BULK-PHASE VISCOELASTIC PROPERTIES OF SEAWATER - RELATIONSHIP WITH PLANKTON COMPONENTS

The viscous and elastic moduli at different shear rates, together with various biological oceanographic properties, were determined in seawa ter from different hydrological layers in the southern North Sea in Ju ne. The biological oceanographic parameters included Phaeocystis and N octiluca abundances, chlorophyll a level (Chl), bacteria, HNAN and agg regate volume fraction. The plankton was jointly dominated by Phaeocys tis sp. and Noctiluca scintillans. Noctiluca abundance showed no corre lation with any other biological or viscoelastic parameter, but Phaeoc ystis abundance correlated strongly. The other biological parameters c orrelated with Phaeocystis and with each other positively and mostly s ignificantly. Overall, viscoelasticity correlated more strongly with C hi than with any other biological parameter. For non-microlayer sample s, the excess complex (viscoelastic) modulus (mu Pa) G(E)= 2.0 x Chi( 1,3) (Chl in mg m(-3)). Viscous and elastic moduli also correlated clo sely with each other. For a given value of Chi, the microlayer samples were 6.5 or 14 times (depending on the estimation method) more viscoe lastic than in bulk-phase samples. Viscoelasticity in samples of settl ed benthic 'fluff' were lower even than bulk-phase samples, but this d ifference was not significant. Comparison with Mediterranean data on v iscoelasticity (Jenkinson, Oceanol. Acta, 16, 317-334, 1993), using pu blished values for phytoplankton biomass (Wiadnyana, J. Rech. Oceanogr ., 17, 16, 1992), suggests that the relationship between Chi (or phyto plankton biomass) and viscoelasticity might be general. This apparent biomodification of the viscosity and elasticity of seawater is discuss ed in relation to its likely impact on turbulence and plankton ecology .