The OC465 coastal survey consisted of a series of transects spanning from just south of Boston to one off Isle au Haut (see map). Surface live counts revealed two population centers of Alexandrium with concentrations in excess of 1000 cells per liter: one southwest of Penobscot Bay, and the other northeast of Cape Ann. These two areas are divided by consistently low concentrations in all but the outermost station of the Casco Bay line—an aspect that we find curious. In any case, the overall resurgence of the western Gulf of Maine Alexandrium population came as quite a surprise, given very low concentrations observed during OC460 (May 1-10) and EN476 (May 26-June 4). We had attributed the unexpectedly low concentrations to a warm and fresh water mass anomaly. Lo and behold, OC465 hydrography reveals the water mass anomaly has lessened, with intermediate and shallower waters having become saltier in the month since our last cruise (see figure). We are very eager to learn if the nutrient environment has also shifted, and how that may have affected vegetative growth of Alexandrium. Data from Dave Townsend’s lab will provide insight into that aspect. In any case, it appears that retreat of the water mass anomaly allowed the Alexandrium population in the western Gulf of Maine to re-establish itself. Another potentially related factor is advection by the coastal current. Initial returns from drifters deployed off Casco Bay suggest the along-coast velocities may have increased from their earlier sluggish state, facilitating increased transport of Alexandrium populations into the western Gulf of Maine.
Ironically, after two months of overpredictions, the forecast model is now underpredicting cell concentrations in the western Gulf of Maine. Clearly this is the year of surprises in the western Gulf!
-on behalf of the OC465 science party