Comparative analysis of mesozooplankton size fraction structure in bivalve aquaculture embayments in Atlantic and Pacific Canadian coastal regions

Coastal embayments are dynamic ecosystems facing environmental and anthropogenic pressures, including bivalve aquaculture and climate change. Mesozooplankton, essential for transferring energy from primary producers to higher trophic levels, serve as indicators of habitat changes. Size structure is a critical trait that reflects local community dynamics, trophic interactions, and ecosystem conditions, offering insights into the functioning and resilience of aquatic environments. This study examines the spatio-temporal variation in mesozooplankton size structure across nine bivalve aquaculture embayments in Atlantic and Pacific Canada from 2020 to 2022. Using high-resolution imaging (FlowCam®) to measure individual zooplankton, we assessed the effects of location, tide, sampling day, season, and aquaculture pressure on the size distribution variation among and within bays. Results indicate that bays with similar size distributions tend to have larger mesozooplankton, while those with more variable distributions are dominated by medium-sized individuals. Significant associations between environmental factors and size variation were observed in four of eleven sampling events. Notably, St. Peters Bay, with the highest aquaculture pressure, showed significant variation associated with station location and sampling day. However, the tide effect was significant only in two sampling events. Seasonal analysis revealed that colder months generally exhibited larger median sizes, with some exceptions influenced by local conditions. Despite high levels of aquaculture pressure in some bays, no consistent association between aquaculture pressure and size variation was found, highlighting the influence of local environmental factors. This study underscores the importance of monitoring mesozooplankton size structure as a bioindicator for effective ecosystem management and targeted conservation strategies. • Mesozooplankton size structure reveals varied biovolume distribution across different aquaculture bays. • Bays with more homogeneous biovolume distributions show more abundance of larger mesozooplankton. • Observed variation in size distributions was significantly related to environmental and seasonal factors but not to shellfish aquaculture. • The bay with more aquaculture pressure exhibited highest variation in mesozooplankton size distributions. • Colder months generally exhibit larger mesozooplankton biovolumes, highlighting seasonal effects.