Drivers of spatial and temporal micro- and mesozooplankton dynamics in an estuary under strong anthropogenic influences (The Eastern Scheldt, Netherlands)

Abstract

The Eastern Scheldt estuary in the Netherlands has been anthropogenically changed by the construction of a storm surge barrier about 30 years ago, affecting abiotic conditions as well as phytoplankton and zooplankton (ZP). As ZP communities have not been sampled in the last decades, it is unclear to what extent they have changed during the last 30 years. We analyzed the micro- and mesoZP composition and background parameters at eight stations in the Eastern Scheldt in spring, summer, and autumn 2018. Additionally, we conducted dilution experiments to estimate phytoplankton growth and microZP grazing rates. Seasonal variability of ZP was higher than spatial variability, but there were differences between stations. In agreement with data from the 1980s, we identified salinity and chlorophyll a as the most important factors in explaining the ZP community. Suspended particulate matter and water retention time were additionally identified for mesoZP, as well as dissolved inorganic nitrogen and phosphate concentration for microZP. We observed some changes in ZP composition with respect to the 1980s, most of which could be explained by the changes induced by the barrier construction. However, increased abundances of the cyclopoid copepod Oithona sp. were likely related to changes in phytoplankton composition due to large-scale bivalve cultivation in the estuary. Phytoplankton growth and microZP grazing rates were variable between seasons, which could be explained by changes in the phytoplankton community during the annual plankton succession. Overall, the observed alterations in the ZP community due to changed background parameters and intense bivalve filtration could affect the microbial loop as well as consumers of mesoZP and thus the functioning of the pelagic food web.