Models of dinoflagellate blooms have been developed from several different perspectives. [ Kamykowski, 1979, 1981] examined the response of a swimming dinoflagellate to internal waves and showed that accumulation of motile and non-motile cells may occur due to an internal wave field, with the accumulation of vertically migrating cells being most significant. These models consider only the physics of the wave field and the swimming behavior of the phytoplankton, without regard to the phytoplankton response to nutrients or light. Others have examined the response of phytoplankton to the flow field of Langmuir cells [ Evans and Taylor, 1980; Watanabe and Harashima, 1986] or to 2-dimensional, cross-frontal circulation [ Franks, 1992], to name just two of many physical systems that have been studied in this theoretical context.
The growth and accumulation of individual harmful algal species in a mixed planktonic assemblage are exceedingly complex processes involving an array of chemical, physical, and biological interactions. Our level of knowledge about each of the many HAB species varies significantly, and even the best-studied remain poorly characterized with respect to bloom or population dynamics. Resolution of various rate processes integral to the population dynamics (e.g., input and losses due to growth, grazing, encystment, excystment, and physical advection) has not been accomplished, but is fundamental to the long-term management of fisheries resources or marine habitats affected by harmful algae. Many of the processes are difficult to quantify in the field because harmful species are often only a small fraction of the biomass in natural samples. The end result is that despite the proven utility of models in so many oceanographic disciplines, there are no predictive models of population development, transport, and toxin accumulation for any of the major harmful algal species in the United States. There is thus a clear need to develop realistic physical models for regions subject to HAB events, and to incorporate biological behavior and population dynamics into those simulations.
The primary method for exploring the details of the interactions of HAB populations with coastal circulation should be through the incorporation of biological and physical field data into circulation models. The physical dynamics of the model can be constrained by the equations used to describe the flow and by physical data gathered in field programs. Numerical experiments can then examine the distribution and fate of cells under a variety of forcing mechanisms. HAB cells can initially be treated as passive tracers, but it will most likely be necessary to include factors such as grazing or physiological adaptation in the models to accurately simulate observed cell distributions. It should then be possible to show, for instance, under what conditions directed swimming behavior will cause cell accumulation and blooms, or to evaluate the relative importance of physical losses (advection, sinking) and biological losses (grazing, cyst formation) in bloom termination. The flexibility of such models allows certain aspects of the field data to be explored in a way that would be impossible through direct sampling. Another advantage is that the behavior and physiology of a single HAB species can be better understood and simulated than that of an entire community. This is an area long neglected in HAB research, and one that should be strengthened in the coming years. The insights to be gained from modeling studies will do much to advance our general understanding of the dynamics and consequences of HABs.