Effects of diet on sinking rates and erosion thresholds of mussel Perna canaliculus biodeposits

Suspension-feeding bivalves produce biodeposits (faeces and pseudofaeces) that have much higher sinking velocities than their constituent particles. Consequently, they cause sedimentation and redistribution of material that might otherwise not be deposited, enhancing the benthic-pelagic coupling of nearshore ecosystems. We quantified the dispersal characteristics (sinking velocity and erosion threshold) of biodeposits produced by the mussel Perna canaliculus, a species with high natural abundance that is also intensively cultured in New Zealand. We examined biodeposits produced by mussels of a wide size range (27 to 114 mm shell length) fed 3 diets (natural [N], algae [A] and silt [S] dominated) in the laboratory. Larger mussels produced bigger biodeposits that sank more quickly, and highly significant relationships between biodeposit size (faecal pellet width and pseudofaeces area) and sinking velocity were derived for each diet (r(2) = 0.43 to 0.79). Biodeposit sinking velocities also varied significantly with diet. Mean sinking velocities of faecal pellets produced on the A diet (0.67 cm s(-1)) were approximately 4x lower than those of pellets produced on the N and S diets (2.46 to 2.71 cm s(-1) respectively). Pseudofaeces produced on the N and A diets settled 50 to 70 % slower than faeces produced on the same diet, but sinking velocities of S diet faeces and pseudofaeces were similar. In situ estimates of biodeposit sinking velocities measured in the Firth of Thames, New Zealand, were comparable to those produced on the N diet. Erosion thresholds were also affected by diet but not by mussel size. Ninety percent of faecal pellets produced on the A diet eroded at a bed shear velocity (u(*)) of 0.28 cm s(-1), half that required to erode the same percentage of faecal pellets produced on the N and S diets. Pseudofaeces generally eroded over a similar but wider range of u(*), and the differences between the 3 diets were less distinctive than for faecal pellets. These results emphasise that mussel biodeposit dispersal (and hence the flux to the benthos) depends on the available diet and mussel size and thus could differ significantly between locations, seasons and the size structure of the population.