Biologically induced modification of seawater viscosity in the Eastern English Channel during a Phaeocystis globosa spring bloom Laurent Seuront a,b, ⁎ , Dorothée Vincent c , James G. Mitchell b a Ecosystem Complexity Research Group, Station Marine de Wimereux, CNRS UMR 8013 ELICO, Université des Sciences et Technologies de Lille, 28 avenue Foch, 62930 Wimereux, France b School of Biological Sciences, The Flinders University of South Australia, GPO Box 2100, Adelaide 5001, South Australia, Australia c Maison de la Recherche en Environnement Naturel, CNRS UMR 8013 ELICO, Université du Littoral-Côte d'Opale, 32 avenue Foch, 62930 Wimereux, France Received 12 November 2004; accepted 26 April 2005 Available online 15 March 2006 Abstract To identify the potential relationship between Pheaocystis globosa bloom conditions and seawater properties, a hydrobiological survey was performed in the inshore waters of the Eastern English Channel over the course of the phytoplankton spring bloom. Chlorophyll concentration, auto- and hetero/mixotrophic composition of protists and standing stock, and seawater viscosity were measured weekly from March to June 2004. The decline of the bloom is characterized by a massive foam formation in the turbulent surf zone. 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 the phytoplankton assemblages observed during the survey. However, significant positive correlations have been found between seawater viscosity and both the size and the abundance of P. globosa colonies. From the correlation patterns observed between chlorophyll concentration and seawater viscosity, we suggest that the rheological properties of seawater are mainly driven by extracellular materials associated with colony formation and maintenance rather than by cell composition and standing stock. © 2006 Elsevier B.V. All rights reserved. Keywords: Seawater viscosity; Phytoplankton; Rheology; Phaeocystis globosa; Spring bloom dynamics; Eastern English Channel 1. Introduction Navier–Stokes equations, as well as any subse- quent models of marine turbulence (Baumert et al., 2005), implicitly assume the seawater medium to be Newtonian, that is without elasticity and with viscosity independent of shear stress (Jenkinson, 1986). Under this assumption seawater viscosity is mainly controlled by its temperature and salinity (Miyake and Koizumi, 1948). In the ocean, viscosity has therefore mainly been thought as controlled by the temperature gradient. However, more recently non-Newtownian or rhelogical properties in seawater have been resolved where the ‘apparent viscosity’ of the fluid is altered by the presence of biologically Journal of Marine Systems 61 (2006) 118 – 133 www.elsevier.com/locate/jmarsys ⁎ Corresponding author. Ecosystem Complexity Research Group, Station Marine de Wimereux, CNRS UMR 8013 ELICO, Université des Sciences et Technologies de Lille, 28 avenue Foch, 62930 Wimereux, France. Tel.: +33 321992937; fax: +33 321992901. E-mail addresses: Laurent.Seuront@univ-lille1.fr, Laurent.Seuront@flinders.edu.au (L. Seuront). 0924-7963/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2005.04.010