Blooms of the toxic dinoflagellate, Alexandrium fundyense in the Casco Bay region of the western Gulf of Maine: Advection from offshore source populations and interactions with the Kennebec River plume

The Casco Bay region, an embayment adjacent to the Kennebec River, has been suggested as a source region for Alexandrium fundyense bloom development in the western Gulf of Maine (GOM). In this study, shipboard observations were acquired within Casco Bay and the nearby coastal waters during the spring of 1998 and 2000. In the early bloom season, low A. fundyense abundances (<100 cells l−1) were observed within the bay, sometimes isolated from A. fundyense populations observed in adjacent coastal waters. When high abundances of A. fundyense (>500 cells l−1) were observed within Casco Bay, they were contiguous with coastal populations observed within the Kennebec/Penobscot river plume and within offshore waters of the western segment of the Maine Coastal Current (WMCC). This general distributional pattern occurred during both study years. Wind directly affected the pathway of the incoming coastal populations. Downwelling-favorable winds generally facilitated bloom formation (and outbreaks of shellfish toxicity) within Casco Bay by enhancing the connection with offshore populations via alongshore and onshore transport of cells from the upstream coastal waters. In contrast, persistent upwelling-favorable winds were associated with low A. fundyense cell abundances (and shellfish toxicity) in Casco Bay by slowing the advance of the coastal population and shifting it offshore with the Kennebec plume front. The striking difference between late season (June) population abundances of the two study years can be explained by a combination of the wind pre-history and interannual differences in large-scale (Gulf-wide) circulation patterns, as evidenced by higher salinities in the coastal waters in 2000 vs. 1998. Advection of A. fundyense cells into Casco Bay and retention, not local growth within the Bay, are likely the dominant processes that typically result in the accumulation of high populations and shellfish toxicity in the Bay. A variety of mechanisms (e.g., circulation underneath or steerage around the Kennebec plume) promote the transfer of cells across the Kennebec/Penobscot plume barrier and into the Bay. These dynamics are complex given the variability of the wind, river inputs, and the Maine Coastal Current structure. Nonetheless, general distributional patterns of A. fundyense and the associated hydrography clearly demonstrate that populations within Casco Bay are not isolated, but instead are part of the large-scale coastal populations that inhabit the western GOM, likely originating from further upstream in the coastal flow. With that knowledge, the ultimate goal of predicting outbreaks of shellfish toxicity in the western GOM based on meteorological and hydrographic conditions may become a reality.