Spring 2006 Update: Fall Chlorophyll Distribution

map map

A general pattern is evident where chlorophyll concentration is greatest in continental shelf waters, intermediate over the deeper slope water, and lowest in the vicinity of the Gulf Stream and Sargasso Sea (see upper map figure showing concentration of chlorophyll during September 2005 from Cape Hatteras to Nova Scotia). High levels of chlorophyll occur in the tidally mixed central areas of Georges Bank and Nantucket Shoals, and in the Middle Atlantic Bight estuaries and coastal waters enriched by estuarine plumes. The high chlorophyll values (3-8 mg m-3) in the northern Gulf of Maine indicate that fall bloom is underway during September. The same geographic range is used to illustrate how chlorophyll conditions during September 2005 compare with the average values for this month, where the average is computed from September data from 1997 through 2005 (see lower map figure showing ratio of chlorophyll concentration). The ratio of September 2005 chlorophyll to the 9-year September mean chlorophyll indicates that throughout much of the inner Middle Atlantic Bight shelf chlorophyll concentrations during September, 2005 were below (2/3rds to1/2) the levels typically observed in surface water during September (1997-2005) whereas in the deeper waters of the Gulf of Maine and over the southern flank of Georges Bank chlorophyll concentrations during September 2005 were relatively greater than those typically observed during September. In the nearshore areas, the phytoplankton fall bloom usually starts in September and peaks during October. These recent data from September 2005 suggest that while the fall bloom is underway it is somewhat diminished relative to other years, and serve to highlight the significant internal and spatial variability of primary production throughout the Northeast Shelf ecosystem.

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