Component of ECOHAB-GOM
|Possible Factors Affecting Alexandrium Dynamics in
the Northern Gulf of Maine
David W. Townsend, Neil R. Pettigrew and Andrew C. Thomas, University of Maine
Another observation reported some time ago in this region of the Gulf lead us to suspect that the Eastern Maine Coastal Current/Plume system was intimately involved with the distribution of Alexandrium along the shoreline of Maine, New Hampshire and Massachusetts. This observation was the "sandwich" phenomenon, where it was noted that a stretch of the Maine coast between the western edge of Penobscot Bay and an area east of Mount Desert Island, Maine, seldom displayed Paralytic Shellfish Poisoning (Figure 3; Hurst and Yentsch, 1981; Yentsch et al., 1986; Shumway et al., 1988). If indeed, as suggested by Martin and White's (1989) results, Alexandrium distributions were highest in the offshore waters away from the immediate shoreline, then it would follow that the offshore-directed surface flow that is commonly observed in the "sandwich" region (Brooks and Townsend, 1989; Bisagni et al., 1995; Pettigrew et al., 1998) is likely tied in some way to processes that effectively exclude Alexandrium from affecting shellfish on these shores. As for why Alexandrium reaches such high cell densities offshore, we suspected nutrients were key, since these eastern Gulf of Maine waters have the highest surface concentrations of inorganic nutrients of any area of the Gulf of Maine region (Townsend et al., 1987).
Northern Gulf of Maine Component of ECOHAB-GOM
Discussion of Early Results
In addition to seeking a better understanding of the interconnectedness of the eastern and western Gulf of Maine waters with respect to transport pathways of Alexandrium, our group is most interested in understanding the possible environmental factors that allow bona fide blooms of Alexandrium to develop, especially blooms with cell densities as high as those reported by Martin and White (1988; >100,000 cells per liter). It has already been shown that species of Alexandrium, and most large dinoflagellates in general, have high light and high nutrient affinities (Epply and Thomas, 1969; Eppley et al., 1969). Thus we have begun to explore the possible relationships between Alexandrium cell densities and these two variables. One example of our initial analyses is given in Figure 10 for our June survey cruise (see data updates at:/ECOHAB/Data-Updates-Eastern-Maine.html). We computed a nondimensional parameter that presents the suitability of light and nutrient fields as they might relate to a phytoplankton population that requires high levels of each. Our approach was to take the ratio of the depth of the 10% surface illumination (based on our PAR profiles at daytime stations) to the depth of the 4 µM nitrate concentration (based on a polynomial fit to each station profile). The result of this analysis for our June cruise, when we saw high abundances Alexandrium over broad areas of the offshore Gulf waters, is given as a surface contour plot (Figure 10). The dark areas in Figure 10 outside of the 0.5 contour line (e.g., with ratios <0.5) represent regions where the combined light and nutrient environments would be expected to be less than optimal for Alexandrium growth. The early light-nutrient model shows promise and generally explains the highest cell densities. The model will be developed further as we incorporate parameters for horizontal advection, and the seasonal changes in water column stratification and the ambient light levels.
We view these results as very encouraging but we do stress, however, that we have completed only the first of three field sampling seasons and that as such our results presented here are still somewhat preliminary. Nonetheless our confidence is growing that we are on the right track and that through continuing analyses of our extensive data sets and with two more field seasons before us we will begin to unravel some of the dynamics of Alexandrium populations in the Gulf of Maine.
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Figure 1. Concentrations of Alexandrium in surface waters of the Bay of Fundy and eastern Maine. Note the highest concentrations (>106 L-1) in the outer Bay of Fundy and in a band roughly coincident with the core of the EMCC (from Martin and White, 1988).