The movements and interactions of water and cells on scales of a few kilometers to a few micrometers is an active research area in the HAB field. At the larger end of this spectrum lies the explanation for enhanced phytoplankton biomass at hydrographic features such as fronts of various types. This enhanced biomass is the result of the interaction between physical processes such as upwelling, shear and turbulence, and physiological processes such as swimming, enhanced nutrient uptake, or biochemical adaptation. Examples of the importance of fronts in phytoplankton bloom dynamics are many, and several prominent studies involve HAB species. Pingree, Simpson and co-workers have demonstrated the linkage between tidally generated fronts and the sites of massive blooms of the toxic dinoflagellate Gyrodinium aureolum [ Pingree et al., 1975; Simpson et al., 1979] in the North Sea. The pattern generally seen is a high surface concentration of cells at the frontal convergence, contiguous with a subsurface chlorophyll maximum which follows the sloping interface between the two water masses beneath the stratified side of the front. The surface signature of the chlorophyll maximum (sometimes a visible red tide) may be 1-30 km wide. Chlorophyll concentrations are generally lower and uniform on the well-mixed side of the front The significance of this differential biomass accumulation is best understood when movement of the front and its associated cells brings toxic G. aureolum populations into contact with fish and other susceptible resources, resulting in massive mortalities [ Holligan, 1979]. This is an example where small-scale physical/biological coupling results in biomass accumulation, and larger-scale advective mechanisms cause the biomass to become harmful.
The biological and physical mechanisms that account for enhanced phytoplankton biomass at fronts are diverse, as they must be to account for the different physical and chemical gradients associated with frontal regions. Many suggested mechanisms relate to differences in nutrient availability and light. High nutrient concentrations are often found at or below the pycnocline, but the rate of supply of nutrients across this interface to the accumulating cells is not well known, nor has the relative importance of recycled versus þnewþ nutrients been quantified with respect to biomass accumulation. Physical processes which can influence nutrient exchange across a front include interfacial friction and Ekman pumping [ Garvine, 1974; Garrett and Loder, 1981], baroclinic eddies [ Simpson, 1981], and tidal stabilization/destabilization [ Demers et al., 1986].
If cells behaved as passive, neutrally buoyant particles, they could not accumulate. Thus for HAB species, many explanations of enhanced biomass accumulation invoke the swimming behavior of the organisms as a mechanism to take advantage of the gradients in nutrient availability in the frontal region. This may manifest itself as vertical migration in which the cells traverse the top layer of the water column in a daily pattern of directed swimming (e.g. Heaney and Eppley, [1981]). Eppley et al., [1968] suggested that downward migration of dinoflagellates at night into deeper, nutrient rich layers at or below the pycnocline allows them to increase their uptake of nitrate, which could then be transformed into cell biomass during photosynthesis when the cells reside in illuminated surface waters during the day. Subsequent work has confirmed that deep nutrients are indeed utilized by some nutrient-starved dinoflagellates (e.g. Cullen et al., [1985]). Alternatively, swimming may simply allow a species to persist at some optimum depth in the presence of vertical currents. One striking observation about G. aureolum blooms is that highly concentrated subsurface layers of cells can sometimes be only tens of centimeters thick ( O. Lindahl, personal communication). The behavioral and physiological strategies that lead to these accumulations are important unknowns.
Clearly, more is involved in the accumulation of cells at fronts and other interfaces than simple enhanced nutrient uptake. Other mechanisms which may be involved include photoadaptation, the inhibition of growth due to wind-driven turbulence, or the different chemical composition of water masses comprising the frontal system. Of special interest is the relative importance of water chemistry (e.g. micronutrients that stimulate growth) versus behavior and hydrography (that physically concentrate cells) as factors responsible for the observed elevated cell concentrations in buoyant plumes and estuarine fronts. Furthermore, Thomas and Gibson, [1990] provide evidence that cell division of one red tide dinoflagellate species is directly affected by levels of turbulence similar to those expected from moderate wind stress at the water surface. If the concept of microscale effects of turbulence on HAB species is a general phenomenon, and if the inhibitory levels of shear are shown to be realistic for conditions commonly encountered in the surface mixed layer, the patchy distribution of HAB species in frontal regions may reflect a microscale level of physical interaction. Before such inferences can be drawn, however, several fundamental oceanographic questions must be addressed. For example, direct physical measurements of turbulence at the scale of the individual algal cell are needed during blooms to determine if natural conditions can in fact cause growth inhibition. Likewise, multiple HAB species must be screened for their susceptibility to turbulence, and the actual physiological mechanism for the turbulence inhibition determined so that sensitive species can be identified. Here again, the practical implications of the basic research on these questions are large with respect to our ability to understand and predict the patterns of blooms.
In general, the physical and biological features of frontal regions have often been studied in isolation, whereas major advances in our understanding of HAB phenomena require cooperative studies in which the distribution, physiology, and behavior of the algae are studied at the same spatial and temporal scales as the physics. In the latter instance, high-resolution measurements of flow and density fields at the interfaces where enhanced algal biomass is observed would provide important insights, especially if they are incorporated into coupled physical/biological models which allow the relative importance of the processes described above to be evaluated.