D.— ZOOLOGY 95 



quently yield an abundant plankton. Or, secondly, though deficient in 

 phosphate, it may bring in large quantities of phytoplankton or zoo- 

 plankton, the product of a former richness in phosphate. This plankton 

 will afford an immediate food-supply for larval fish and other animals, 

 and when it dies down the phosphate will be regenerated and will serve 

 for further plankton production in the future. 



It is thus what we may call the biological condition of the water that 

 is of importance, and this no doubt is to some extent determined by the 

 season of the year. At times, in summer, the surface water may be 

 largely devoid of both plankton and phosphate and an influx of such 

 water, even though its high salinity may indicate an oceanic origin, will 

 bring no improvement to biological conditions and may indeed be harm- 

 ful. In winter, when the thermocline has broken down and surface 

 phosphate has been renewed by convection and by stormy weather, an 

 influx may prove of advantage. But it is perhaps more probable that 

 upwelled water, rich in the nutrient salts which are always to be found 

 in the lower layers of the ocean, is the potent source of surface enrichment, 

 and of the conditions in which such upwelling occurs we are very largely 

 ignorant. We lack the necessary data and can merely speculate on what 

 may be happening from analogy with what is known in other areas. 



Some twenty-five or thirty years ago Mr. D. J. Matthews ^^ published 

 a series of papers on the physical conditions in the English Channel and 

 adjacent waters, and to this day his work remains one of our principal 

 and most valuable sources of information. He showed that to the south 

 of Ireland there is an extensive cyclonic or counter-clockwise circulation, 

 which may at times reach as far south as 48^° N., and nearly a quarter of 

 a century ago he suggested that this circulation might prove of consider- 

 able biological importance. * If the strength of the cyclonic system varies 

 from year to year, so will the character of the water at any place within 

 its influence, such as the areas of the drift-net fishing off the mouth of 

 the English Channel and off the south coasts of Ireland.' There can 

 scarcely be a doubt that the vagaries of this circulation have a profound 

 effect on conditions in the Channel. If we possessed, as unfortunately 

 we do not, a continuous series of observations on the oceanographic 

 conditions to the south of Ireland and on the edge of the continental 

 slope, the variations in this cyclonic system could be traced, and even 

 though it might then appear that the ultimate causes of the present 

 depression are linked with changes in the Atlantic circulation, and thus 

 still out of reach, the information which would be gained would un- 

 doubtedly throw new light on the problem. 



I have dwelt at some length on these events in the Plymouth area 

 because they afford a good example of a long-period fluctuation and 

 illustrate the way in which observations drawn from widely different 

 lines of inquiry are linked together. From other sources also there is 



1' D. J. Matthews, ' Report on the Physical Conditions in the Enghsh Channel 

 and adjacent Waters in 1903 : in 1904 and 1905 : in 1906,' Internal. Invest. Alar. 

 Biol. Assoc, Rep. I, 1905 ; Rep. II, part ii, 1909 ; Rep. Ill, 1911. ' The Surface 

 Waters of the North Atlantic Ocean, South of 60° N. Lat., Sept. 1904 to Dec. 

 1905,' ibid., Rep. II, part i, 1907. ' The Salinity and Temperature of the Irish 

 Channel and the waters South of Ireland,' Fisheries, Ireland, Sci. Invest., 1913, 

 iv. 1914. 



