Part
2, Sec. 2B2a
Sanctuary
Resources - Natural Resources
Phytoplankton
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
underway by mid- to late-March in the area between Cape Ann
and Cape Elizabeth. The bloom usually starts in 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.
Low 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 mixed layer to shoal.
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