220 POPULATIONS OF THE SEA 



tions were also towing Plankton Recorders. Each dot or symbol 

 indicates the position of a Recorder sample. The presence of three 

 species is shown. It is clear that the separate distributions could 

 be explained in terms of the tolerance of these species to environ- 

 mental conditions or of their recruitment from different water 

 masses. The boundaries between the water masses corresponded 

 to the boundaries between the distributions of the plankton. Two 

 of the species were confined to the off-shore oceanic water. One 

 of them {Calanus minor) was restricted to the warmer part and 

 the other {Pleiiromamma) was found mostly in the colder end of 

 the oceanic water but with some o\'erlap into the warm water. 

 The third species, Sagitta elegans, occurred over a wide tempera- 

 ture range but only in the mixed shelf water; it is, of course one of 

 the best-known indicators of the presence of Atlantic coastal 

 water in the North Sea. None of these three species was found in 

 the central North Sea during these two months. This is a very 

 simple example, taken from Bary (1959b) and is chosen to il" 

 lustrate the potentialities of this method of expressing the shape 

 of distribution. 



Concluding Remarks 



It would be tempting at this stage to consider other techniques 

 by which distributions could be related to various characters of 

 the environment. It is impossible to generalize, however. A great 

 variety of valuable work is already being carried out, and different 

 techniques meet the needs of different situations. A few examples 

 will suffice. Brandhorst (1958) has shown an inverse relationship 

 between the depth of the thermocline and the size of the standing 

 crop of plankton in the eastern tropical Pacific. A little further 

 west, in the central Pacific, King (1954) has shown that upwelling 

 (resulting from the equatorial divergence) provides nutrients for 

 the phytoplankton and, through the zooplankton, for the tuna 

 which is so plentiful in this region. In the benthos, Vinogradova 

 (1959) has related zoogeographical regions to physical features 

 such as underwater ridges. Taylor et al. (1957) have shown the 

 long-term relationship between climatic trends and the distribu- 



