Phytoplankton community structure responses to episodic summer storms in a temperate coastal ecosystem

Extreme events potentially modify the physical and biogeochemical environment resulting in dramatic changes in phytoplankton community structure. In this study, the impact of ten well-identified storms on phytoplankton communities was explored in a productive temperate coastal ecosystem, the eastern English Channel (EEC). Our study targeted the summer season, which is typically characterised by low nutrient availability, yet transient phytoplankton blooms often occur. From 2012–2022, low-frequency (weekly to fortnightly) flow cytometry measurements of phytoplankton abundance were combined with high-frequency meteorological data (precipitation and wind) and river flow rates as a proxy for riverine influence. Storm impacts occurred in three distinct forms: high river inflow events, high wind stress-low inflow events, and low wind stress-low inflow events. Regardless of wind conditions, high inflow storms favoured diatoms, as nutrient-rich river plumes supported their growth. In contrast, low inflow events paired with strong winds enhanced vertical mixing and nutrient availability, favouring nanophytoplankton. Finally, weak winds and relatively low-nutrient conditions favoured picophytoplankton ( Synechococcus and picoeukaryotes). Across years, storms repeatedly reset seasonal succession and maintained environmental heterogeneity, leading to transient monospecific peaks of phytoplankton. These findings highlight storms as recurrent structuring forces in the EEC, mediating nutrient availability and driving shifts in phytoplankton composition; as such, storm-driven dynamics might be an important input to prediction models for phytoplankton.