Nutrient dynamcics in the Georges Bank -- Gulf of Maine region

David W. Townsend, Nathan Rebuck and Maura Thomas
School of Marine Sciences, University of Maine, Orono, ME 04469

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Abstract

The source of dissolved inorganic nutrients to the Gulf of Maine is via deep slope waters that enter through the Northeast Channel. It is those same deep-water nutrients, from the interior of the Gulf of Maine, that are mixed onto the western half of the northern flank of Georges Bank. We reviewed aspects of the nutrient data base available for analyses in the region, and presented some time-series plots. We also reviewed the limitations of those data, as well as a number of factors and processes which, individually and combined, will be important in formulating biological-physical models in the region. The main points in this presentation were:

1. Nutrient climatologies (for the purposed of model initial conditions) produced from the existing database will be in error for the colder months of the year, because of low density data coverage. Instead, we suggest that we "synthesize" a logical nutrient field for initial modeling conditions;
2. The magnitude of nutrient fluxes, first into the Gulf of Maine and then vertically upward into the euphotic zone, whether to surface waters of the broader Gulf of Maine or Georges Bank, is dependent on passage into and through the Intermediate Water layer in the Gulf;
3. Deep slope waters that enter the Gulf of Maine are presumed to be of Labrador Slope Water (LSW) origin, or Warm Slope Water (North Atlantic Central Water) origin. However, there is evidence of a layering of both those slope waters in the NE Channel, a layering that remains identifiable well into the interior of the Gulf of Maine. That is, the Gulf of Maine appears to have four, not three, water masses: Maine Surface water, Maine Intermediate Water, and 2 types of Maine Bottom Water.
4. Analyses of historical data trends over the last 10 years, using only data from samples with greater than 34 ppt salinity and > 100m depth, show a decline in temperature and nitrate, and nitrate:silicate ratios. Dramatic declines in each are also clearly seen in the 1998 LSW intrusion.
5. Silicate concentrations in the deep, salty waters exhibits a sigmoid shape over the 10 year time frame (which has yet to be explained);
6. Nitrogen limitation ultimately controls zoolplankton production on Georges Bank;
7. Alexandrium spp (the red tide dinoflagellate) was blooming on Georges Bank this past spring-summer (2007: D. Anderson, D. McGillicuddy et al., study; cell counts from Anderson's lab); The point here: species-species interactions may be important in understanding the dynamics of plankton production on Georges Bank.