Long-term phytoplankton dynamics in the Zeeschelde estuary (Belgium) are driven by the interactive effects of de-eutrophication, altered hydrodynamics and extreme weather events

We studied how changing human impacts affected phytoplankton dynamics in the freshwater and brackish tidal reaches of the Zeeschelde estuary (Belgium) between 2002 and 2018. Until the early 2000s, the Zeeschelde was heavily polluted due to high wastewater discharges. By 2008, water quality had improved, resulting in lower nutrient concentrations and higher oxygen levels. Since 2009, however, increased dredging activities resulted in altered hydrodynamics and increased suspended sediment concentration. The combined effects of these environmental changes were reflected in three marked transitions in phytoplankton community composition. Assemblages were dominated by Thalassiosirales and green algae (especially Scenedesmaceae) until 2003. The period 2003–2011 was characterized by the wax and wane of the centric diatoms Actinocyclus and Aulacoseira, while in the period 2012–2018 Thalassiosirales and Cyanobacteria became dominant, the latter mainly imported from the tributaries. Phytoplankton biomass increased sharply in 2003, after which there was a gradual decline until 2018. By 2018, the timing of the growing season had advanced with about one month compared to the start of the study, probably as a consequence of climate warming and intensified zooplankton grazing pressure. Our study shows that de-eutrophication (during the 2000s) and morphological interventions in the estuary (in the 2010s) were dominant drivers of phytoplankton dynamics but that the main shifts in community composition were triggered by extreme weather events, suggesting significant resistance of autochthonous communities to gradual changes in the environment.