332 DISCOVERY REPORTS 



shown the various catches of macroplankton organisms, which appear to exhibit ex- 

 clusion (i.e. all those except Antarctomysis), together with Calanus propinquus, Clauso- 

 calanns and Ctenocalanus, taken in the surface nets both N ioo H and N 70 H ; for each 

 organism catches of over 100 are shown as circles and those of over 50 as squares. The 

 lower figure shows similar catches but in the lower of the three nets. We see an indica- 

 tion that the poorer the phytoplankton the longer in time from midnight may organisms 

 be found in numbers at the surface. In the area of the figure bounded by the line ABC 

 there are eighteen stations, at nine of which organisms are found in numbers of over 50 

 or 100 at the surface; in the area ABD there are twelve stations, at none of which are 

 such numbers of organisms found. Yet the lower figure shows that such organisms are 

 present at these stations, though lower in the water. 



Our material was not collected with a view to showing the depth relationship of zoo- 

 plankton to phytoplankton ; it is clear that to obtain definite evidence on this point a 

 series of special hauls should be taken at different times of the day and night at a number 

 of stations in different concentrations of phytoplankton, and that the details of the 

 vertical distribution of the phytoplankton should also be studied. 



Whilst this explanation must remain for the present a hypothesis, it may be well, as 

 a possible guide for future experiment and observation, to consider various factors which 

 might bring about such a modification in vertical migration in relation to the phyto- 

 plankton. 



It is clear that the simple physical presence of a large number of plant cells in the 

 water could not in itself be the cause of such a modification. Whilst it is theoretically 

 possible to imagine plant cells so numerous in the water as to interfere with the 

 locomotion or other activities of the plankton animals, we do in fact observe the 

 animals coming up into the denser phytoplankton zones during the night but avoiding 

 them during the day. 



The actual reduction in phosphates cannot be the cause. We see different ranges of 

 phosphate values in different areas at different seasons ; the zooplankton is only corre- 

 lated with a relative reduction of phosphates and only so long as it is corresponding with 

 phytoplankton production. In the English Channel, where the phosphate content is 

 reduced to negligible quantities during the summer, the animal plankton flourishes, 

 and, as Russell has shown, may even come to the surface in the daytime. 



In 1912 a number of the plaice in a pond at the fish hatchery at Port Erin died from 

 a severe epidemic which produced an ulceration of the skin. Moore, Prideaux and 

 Herdman (1915) examined the water chemically; they found it was highly alkaline and 

 that the water was green from the presence of " floating monocellular algae and a minute 

 green flagellate Infusorian in great profusion ". They came to the conclusion that " photo- 

 synthesis by green algae causes a marked diminution in hydrogen-ion concentration in 

 sea water, and in confined volumes of water this variation in the direction of increased 

 alkalinity may act as the inducing or favouring cause for pathological conditions and 

 disease". This led them to make a study of the seasonal variations in the hydrogen-ion 

 concentration (pH) in the sea, and they found that there was a distinct increase in 



