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We investigated how environmental factors initiate Heterosigma akashiwo and Skeletonema costatum blooms from resting stages in bottom sediments in a shallow port over 2 yr. Using field-collected sediments, we also conducted laboratory experiments on how light intensity affects germination of resting stages and growth of the germinated cells. Both phytoplankton species bloomed only in summer, when water temperature and solar radiation were high enough for growth. All three blooms of H. akashiwo and the earliest bloom of S. costatum in a year occurred right after transmission of strong light (.200 mmol quanta m 22 s 21 ) to the bottom layer and a peak occurred in dissolved inorganic phosphorus (DIP). In the laboratory, resting stages of H. akashiwo and S. costatum germinated even in dim light (20 and 65 mmol quanta m 22 s 21 , respectively), but germinated cells required stronger light of .130 and 280 mmol quanta m 22 s 21 , respectively, for rapid growth. This value is much higher than the threshold for survival, and is higher than the half-saturating light intensity for growth of vegetative cells. Abundance of the resting stages of both species in the sediments rapidly increased during blooms and logarithmically decreased during nonbloom periods, suggesting that resting stages are continuously consumed. For both species, our results suggest that blooms initiate when transmission of sufficient light permits: first, germination of cells from the sediment; second, rapid growth of these germinated cells. Temperature and DIP must also exceed a facilitating threshold.

The goal of this study was to examine the significance of allelopathy by the raphidophyte Heterosigma akashiwo in a multispecies phytoplankton community in the field. Towards this aim, we sought allelochemicals of H. akashiwo, which had allelopathic effect both in laboratory experiments and in the field. As an initial step, we showed that the allelopathic effects of H. akashiwo filtrate were both species-specific and dependent upon the cell density of the target species. Secondly, we found for the first time that extracellular, high-molecular weight allelochemicals [that is, polysaccharideprotein complexes (APPCs)] were produced by a marine phytoplankton species, H. akashiwo. Thirdly, we indicated that the purified APPCs selectively inhibited the growth of the diatom Skeletonema costatum that is a major competitor of H. akashiwo, and thereby tended to promote the formation of monospecific H. akashiwo blooms. Furthermore, we demonstrated that the inhibitory effect of APPCs on the growth of the diatoms was determined by binding to the cell surface of the target species. Finally, we succeeded in the detection of APPCs in the field samples at concentrations exceeding their experimentally determined action threshold during the H. akashiwo bloom. Strategies for ecosystem control, including mitigation of harmful algal blooms (HABs), should take into account that red-tide organisms like H. akashiwo are already part of complex webs involving inter-specific allelopathic inhibition and ecosystem control during their dense blooms.

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 1-1 (Chi in mg m J). 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.