210 POPULATIONS OF THE SEA 



ably fairly close to Smayda's classification. But, with the excep- 

 tion of a few organisms with very restricted distributions, none 

 of these terms can be applied too rigidly to the plankton. It is 

 for reasons such as these that Fig. 2 has been prepared as a con- 

 tinuous series with the addition of the noncommittal terms 

 oceanic, neritic, and intermediate. 



Hitherto, planktonic indications of water movement have 

 usually been sought from the evidence about a few relati\'ely rare 

 organisms. But there is an appreciable hazard in the deduction of 

 water movements or other environmental changes, from observa- 

 tions of a single species. The distribution and abundance of an 

 organism reflect many factors of mortality and survival, or ag- 

 gregation and dispersal, of which water movement is only one. 

 It seems likely that some of these difficulties would be reduced 

 b}^ studying the total species composition, and it should be possi- 

 ble to make use of information about the common species which 

 make up the bulk of the plankton and which are, perhaps, most 

 important in studies of fluctuations in the commercial fisheries. 

 Abramova (1956) studied the abundance and species composition 

 of the plankton as part of an ecological study of the herrings of 

 the North Atlantic. She described cold and warm water forms in 

 the plankton and was able to demonstrate the value of a wide 

 variety of species in providing biological indications of water 

 movement. 



Indications of environmental change or water movement in 

 difi"erent parts of the Recorder Sur\ey would be provided by 

 groups of organisms selected from different parts of the series. 

 For example, in studying conditions in the northern North Sea, 

 groups of organisms at the oceanic end of the intermediate series 

 would be compared with species from the upper end of the neritic 

 series. In Glover (1957), I attempted to use the species balance of 

 the plankton in this way. Figure 3 shows my earlier results brought 

 up to date. 



During the past ten years samples were collected, with the Hardy 

 Plankton Indicator, by fishermen taking part in the summer her- 

 ring fishery in the northwestern part of the North Sea, a fishery 

 which lies in the path of the inflow of water from the Atlantic. From 



