Geoscience Reference
In-Depth Information
these elements together, blasting nutrients from the sea bottom into the sunlight, or photic zone, thus kick-
starting the chain of events that creates the classic marine food web: phytoplankton, shrimp, herring, whales.
Phytoplankton, or algal blooms occur under special conditions, whenever there is an adequate supply of
both nutrients and sunlight. In summer the surface waters of the gulf are warm, but the nutrients are located in
its deep basins. As the amount of sunlight declines, however, the surface waters begin to cool, and as they do,
they grow heavier and begin to sink. This process, known as convection overturn, has been compared to turn-
ing over a garden, bringing nutrients from the deeper layers toward the surface. When these nutrients meet the
light, there is an explosion of growth—an algal bloom. This occurs in the spring and fall in the central basins
of the gulf, with dramatic results. Wherever you find phytoplankton blooms you will also find fish, seabirds,
and whales feeding.
The link between the primary producers and the higher marine life forms is the zooplankton, a mixed group
of organisms that includes tiny crustaceans, worms, molluscs, and larval fish. The most dominant group is the
copepods, small shrimplike crustaceans numbering 7,500 species globally and occurring on land and in fresh-
water as well as in marine habitats. Copepods are generally the dominant group in marine zooplankton com-
munities, where they primarily graze on diatoms and other large phytoplankton by filtering them from the sea-
water. They can also be carnivorous, however, preying on protozoa and other smaller zooplankton, including
larval fish. Although they are able to swim freely, they cannot make headway against a current. But zooplank-
ton undergo daily vertical migrations, a defensive behavior designed to prevent predators from seeing them.
(Older animals—especially egg-bearing females—may also gain an energetic advantage by spending their
days in deeper waters, because their rate of oxygen consumption drops so much.) During the day they concen-
trate in deeper, dimmer water, and at night they migrate into the upper water layers, where their phytoplankton
food items are more abundant. Gulf of Maine fishermen call the reddish-brown adults “cayenne” when they
appear in great swarms on the surface.
Although the Gulf of Maine as a whole is one of the most productive marine regions in the world, Georges
Bank is particularly so, since it is shallow and has a ready supply of minerals and nutrients on its dunelike
seabed. For its size, Georges may be the richest marine area in all of the temperate and high latitudes of the
world's oceans. It is so productive that one Russian trawler captain described it in his memoirs as “an oceanic
miracle.”
Since the early part of the 20th century, there has been a concerted fishery throughout the year on Georges
Bank, which has revealed the seasonality of fish movements. Traditionally, there was a winter fishery for fresh
haddock and other mixed species, a spring halibut fishery that often began in late winter and extended into the
summer months, a summer fishery for cod, and a swordfish fishery in June, July, and September. Since World
War ii, however, the fishery for the sedentary scallop has become dominant economically.
Georges's nurturing capacity is obvious even to the casual observer. I spent a week on a scallop dragger on
the bank, and at any given time I might see whales feeding or shark or swordfish fins knifing the waves, and
always hundreds of shearwaters (breeders from the Southern Hemisphere summering in the gulf during the
austral winter) skimming the waters—all signs of the fishy riches lurking below the surface. When the scallop
drags were dumped onto the decks, a rogue's gallery of sea-bottom creatures was revealed: yellow-and-
orange-spotted deep-sea skates and rays, conger eels as thick as a grown man's arm, monkfish—“sea mon-
sters” with enormous gaping mouths for swallowing prey whole—as well as various benthic invertebrates, in-
cluding the sought-after scallops and a by-catch of lobsters and groundfish such as haddock.
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