Plant Nutrients in the Sea

Abstract

IN a recent issue of Nature¹, Prof. J. Ritchie speculates "upon a day . . . when the International Fisheries Commission of the nations bordering the North Sea may discuss, along with its programme of researches, the allocation of sums to be contributed by each nation for chemical nutrients in the assurance that these will support a larger fish population and an increased fishing fleet in the North Sea". Prof. Ritchie surely does not realize the vast quantities involved. The English Channel is not a specially rich fishing ground, but calculations based upon the annual phosphate cycle show that the minimum value for the phytoplankton crop is about 1,400 metric tons wet weight per square kilometre each year². The annual phosphate cycle involves the complete removal of this substance from the surface waters and a lesser removal in the deeper water, on account of the reduced illumination. But under each square metre the water column yields to vegetation about 1 gm. of phosphorus, equivalent to about 7 gm. of calcium phosphate. Thus each square kilometre of sea has an annual turnover of about seven tons. To put any appreciable fraction of this quantity into the English Channel would involve a vast expenditure. Moreover, such addition would soon be dissipated into the ocean.