Earlier today we finished our survey of Georges Bank (see map). Surface waters are virtually devoid of Alexandrium cells, with a non-zero finding at only one station. During the survey we occupied a pump station at the FDA shellfish monitoring site on Cultivator Shoal, despite a live count of zero in that location. Given the near-absence of Alexandrium on the bank, we decided not to occupy the final transect of the survey on the Northeast Peak in order to leave more time for our coastal survey. We are currently in transit westward to commence the coastal survey at the Cape Ann line.
As we leave Georges Bank for the final time during GOMTOX, we have been reflecting on what has been learned thus far. Data from this cruise suggest that termination of the Georges Bank Alexandrium bloom in 2010 took place at approximately the same time as in 2008—- and both years exhibit what appears to be a seasonal bloom from May to August. Moreover, some clues to the striking interannual variability are beginning to emerge. Water mass analysis suggests that the times when Alexandrium was most abundant on Georges Bank (May, June 2007; June 2008), the water was relatively cold and salty. In contrast, low Alexandrium abundance was characterized by waters that were relatively warm and fresh. These warm and fresh anomalies have at least two potential origins. The drastic decline of the Alexandrium population from June to July 2008 was accompanied by a warm and fresh anomaly that was confined to near-surface waters, and analysis of the climatological mean temperature and salinity fields suggest an origin in the western Gulf of Maine. In 2010, a warm/fresh anomaly was present at depth on Georges Bank, as well as in the deep basins of the Gulf of Maine.
To first order, the GOMTOX findings are consistent with a “leaky incubator” model for Alexandrium populations on Georges Bank. When Georges Bank is relatively isolated from its surroundings (cold and salty), Alexandrium populations thrive. When large volumes of warm and fresh waters flow onto Georges Bank, two factors potentially hinder Alexandrium populations: (1) dilution of the Georges Bank population with low-Alexandrium water, and (2) inoculation with relatively low-nutrient water that is unfavorable for Alexandrium growth. We will see if this straw-man conceptual model holds up to further scrutiny when the full data sets are in hand.
In the meantime, its off to the coastal Gulf of Maine!
On behalf of the OC467 Science Party