PHYTO- AND ZOOPLANKTON INTERRELATIONS 279 



influence over the herring and the pelagic animals upon which they feed, as very little 

 animal life was found amongst it ". The fact that the herring in the East Anglian autumn 

 fishery are not feeding (Hardy, 1924; Savage, 193 1) shows that the effect on the fish can 

 be a direct one. It should be mentioned that it has long been a tradition amongst herring 

 fishermen that they will not catch herring in what they call "stinking" or "weedy" 

 water, and such water has been shown to be due to dense phytoplankton. 1 



Fish (in 1925) published important observations on the correlation of zoo- and phyto- 

 plankton in the Woods Hole region of America. He writes as follows: 



Normal diatom maxima have no noticeable effect on the larger planktonic forms. When the un- 

 usually large swarms of phytoplankton appear, however, the zooplankton decreases rapidly and may 

 even totally disappear for a time. Such conditions are often found during the summer maxima of 

 Rhizosolenia semispina. Usually the winter maxima do not affect the larger forms. In the winter of 

 1922-3 the phytoplankton and zooplankton were both abundant at the same time. At this time 

 Rhizosolenia alata was the dominant diatom. In 1923-4 Nitzschia seriata occurred in such abundance 

 that the zooplankton disappeared almost entirely from November 16 until February 1. During this 

 period top and bottom collections in the shallow water of the bay and sound yielded nothing but 

 diatoms. The zooplankton was found to be fairly abundant in the deeper waters at the western end 

 of the Sound. As soon as the diatoms declined in numbers the larger forms returned to the shallow 

 water. 



His curves showing the relative abundance of zoo- and phytoplankton are of great 

 interest. The coincident abundance of the two in the winter of 1922-3 is for a short 

 space of time at the end of December and beginning of January. This might possibly 

 be of significance in that this is the time of minimum light intensity and so minimum 

 photosynthesis. 



In his general conclusions Fish writes : 



The pelagic diatoms exert a very great influence on the zooplankton. When the greatest maxima 

 appear most of the zooplanktonic forms disappear. There are possibly two reasons for this. First, 

 the common species having these swarming periods do not form the food of the zooplankton so far 

 as I have been able to determine. During the maxima of the larger diatoms the smaller members of 

 this group which are eaten by pelagic animals disappear, causing a scarcity in the food supply. This 

 may account for the similarity in the time of disappearance of the larger forms and the small diatoms. 

 Second, the great numbers of the diatoms filling the water apparently cause conditions unfavourable 

 for animal life of any sort. The macroplankton seems to be literally choked out. This, however, is 

 hardly probable, and is offered merely as a possible explanation. 



In our survey it is the smaller forms of diatoms, such as the very minute Chaetoceros 

 socialis, that have an inverse correlation with the zooplankton as much as the larger forms. 



In this review of earlier work mention should also be made of the observations of 

 Herdman (1907) upon the vertical distribution of the zoo- and phytoplankton, which is 

 quoted in full on p. 324. He showed that when the diatoms were more abundant in the 

 bottom waters the animals were at the surface and vice versa. 



There are then two ways of considering this inverse correlationship between the plank- 

 tonic plants and animals. The view first put forward by Castracane in 1885 that possibly 

 the phytoplankton is abundant because of a shortage of animal plankton to keep it in 



1 See Addendum note on p. 364. 



