Phytoplankton community structure responses to episodic summer storms in the eastern English Channel

Extreme events can rapidly alter the physical and biogeochemical environment, triggering pronounced shifts in 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). Summer was selected as the focal season because it is typically nutrient‑poor, and major phytoplankton shifts or blooms are not expected. Low‑frequency (weekly to fortnightly) flow cytometry measurements of phytoplankton abundance were combined with high‑frequency meteorological (precipitation, wind) and hydrological records from 2012 to 2022. Additional biogeochemical and phytoplankton data were obtained from French National Observing Systems (SNOs): SNO SOMLIT and SNO PHYTOOBS (biweekly sampling) and SNO COAST‑HF (10–30 min resolution).

Storm impacts emerged in three distinct categories, illustrated by representative events: a high river inflow storm (02 July 2016), a high wind stress–low inflow storm (13 July 2015), and a low wind stress–low inflow storm (06 June 2022). High inflow storms, regardless of wind intensity, enhanced coastal advection of nutrient‑rich river plumes, sustaining diatom dominance and, under strong winds, promoting nano‑sized taxa such as nanophytoplankton and cryptophytes. Under low inflow conditions, limited riverine nutrient supply increased reliance on regenerated nutrients from vertical mixing. When paired with strong winds, these conditions favoured nanophytoplankton growth, whereas short, low‑wind storms supported pico‑sized phytoplankton, particularly Synechococcus spp. and picoeukaryotes.

Across years, storms repeatedly reset seasonal succession and maintained environmental heterogeneity, generating transient monospecific peaks (e.g., Leptocylindrus danicus) and shaping community trajectories throughout summer. These findings highlight storms as recurrent structuring forces in the EEC, mediating water-column structure (stratification versus mixing) and associated nutrient availability, thereby driving rapid shifts in phytoplankton composition.