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

Abstract

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 4× 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.

Sedimentation rates in a suspended mussel farm (Great-Entry Lagoon, Canada): biodeposit production and dispersion

“…As noted by Giles & Pilditch (2004), sinking velocity was best correlated with faecal pellet width. Thus, measures of pellet width are more important for understanding sinking velocity than are other measures of pellet size.…”

“…Little is known about biodeposit quality (Navarro & Thompson 1997), biodeposit production rates (Kautsky & Evans 1987), or their potential for dispersion (Miller et al 2002, Giles & Pilditch 2004, Hartstein & Stevens 2005. Further, empirical relationships between biodeposit size and sinking velocity are poorly estimated by simple sinking velocity equations, such as Stoke's law, as has been shown by Chamberlain (2002) and Giles & Pilditch (2004). A better understanding of the relationship between these factors is necessary in order to make accurate predictions of benthic loading and subsequent effects on the local environment (Henderson et al 2001).…”

Influence of filtering and biodeposition by the cultured scallop Chlamys farreri on benthic-pelagic coupling in a eutrophic bay in China

“…Since faeces and pseudofaeces are voided from the animal body as mucus-bound aggregates, they are larger and more prone to sedimentation than the nonaggregated particles from which they are formed, and deposit at rates up to 40 times that of nonaggregated particles (Kautsky & Evans 1987, Widdows et al 1998, Giles & Pilditch 2004. Consequently, the filtering-biodeposition process can be extremely important in regulating water column processes and pelagicbenthic coupling when bivalves are abundant in coastal waters , Newell 2004.…”

Fecal production by sea urchins in native and invaded algal beds

“…Diet can also affect the settling and critical shear velocities of fecal pellets by influencing their shape, size, and density (Taghon et al 1984, Turner 2002. For example, the presence of inorganic material, such as CaCO 3 in M. membranacea, may decrease horizontal advection and increase deposition (Miller et al 2002, Giles & Pilditch 2004.…”