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

Experimental and field studies were carried out to characterise biodeposit dynamics in a suspended mussel Mytilus edulis L. farm in Great-Entry Lagoon, eastern Canada. We assessed: (1) the quantity and quality of biodeposits produced by different age classes of mussels, (2) the size-dependent sinking velocity of faeces and (3) the variation in sedimentation rates at different spatial and temporal scales. Individual 0+ mussels produced on average only 63% of the mass of biodeposits (32.4 mg dry wt d(-1) ind.(-1)) that 1+ mussels did (51.5 mg dry wt d(-1) ind.(-1)). In contrast, the amount of biodeposits produced per unit body weight (dry weight of soft tissue) was greater for 0+ than for 1+ mussels. Faecal pellet sinking velocity ranged from 0.27 to 1.81 cm s(-1) for mussels ranging in size from 3 to 7 cm, and was best correlated with faecal pellet width. Sedimentation rates were greater within the farm than at reference sites, supporting the hypothesis that mussel farming increases sedimentation rates. Variations in sedimentation were also observed at small spatial scales and through time. Prior to the harvesting of 1+ mussels, sedimentation rates directly under the 1+ mussel lines were about twice those 10 m distant, between the lines, and in other zones (reference sites and sites in the lease with 0+ mussels). These observations and sedimentation patterns along transects leading away from the mussel farm suggest that biodeposits from the farm are not dispersed broadly. The estimated initial dispersal of faecal pellets ranges from 0-7.4 m (1+ mussels) to 7-24.4 m (0+ mussels).