Stellwagen Bank Final EIS and Management Plan 



Page 18 



approximately 300 meters (984 ft.) in length, 

 occmring over 8 to 10 minutes, with amplitudes of 

 up to 30 meters (98 ft.) being exhibited. 

 Overturning of the waves has also been acoustically 

 recorded over Stellwagen Bank, in deep Bay center 

 waters (80 meters or 262.5 ft.), and during 

 dissipation in shallow western bay waters. 



Phytoplankton (chlorophyll) and zooplankton are 

 carried up and down by the overturning and mixing 

 action of internal waves, causing the vertical 

 distributions of plankton to be altered. Within time 

 periods of approximately 10 minutes, displacement 

 of plankton by as much as 30 meters occurs twice 

 daily during late summer months in Massachusetts 

 Bay. Thus, hght levels experienced by 



phytoplankton cells may vary from 0.1 to 26% of the 

 ambient surface illumination. Such rapid changes in 

 hght are thought to alter fluorescent yields of plant 

 cells, affecting in turn, primary productivity of the 

 Bay generally. 



Water temperature and salinity are seasonally 

 variable in the Gudf of Maine. During winter 

 months. Gulf waters are coldest in shallow areas, 

 with little temperature differences exhibited in high 

 salinity waters along the eastern and western parts 

 of the coast. 



2. Natural Resources 



a. Phytoplankton 



The seasonal presence of more than 675 species 

 of phytoplankton has been documented in coastal 

 waters of the northeastern United States (Marshall 

 and Cohn, 1982). Although several similar species 

 are common throughout this area and throughout 

 the year, highest concentrations of phytoplankton 

 cells are associated with peak productivity (or 

 outburst) periods occurring from roughly December 

 through late March/early April. A second, less- 

 marked growth period also occurs generally during 

 July and August. In all seasons, diatoms generally 

 dominate phytoplankton species. 



Relative to Stellwagen Bank, periods of the year 

 when highest phytoplankton concentrations are 

 exhibited include: December through early April 

 (highest concentrations); and in August (Marshall 



and Cohn, 1982). The seasonal cycle of 

 phytoplankton abundance is tied to this spring 

 bloom, and is similar to those found in boreal 

 waters throughout the world. 



Phytoplankton abundance is low in the winter; 

 sparse flora are dominated by Coscinodiscus and 

 Ceratium . Spring bloom is well imderway by mid- 

 to late-March in the area between Cape Ann and 

 Cape Ehzabeth. The bloom usually starts m 

 western Georges Bank waters, and by mid-April, 

 peaks in Massachusetts Bay; in eastern coastal 

 waters off Nova Scotia; and over eastern Georges 

 Bank. By late-April, bloom has peaked over 

 southern areas of the western basin and by early 

 May over northern parts of the western basin and 

 the northern coastal waters. 



The annual phytoplankton cycle divides the Gulf 

 of Maine into two areas: 1) northern coastal belt to 

 Bay of Fundy, Georges Bank, and eastern coastal 

 area off Nova Scotia; and 2) all other Gulf of Maine 

 waters. Nantucket Shoals, Georges Bank, south- 

 western Nova Scotia, and the Maine coast northeast 

 of Penobscot Bay are identified as highly productive 

 in summer; and high biomass extends to the 

 southwest over Jeffreys Basin and Ledge. 



Thalassiosira nordenskioldii bloom for 2 to 4 

 weeks in the southwestern and eastern Gulf of 

 Maine. This bloom is succeeded by a 4 to 6 week 

 bloom of the genus Chaetoceros throughout the 

 Gulf of Maine. Following late spring, moderate 

 increases occur in phytoplankton cells during July 

 and August over the central Gulf basin. From year 

 to year, there is variation in the dominating genus: 

 diatoms may dominate the flora, and in other years, 

 Pontosphaera may replace the diatoms. 



Phytoplankton blooms may occur when critical 

 depth (above which total photosynthesis is greater 

 than total respiration) is equal to or exceeds the 

 mixed layer depth, assuming a sufficient supply of 

 nutrients. Lx)w winter phytoplankton growth in the 

 Gulf results from low light levels, which produce 

 shallow critical depth. Rising light levels in spring 

 deepen the critical depth as increased temperature 

 and fresh water runoff cause the mbced layer to 

 shoal. 



