C.3.1 Alexandrium Biogeography. The Alexandrium species distribution is not uniform in the GOM (Fig. 1). Cells are broadly dispersed from the Bay of Fundy to Massachusetts Bay (Anderson et al. 1994), but become sparce between Cape Cod and New Jersey, the southern biogeographic limit of Alexandrium in eastern North America. Two closely related species are linked to PSP within the GOM: A. tamarense and A. fundyense (Anderson et al. 1994). Within the eastern GOM, all species examined to date have been A. fundyense, whereas both species are found in western waters. (For convenience, the term Alexandrium will be used hereafter to refer to this species complex).

Five habitats have been identified in which Alexandrium blooms occur within the GOM (Fig. 2). Some habitats have early onset (e.g. Cape Cod salt ponds), others have mid-season (WMCC) or late-season (EMCC) outbreaks. One goal of ECOHAB-GOM is to determine the ecological and hydrographic mechanisms that underlie these differences, focusing on the largest and most significant regional habitats - the EMCC and the WMCC. Georges Bank will also be studied because of its rich shellfish resources and its location "downstream" as a potential "sink" for toxins produced in the two coastal current habitats.

C.3.2 Alexandrium Biology. Under normal conditions, Alexandrium cells divide asexually to produce daughter cells, forming "blooms". Nutrient limitation induces sexual reproduction in which gametes form and fuse to produce zygotes that fall to the sediments as resting cysts - dormant cells that overwinter and germinate to initiate new blooms (Anderson and Wall, 1978). The cyst stage can be critical in bloom dynamics: the location of "seed beds" can determine sites for bloom initiation, and cyst germination and formation can influence bloom initiation and decline. Unfortunately, quantitative Alexandrium cyst dynamics have received little scientific attention, especially in open coastal waters.

C.3.3 The Circulation of the Gulf of Maine. Alexandrium species distribution and patterns of PSP are closely linked to the large- and small-scale circulation of the GOM. The overall circulation (Fig. 1) tends to be cyclonic (Bigelow 1927; Brooks 1985), with southwestward flow along the coast of Maine. The near-surface currents are influenced by inflow from the Scotian shelf, which provides cold, low-salinity water to the northeastern Gulf. This flow continues past the mouth of the Bay of Fundy and feeds the EMCC, with additional input of freshwater from the St. John River and other sources. The EMCC bifurcates near Penobscot Bay, with a portion branching seaward and continuing around the Jordan Basin gyre and the remainder continuing down the coast to feed the WMCC (Brooks and Townsend 1989). Little is known about this bifurcation, whether there are times when the EMCC follows exclusively one route or the other, or the seasonal variation in the trajectory of the flow. Nor are the dynamics well understood, for instance how wind forcing and freshwater inflow from the Penobscot affect the trajectory. A better understanding of the EMCC trajectory is important with regard to seed populations and transport pathways of Alexandrium cells or cysts from Canada and eastern Maine, to western Maine, Massachusetts, and Georges Bank. To the west of Penobscot Bay, the along-shelf flow becomes re-established as the WMCC (Geyer et al., submitted; Franks and Anderson 1992a). This current is driven in part by Gulf-scale circulation and by the freshwater sources, notably the Penobscot, Kennebec and Androscoggin Rivers, and is strongly influenced by winds (Franks and Anderson, 1992a; Geyer et al., submitted). The influence of the WMCC on Alexandrium has been studied extensively (e.g., Anderson and Keafer 1992, Franks and Anderson 1992a,b;) and provides a major framework for this study. The WMCC intermittently feeds Massachusetts Bay, and thence continues past Cape Cod, bifurcating to feed Georges Bank to the east and the mid-Atlantic Bight to the west. This connection implicates the WMCC Alexandrium populations in the PSP observed on Georges Bank.

C.3.4 Alexandrium and the EMCC. A substantial PSP database exists for the EMCC, but little is known of Alexandrium motile cells or cysts. The Bay of Fundy, to the north, has severe PSP and both cell and cyst numbers are extremely high (White and Lewis 1982). Patterns of offshore toxicity and Alexandrium distributions are unknown for the EMCC, except for a 1980 report (Martin and White 1988) of a continuous band of Alexandrium cells from Canada to central Maine (Fig. 3). These observations, in addition to reports of highly toxic offshore shellfish populations (Shumway et al, 1988), implicate the EMCC as a transport pathway and Canadian waters as a source of cells. The influence of the EMCC on Alexandrium blooms has not been established, but is likely to be significant in population growth and transport. A related issue is a feature called the "PSP sandwich", a term used by Shumway et al. (1988) to describe a toxin-free zone at the end of the EMCC near Penobscot Bay, bounded on the east and west by areas with recurrent PSP (Fig. 2). (PSP has been detected in the Penobscot area once in 24 years). No Alexandrium cysts have been found there (Lewis et al. 1979). The best explanation for the toxin-free anomaly is that the EMCC deflects from shore near Penobscot Bay (Fig. 1; Brooks and Townsend 1989), carrying cells into offshore waters. Those cells can be carried back towards shore and join the WMCC (a linkage we will explore), but are more likely to remain offshore or be entrained into a gyre that would bring them back to downeast Maine. Pettigrew (unpub.) documented a drifter track that would take cells in the EMCC directly to Georges Bank, bypassing the WMCC.

Tidal mixing is another important process within the EMCC. Tidal ranges >6m create high tidal current velocities and significant mixing. This may be important with respect to cyst resuspension and bloom initiation, and also because fronts often form in the zone between tidally mixed and thermally stratified waters (Townsend et al. 1987), and these are often sites of dinoflagellate accumulation (Franks 1992). Alexandrium blooms within the EMCC region may thus be linked to the formation, persistence, movement, and breakdown of fronts.

In summary, we are faced with several plausible hypotheses concerning the mechanisms underlying the recurrent outbreaks of PSP in the EMCC. A major objective of ECOHAB-GOM is to document the distribution and abundance of Alexandrium in the EMCC region and to identify the oceanographic mechanisms that regulate population dynamics and patterns of PSP.