Horizontal Ocean Currents
As an outcome of the phytoplankton collections taken by the Challenger from the various seas of the world, it appeared that in general no essential differences existed between the pelagic flora of different seas; but later, as a result of the more detailed plankton work of the German Plankton Expedition, Schütt concluded that different ocean currents are inhabited by characteristic types of floating plants.
Cleve (1900) was of the opinion, after studying extensive plankton collections from many separate areas, that plankton species endemic to separate geographic areas (each with its own set of environmental conditions) are transported from these areas by water currents to remote regions, depending upon the hydrographic conditions prevailing. Accordingly, the procedure of investigation should be first, to determine the geographic distribution of the separate species and to learn the conditions of life required by the principal plankton associations. Then, according to the theory, the origin of various water masses could be determined by a biological analysis of the waters to ascertain the type of plankton organisms they support. Cleve recognized a number of plankton communities with dominant types characteristic of separate ocean regions wherein the water has certain uniform characteristics with respect to the physical characters, but the hope of utilizing the diatom plankton types as indicators of water origin has not been fully realized. The nature of the life history of diatom populations, their prompt response by increase or decrease to fluctuating conditions of nutrients and other variable environmental factors, must make them of less value as direct indicators of ocean currents than are the planktonic animals with longer, more tenacious lives. Certain species of Ceratium have proved of special value in characterizing water masses, for example, the fluctuating flow of Arctic and Atlantic waters around Newfoundland (p. 865).
The diatom plankton types of Cleve are of great significance, however, as biological types (p. 793), each with different requirements of life which make them respond in succession to changes of season as living conditions become favorable to their specific needs.
From the strictly biological aspect, we are especially interested in the dispersal of species that is brought about by means of ocean currents. The widespread distribution of most species is evidence in itself of the operation of this type of dispersal. Many species are common to widely separate areas where living conditions are similar during at least a portion of the year. The diatom Asterionella japonica was not recorded from the waters of Romsdalsfjord, Norway, during investigations covering the
It is not uncommon to find small numbers of neritic diatoms that have been carried far seaward with outgoing currents. They may even reproduce for a time by fission but they gradually become abnormal, with weakly silicified shells and diminutive in size; some may form resting spores which drift about. Just what are the oceanic factors that are unfavorable to neritic species is not clearly known. It may be a matter of nutrition or salinity, though, with respect to the latter, many neritic species are doubtless euryhaline.
The most popularly known drift of marine plants is that of the higher alga Sargassum. Although naturally an attached littoral plant, Sargassum may be torn from its moorings along the shore and float far to sea with the currents. Under these conditions it thrives vegetatively but is unable to reproduce except by fragmentation. Its tendency to accumulate in the slowly flowing cyclonic circulation in the North Atlantic has given the name Sargasso Sea to that relatively quiet halistatic area. Commonly, other littoral seaweeds and animals live attached to or among the branches and gas-filled floats of the Sargassum.