Influence of suspended mussel farming on planktonic communities in Grande-Entrée Lagoon, Magdalen Islands (Québec, Canada)

Mussel culture has been on-going since 1984 in Grande-Entrée lagoon (Magdalen Islands, Gulf of St. Lawrence, Canada). This industry has grown approximately 10-times, to about 180 tons y⁻¹ in 2006. The goal of this study was to examine the influence of mussel farming in this lagoon, focusing on the planktonic communities. The influence of the suspended mussel longlines on surrounding environment was analysed by comparing two stations inside and outside of the mussel farm over a 4 month period during the summer of 2003. No significant differences were found between the two stations for particulate matter and nutrients. There was no significant reduction in phytoplankton concentrations within the mussel farm, both in terms of total chlorophyll a concentrations or the biomass of specific phytoplankton groups determined by microscopy. However, phytoplankton productivity was enhanced inside the farm, where mussels likely helped to supplement the low concentrations of nutrients locally present. Heterotrophic plankton was dominated by ciliates, which represented the largest carbon resource among the various plankton groups. Overall, there was no significant difference between the two stations for ciliates, but their biomass was lower at the mussel farm on several occasions. The biomass ratio of planktonic autotrophs to heterotrophs was also occasionally larger inside the farm, associated with the lower heterotrophic biomass. Path analysis showed a negative interaction between the farmed mussels and ciliates and heterotrophic nanoflagellates in GEL, consistent with an inverse seasonal trend between these organisms. An estimate of the influence of farmed mussels on ciliates, phytoplankton and daily primary production rates was however low (maximum=16% removal for ciliates), not sufficient to be detectable as a difference between the stations inside and outside the farm. Comparisons with historic records in GEL indicate that, even though mussel culture has grown by about 10 times in this environment since 1984, there is still no significant change in plankton abundance or composition. In summary, the local mussel production is probably not high enough to exert a significant control on plankton composition and biomass at the spatial and time scales studied here, although it can enhance phytoplankton productivity. The occasional decrease in ciliates at the mussel farm in comparison to the outside suggests that future increases in mussel production could affect these organisms.