. The depths of the ocean; a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. Oceanography. VI PELAGIC PLANT LIFE 379 104 35-37 found abundance of plankton in April 1910, algae being present in large quantities as deep down as they have been known to occur, that is to say as far down as sufficient light penetrates. We can appreciate the difference between these conditions and the conditions in coastal areas like the Christiania fjord, if we remember that the nutritive substances in the
. The depths of the ocean; a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. Oceanography. VI PELAGIC PLANT LIFE 379 104 35-37 found abundance of plankton in April 1910, algae being present in large quantities as deep down as they have been known to occur, that is to say as far down as sufficient light penetrates. We can appreciate the difference between these conditions and the conditions in coastal areas like the Christiania fjord, if we remember that the nutritive substances in the first case may rise up from the deep water, while in the second they are derived from the surface through the admixture of fresh water. Vertical circulation is regulated by differences in tempera- ture at the surface, due to summer and winter, which are sufficient to in- crease the density ^ ^^ J^, J^ of the upper layers till it equals the density lower down, and if cir- culation is to have any effect in the open sea, the sur- face-layers must 150 be able to sink to a depth of at least 200 to 300 metres. The greater the difference in tern- 25° perature between summer and win- ter, the more effective will ver- tical circulation generally be. Assuming, then, that our view is correct, namely that plant production in the sea is mainly regulated by the amount of dissolved nutritive substances, we must expect to find plankton produced in abundance in coastal areas to which large rivers convey nourishment from the land, and in oceanic areas where vertical circulation takes place on a large scale, or where ascending currents bring up the deeper water-masses. Where vertical circulation is the controlling influence, the greatest profusion will be at seasons when the temperature of the surface reaches its minimum ; that is to say, generally in winter, or in higher latitudes in the early months of spring. It would be possible to test the truth of this theory if we could. 200m Fig. 252.—
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