Phvtoplankton Distribution in Early Spring 

 May 1979 



The CZCS pigment image (46-2) shows the high diversity of 

 phytoplankton distribution in the coastal and offshore region from Cape 

 Hatteras to the Gulf of Maine. The region can be subdivided into four 

 areas, based on pigment content, as identified in the locator map (46-1). 

 Area 1 consists of shallow coastal waters (less than 25 m) which extend 

 from Cape Hatteras to Cape Cod and over Nantucket Shoals and Georges 

 Bank. In this area, the pigment concentrations are generally greater than 

 2 mg m'-l Area 2 consists of deep coastal waters which e.xtend from the 

 edge of the Continental Shelf, circumnavigating Georges Bank, and into 

 the eastern regions of the Gulf of Maine. Pigment concentrations in this 

 area range from 0.2 to 0.5 mg m'^. These are generally lower than the 

 values found on either side of this area. Area 3 encompasses all slope and 

 basin waters greater than 100 m deep, with the seaward limit bemg the cold 

 wall of the Gulf Stream. It is in this area where the largest diversity in 

 pigment concentration occurs: ignoring the intrusive features, the pigment 

 content ranges from 0.5 mg m ' to values in e-xcess of 2 mg m'\ Area 4 is 

 characterized by the most prominent feature observed in the image: the 

 frontal cold north wall of the Gulf Stream. This strong frontal feature 

 extends from Cape Hatteras meandering eastward to the waters of the 

 open North Atlantic. 



Some of the Gulf Stream meanders form eddies which are large rotating 

 cells of water composed of Gulf Stream. Sargasso Sea. and slope water. A 

 warm-core ring, seen in Area 3, is a good example. This is an anticyclonic 

 eddy consisting of a central core of Gulf Stream or Sargasso Sea water 

 surrounded by slope water. A cold-core ring (cyclonic eddy) is also seen in 

 the lower part of the image. This ring was spawned from an encirclement 

 of coastal water by Sargasso Sea or Gulf Stream water. The difference in 

 the pigment content of the water masses, which compose the eddies, allows 

 them to be identified by ocean color. The four pigment-content areas are 

 clearly observed in the enlargement (47-1). 



The high diversity of phytoplankton distribution can be explained in 

 terms of variation in the growth of phytoplankton, which is regulated by 

 light and the vertical mixing in the water columns. The amount of vertical 

 mixing regulates the mean light intensity reaching phytoplankton which, 

 in turn, controls their growth. A simple rule of thumb is that water 

 columns which are deep and well-stirred, which is the case of those in 

 Area 2. will promote less phytoplankton growth than those where the 

 mixing depth is retarded by the shoalness of the bottom, which is the case 

 in Area 1. The energy for stirring of these water columns can come from 

 water movements by tides or from other sources across the bottom and/or 

 wind blowing across the surface of these waters. In shallow waters, the 

 tidal components may be the most important during this period. Area 3 

 differs from Areas 1 and 2 in that the bottom depths are greater and hence 

 the frictional forces which stir the column are much less. In Area 3, the 

 surface layers become more buoyant than the underlying waters because 

 of an increase in solar radiation coinciding with a decrease in surface wind 

 mixing. The combination of these events promotes an intense growth of 

 phytoplankton activity. Area 3 is scarred by water mass intrusions from 

 the coast and Gulf Stream. The fluid motions associated with these 

 intrusions tend to inhibit or destroy buoyancy which essentially retards the 

 onset of the phytoplankton blooms. 



46-1. Locator Map for 46-2 and 47-1. 



46-2. Nimbus-7. Orbit 2701. 7 May 1979. CZCS Pigment Image. 



46 



i, 



