In the Eastern English Channel, the spring phytoplankton bloom is dominated by Phaeocystis sp. (Prymnesiophyceae), which at the peak of the bloom forms characteristic large colonies, supported by a mucopolysaccharide matrix produced by swarming cells. The decline of the bloom is characterized by the accumulation of foam in the turbulent surf zone of beaches, suggesting potential changes in the rheological properties of seawater, potentially due to cell senescence and colony degradation. To identify this potential relationship between bloom conditions and seawater properties, chlorophyll concentration, auto- and hetero/mixotrophic protists composition, standing stock and biomass, and seawater viscosity were investigated weekly in the inshore waters of the Eastern English Channel over the course of the spring phytoplankton bloom, from March to June 2004, characterized by a massive foam event. Before foam formation, seawater viscosity significantly increased, showing a significant positive correlation with chlorophyll concentration. In contrast, after foam formation this correlation was negative, seawater viscosity kept increasing despite a sharp decrease in chlorophyll concentrations. No significant correlation has been found between seawater viscosity and the composition of phytoplankton assemblages. However, significant positive correlations have been found between seawater viscosity and both the size and the abundance of Phaeocystis colonies. These results indicate the existence of a match/mismatch dynamics between chlorophyll concentration and seawater viscosity before and after foam formation. Before foam formation, seawater viscosity was correlated to chlorophyll concentration as dominant Phaeocystis cells are embedded in the mucilaginous colonial matrix. Conversely, after foam formation Phaeocystis cells are released from their colonial form. The negative correlation between chlorophyll concentration and seawater viscosity then suggests that the rheological properties of seawater are mainly driven by extracellular materials associated with colony formation and maintenance rather than by cell composition, standing stock and biomass. While additional experiments are needed to ensure the relevance and the generality of the present results, the observed biomodification of seawater viscosity is discussed in relation to its likely impact on the biophysical properties of the marine environment.
