high inorganic seston loads on prey selection by the suspension-feeding bivalve, Atrina zelandica

Suspension-feeders are discriminate feeders, selecting prey by size, shape and food quality, this discriminate feeding behaviour has important consequences for the modelling of growth rates, population dynamics and ecosystem change. When given natural seston, the large pinnid bivalve Atrina zelandica (Gray), fed on picophytoplankton (< 2 μm), larger phytoplankton (2–270 μm) and microzooplankton, but preferentially selected algal species within the 2–20 μm size fraction. Selection for ingestion was based on food quality, with morphotype, carbon content and potential toxicity also being important. Microzooplankton were readily ingested and represented 48% of the available diet in terms of carbon indicating they play an important role in the diet of Atrina. Mussels were subsequently fed a selected cultured algal diet consisting of three different types of 2–20 μm sized phytoplankton to assess differences in prey selection and grazing efficiency. When high inorganic suspended sediment concentrations of 500 mg/l were added in addition to the cultured algal diet this caused Atrina to increase filtration and rejection rates and reduced the efficiency of Atrina to select food in all cases. More importantly, there were changes in the species preferentially selected for ingestion. Our results suggest that as well as reducing feeding efficiency increased suspended sediment concentrations may affect prey selection and therefore have consequences for benthic–pelagic coupling beyond that of reduced removal and deposition rates.