CH. VIl] THE DISTRIBUTION OF THE PLANKTON 147 



day when the sky is overcast is also to be attributed to the 

 heliotropism of the planktonic organisms^. 



Temperature changes in the surface layers of the sea produce 

 vertical plankton movements either because the organisms are 

 affected by these per se, or because convection currents are set up, 

 or because changes in the viscosity of the sea water are the results 

 of temperature changes. Apart altogether from general seasonal 

 changes, which affect large sea areas simultaneously, we find that 

 comparatively rapid changes in the temperature of restricted areas 

 of the sea are the result of local changes in the direction of 

 the wind, or are simply diurnal changes. The daily range of 

 temperature at the surface is from 1 to 2° C, the surface being 

 hottest at about 4 p.m. and coldest at about sunrise. Much more 

 considerable changes are produced by changes in the direction of 

 the wind. In the Irish Sea an east wind lasting several days will 

 cool the surface water several degrees, and a south to south-west wind 

 will as rapidly warm it. The change produced by the wind does 

 not only affect the surface, but will also affect the lower layers, for 

 the superficial film of water is rapidly removed by the force of the 

 wind and a new layer is exposed. If the wind is blowing offshore 

 the surface water is drifted away from the land and deep water 

 flows inshore as an undercurrent to take its place. Conversely if 

 the wind is blowing on-shore, surface water is banked up at the 

 beach and then flows away as an undercurrent. If the wind lasts 

 for some time then all the water in a moderately deep sea will 

 by-and-by be cooled or warmed as the case may be. Ultimately 

 then a uniform mixture is produced, but for a short time after the 

 change of wind the temperature difference between the surface and 



1 Gough (Fishery and Hydrographic Investigations in the North Sea and 

 Adjacent Waters, Report 2, Southern Area, Cd. 2670, 1905, p. 345) has made some 

 instructive observations of these movements due to light changes and temperature 

 changes. He made a number of hauls in the English Channel during the 24 hours 

 by vertical and horizontal nets. The samples were taken every two hours. The 

 composition of the hauls made by the vertical nets did not vary much during the 

 whole period of day and night. But the horizontal hauls shewed considerable 

 differences. Euchilota pilosella, Obelia sp. and Tomopteris helgolandica came to 

 the surface only at night. Calanus finmarchicns swarmed at the surface during 

 the hottest part of the day, but at night it dispersed through all layers of the sea. 

 Next day it did not come to the surface, but the weather had changed, the sk}' was 

 overcast and a strong wind was blowing. 



10—2 



