Publication Date:
2022-07-21
Description:
Plankton pump samples and plankton tows (size fractions between 0.04 mm and 1.01 mm) from the eastern North Atlantic Ocean contain the following shell- and skeleton-producing planktonic and nectonic organisms, which can be fossilized in the sediments: diatoms, radiolarians, foraminifers, pteropods, heteropods, larvae of benthic gastropods and bivalves, ostracods, and fish. The abundance of these components has been mapped quantitatively in the eastern North Atlantic surface waters (Fig. 4) in October - December 1971. More ash (after ignition of the organic matter, consisting mostly of these components) per cubic meter of water is found close to land masses (continents and islands) and above shallow submarine elevations than in the open ocean. Preferred biotops of planktonic diatoms in the region described are temperate shallow water and tropical coastal upwelling areas. Radiolarians rarely occur close to the continent, but are abundant in pelagic warm water masses, even near islands. Foraminifers are similar to the radiolarians, rarer in the coastal water mass of the continent than in the open ocean or off oceanic islands. Their abundance is highest outside the upwelling area off NW Africa. Molluscs generally outnumber planktonic foraminifers, implying that the carbonate cycle of the ocean might be influenced considerably by these animals. The molluscs include heteropods, pteropods, and larvae of benthic bivalves and gastropods. Larvae of benthic molluscs accur more frequently close to continental and island margins and above submarine shoals (in this case mostly guyots) than in the open ocean. Their size increases, but they decrease in number with increasing distance from their area of origin. Ostracods and fish have only been found in small numbers concentrated off NW Africa. All of the above-mentioned components occur in higher abundances in the surface watet than in subsurface waters. They are closely related to the hydrography of the sampled watet masses (here defined through temperature measutements). Relatively warm water masses of the southeastern branches of the Gulf Stream system transport subtropical and southern temperate species to the Bay of Biscay, relatively cool water masses of the Portugal and Canary Currents carry transitional faunal elements along the NW African coast southwards to tropical regions. These mix in the northwest African upwelling area with tropical faunal elements which are generally assumed to live in the subsurface water masses and which probably have been transported northwards to this area by a subsurface counter current. The faunas typical for tropical surface water masses are not only reduced due to the tongue of cool water extending southwards along the coast, but they are also removed from the coastal zone by the upwelling subsurface water masses carrying their own shell and skeleton assemblages. Tropical water masses contain much more shelland skeleton-producing plankters than subtropical and temperate ones. The climatic conditions found at different latitudes control the development and intensity of separate continental coastal water mass with its own plankton assemblages. Extent of this water mass and steepness of gradients between the pelagic and coastal environment limit the occurrence of pelagic plankton close to the continental coast. A similar water mass in only weakly developed off oceanic islands.
Type:
Article
,
PeerReviewed
Format:
text
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