Diversity of harmful algal blooms in coastal waters Bloom dynamics of toxic AEexandrium species in the northeastern U . S . L 3

Coastal waters of the northeastern U.S. are subject to rccurrcnt outbreaks of paralytic shclllish poisoning (PSP) caused by toxic dinoflagellates in the genus Alexandrium. PSP is not uniform across the large region, but instead reflects Alexandrium growth and toxin accumulation in five separate habitats or zones defined by circulation patterns and the discontinuous distribution of the dinollagellatcs. Each of these habitats has a unique set of environmental and oceanographic forcings that dcterminc the timing and extent of bloom development and transport and that regulate the extent of genetic cxchangc with adjacent populations. Several habitats (e.g. the southwcstcrn Gulf of Maine, Massachusetts Bay, and Georges Bank) arc linked hydrographically and may share the same Alexandrium population via large-scale transport in a coastal current, whereas the other two habitats (eastern Maine and southern salt ponds-embaymcnts) seem to be isolated and have little or no hydrographic or genetic linkage to adjacent regions during bloom seasons. My paper provides an overview of the regional ecology and oceanography of Alenandrium through a focus on these hvc subpopulations. Issues that relate to PSP and AZenandrium dynamics throughout the world are highlighted, including species dispersal, the role of cysts and “initiation zones” in bloom dcvclopment, and the influence of largeand small-scale hydrography on population development and transport. The ability of Alexandrium to colonize multiple habitats and to persist over a large region is emphasized in recognition of the adaptability and resilience of this important organism. In many parts of the world, paralytic shellfish poisoning (PSP) is a recurrent and serious problem associated with blooms of toxic dinoflagellates in the genus Akxandrium. The potent neurotoxins produced by these organisms are accumulated by filter-Feeding shellfish and other grazers and are passed on to humans and other animals at higher trophic levels, leading to illness, incapacitation, and even death. AZexandrium causes toxicity in many different hydrographic and climatic regimes, from temperate to tropical (e.g. Cembella et al. 1988; La Barbera-Sanchez et al. 1993). This ecological distribution reflects a genetic diversity that is far more extensive than simple morphology might imply. Scholin et al. (1994, 1995) clearly demonstrated that even though a single morphospecies such as Alexandrium tamarense can be found in widely different geographic zones, isolates from those different regions have unique genotypes based on ribosomal RNA sequences. Acknowledgments Special thanks to D. M. Kulis and B. A. Keafer for many years of field and laboratory assistance underlying much of the work prcscnted here. Thanks also to J. Hurst and D. Whitakcr for providing shellfish toxicity data. This research was supported by grants from the National Science Foundation (OCE 94-15536), the National Oceanic and Atmospheric Administration (NA36RM0190) through the Gulf of Maine Regional Marine Research Program, the National Sea Grant College Program Office (NA90-AA-D-SG480; WHOI Sea Grant Project R/B12 1), and the Massachusetts Water Resources Authority. Contribution 9389 from the Woods Hole Oceanographic Institution. Another reason for growth success across such a variety of habitats is that many Alexandrium species have a cyst stage in their life histories. This allows the organism to remain dormant in bottom sediments through temperature extremcs, with seasonal germination inoculating vegetative cells into the water column only during intervals where temperature and light are suitable for growth. Population development is thus possible in more locations than would otherwise be the case if year-round persistence in the plankton were the only means for survival. PSP is a relatively new phenomenon in the northeastern United States (see below), but is now recurrent and widespread, affecting vast expanses of the Gulf of Maine coastline (Anderson et al. 1.982). PSP outbreaks and the causative Alexandrium blooms are not uniform throughout the region, however. Rather, there are several discrete hydrographic rcgimes or habitats that support Alexandrium blooms, each with a unique set of environmental and oceanographic Forcings. Genetic exchange across these boundaries variessome habitats arc virtually isolated, whereas others exchange cells frequently. In all of these habitats, Alexundrium blooms are seasonal and recurrent in nature. Given the temperate climate of the region and the general absence of Alexandrium cells and toxins during winter (e.g. Anderson and Morel 1979; Shumway et al. 1988), blooms are heavily dependant on benthic cysts for re-inoculation. The mechanisms of bloom initiation and the quantitative importance of cyst seedbeds differ among habitats, however, and toxicity episodes are variable with respect to magnitude, frequency, duration, and type (Anderson et al. 1994).