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

Abstract. Extreme events potentially modify the physical and biogeochemical environment resulting in dramatic changes of phytoplankton community structure. In this study the impact of 10 well identified storms on phytoplankton communities was explored in a productive coastal temperate ecosystem, the eastern English Channel (EEC). We focussed in summer because it is a low nutrient season where phytoplankton major shifts and blooms are not expected. Low-frequency (weekly to fortnightly) flow cytometry measurements of phytoplankton abundance were combined with high-frequency meteorological data (precipitation and wind) and hydrological records from 2012 to 2022. Storm impacts occurred in three distinct forms: high river inflow events, high wind stress–low inflow events, and low wind stress–low inflow events. High inflow storms, whether accompanied by weak or strong winds, promote coastal advection of riverine plumes enriched in nutrients, sustaining diatom dominance and, under high wind stress, favouring nano-sized phytoplankton (nanophytoplankton and cryptophytes). In contrast, low inflow conditions limit riverine nutrient supply, shifting reliance to regenerated nutrients from vertical mixing. When combined with strong winds, these conditions support nanophytoplankton growth, while short, low-wind storms favour pico-sized phytoplankton (Synechococcus spp. 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 across the summer months.