Bulk-phase viscoelastic properties of seawater relationship with plankton components

Abstract

The viscous and elastic moduli at different shear rates, together with various biological oceanographic properties, were determined in seawater from different hydrological layers in the southern North Sea in June. The biological oceanographic parameters included Phaeocystis and Noctiluca abundances, chlorophyll a level (Chi), bacteria. HNAN and aggregate volume fraction. The plankton was jointly dominated by Phaeocystis sp. and Noctiluca scintillans. Noctiluca abundance showed no correlation with any other biological or viscoelastic parameter, but Phaeocystis abundance correlated strongly. The other biological parameters correlated with Phaeocystis and with each other positively and mostly significantly. Overall, viscoelasticity correlated more strongly with Chi than with any other biological parameter. For non-microlayer samples, the excess complex (viscoelastic) modulus (jiPa) C*E = 2.0 x Chi (Chi in mg m). Viscous and elastic moduli also correlated closely with each other. For a given value of Chi. the microlayer samples were 6.5 or 14 times (depending on the estimation method) more viscoelastic than in bulk-phase samples. Viscoelasticity in samples of settled benthic 'fluff were lower even than bulk-phase samples, but this difference was not significant. Comparison with Mediterranean data on viscoelasticity (Jenkinson. Oceanol. Acta, 16,317-334.1993), using published values for phytoplankton biomass (Wiadnyana,/ Rech. Octanogr., 17,1-6,1992), suggests that the relationship between Chi (or phytoplankton biomass) and viscoelasticity might be general. This apparent biomodlfication of the viscosity and elasticity of seawater is discussed in relation to its likely impact on turbulence and plankton ecology.