Assessing coastal zooplankton in the St. Lawrence estuary: spatio-temporal patterns of taxonomic and functional biodiversity.

Biodiversity assessment promotes information on the state of an ecosystem. Zooplankton, as a sentinel group at the basis of aquatic food webs, are, thus, an important component to monitor for ecosystem conservation and management. For the first time, we characterized biodiversity of coastal zooplankton along the shallow Northern shoreline of the lower St. Lawrence estuary (LSLE) using an integrated taxonomic and trait-based approach. For 3 years (2019-2021), in July and October, the zooplankton community and environmental parameters were sampled at < 35 m depth. Mesozooplankton were identified at the lowest possible taxonomic level and assigned functional traits. Community structure and diversities revealed high spatio-temporal variations among three different geographic sectors and between seasons, mainly driven by water temperature, Chlorophyll-a concentration and less by salinity. Hotspots of taxonomic and functional diversities occurred in different sectors in the same month, underlining the complementarity of the two approaches. Seasonal shifts in functional diversity hotspots highlight how environmental variability affects biodiversity beyond taxonomic metrics alone. The results of our study in the LSLE establish a first robust baseline to improve our understanding of zooplankton dynamics in the coastal LSLE, to allow future tracking of ongoing change due to the increase of anthropogenic activities and climate changes and to support future monitoring efforts.