Factors influencing biodeposit production by the New Zealand freshwater mussel <i>Echyridella menziesii</i>

Laboratory experiments on the New Zealand freshwater mussel <i>Echyridella menziesii</i> were used to investigate the short-term effects (7–8 days) of food type on rates of biodeposition and benthic substrate respiration. Post-feeding biodeposition rates ranged from 0.34 to 1.52 mg g⁻¹ h⁻¹ (mean = 0.50 mg g⁻¹ h⁻¹) and were unaffected by the addition of toxin-producing <i>Microcystis</i>. Addition of suspended sediment (30 mg L⁻¹) visibly altered substrate composition, and increased total and inorganic biodeposit production rates by 24–33% compared to mussels fed commercial phytoplankton stock. Biodeposition rates of mussels in lake bed substrates were 38% higher than those in silica sand for identical feeding regimes, suggesting that a significant proportion of material produced in this experiment could have been derived from feeding on organic matter in the lake bed sediments. Respiration rates were higher in treatments with <i>Microcystis</i> but were unaffected by the presence of mussels. This laboratory study suggests that biodeposition by <i>E. menziesii</i> is resilient to short-term exposure to <i>Microcystis</i>, and highlights the ability of mussels to alter benthic substrate composition by incorporating suspended sediment into substrates.