Toward a theory of biological‐physical control of harmful algal bloom dynamics and impacts

Abstract

A growing body of laboratory, field, and theoretical work suggests that the dynamics of harmful algal blooms and their impacts on other organisms are frequently controlled not only by physiological responses to local environmental conditions as modified by trophic interactions, but also by a series of interactions between biological and physical processes occurring over an extremely broad range of temporal and spatial scales. All too frequently, major gaps in our ability to identify, measure, and model the underlying biological and physical processes and their interactions over the appropriate temporal and spatial scales have prevented the quantitative assessment of the importance of these factors in causing past blooms and the development of predictive models of bloom dynamics and impacts. For these reasons, we have combined fluid continuity equations with a conservation equation for population dynamics to quantify how biological and physical processes and their interactions affect the population dynamics of harmful algae and their potential impact on other organisms. Applications of the resulting numerical and conceptual models to toxic algal blooms in upwelling systems and pycnocline layers suggest not only that bloom dynamics and impacts are sensitive to biological-physical interactions occurring at multiple scales, but also that such interactions may be critical components of the life-history strategies of these organisms.Algal blooms that result in harm to other organisms vary dramatically in the relative importance of biological and physical processes and their interactions in controlling their dynamics and impacts. At one extreme are high biomass blooms of nonmotile algae that form in poorly flushed, nutrient-enriched coastal waters when a species out-competes other species for light and nutrients and(or) escapes control by grazers. Although physical conditions set the stage for such blooms, physiological responses to local conditions and trophic interactions control the persistence and impacts of the harmful algal bloom (HAB). For example, although the persistent Aureococcus anophagefSerens bloom in Laguna Madre seems to have been triggered by an unusual freeze that killed other trophic levels and released nutrients, the bloom has persisted for 5 yr by out-competing other algae and suppressing grazing (Buskey and Stockwell 1993). At the other extreme are blooms of motile algae that appear in stratified, physically dynamic coastal waters far more rapidly and(or) reach concentrations far higher than can be explained by local growth in response to average conditions in the region where impacts are observed (see Ryther 1955;Seliger 1993). In such systems, bloom dynamics and impacts seem to be dominated by interactions between biological and physical processes that occur over a broad range of temporal and spatial scales. Good examples of this are the blooms of Prorocentrum mariae-Zebouriae that occur in fronts in upper AcknowledgmentsThis work was supported by grants from ONR Biological and Chemical ...

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