Blooms of the toxic dinoflagellate Alexandrium fundyense are annually recurrent phenomena in the Gulf of Maine during the spring and summer months. Toxins produced by A. fundyense accumulate in the tissues of filter-feeding shellfish such as mussels and clams. Human ingestion of these contaminated shellfish can lead to Paralytic Shellfsh Poisoning (PSP), a potentially fatal illness. Understanding the factors that determine the distribution and abundance of A. fundyense is therefore of considerable economic and public health interest. Existing data and models demonstrate strong interconnections between A. fundyense populations and the Gulf of Maine coastal current system. Indeed, coupled physical-biological models of population dynamics and hydrodynamic transport are able to simulate some of the general features of the observed A. fundyense distribution. However, such models are sensitive to the choices of the input parameters (e.g. growth rate, swimming speed, germination rate, etc). Curiously, the ability of the models to match the observations occurs in a range of this parameter space that is relatively narrow with respect to the envelope of reasonable values defined by laboratory and field measurements. One possible explanation for this apparent inconsistency is genetic heterogeneity in the natural populations. We hypothesize that the aggregate distribution of A. fundyense is composed of a mosaic of genetically distinct subpopulations, each with different physiological and/or behavioral responses to environmental conditions. The goal of this project is to understand the hydrodynamic and biological controls on these populations, their toxin production, and how these factors ultimately determine fluctuations in shellfish toxicity.