18. BIOLOGICAL SPECIES, WATER-MASSES AND CURRENTS 



M. W. Johnson and E. Brinton 



1. Introduction 



Oceanographers have long been aware of the existence in the oceans of areas 

 that can be more or less clearly defined as "water-masses". These areas are 

 recognized because of individual characteristics, particularly with respect to 

 the temperature-salinity relationship as established by T-S curves for sub- 

 surface water. For fullest discussion the reader is referred to The Oceans 

 (Sverdrup, Johnson and Fleming, 1942). 



It is also recognized by marine biologists that of the vast array of marine 

 organisms many are sensitive to small changes in environmental conditions 

 during all or part of their life history. Salinity and temperature are outstanding 

 factors that are most clearly demonstrated as environmental variables having 

 critical ranges for the lives of many species. In some species, notably the oceanic, 

 the range of tolerance for fluctuations in these factors is relatively small. Such 

 species are known as either stenohaline or stenothermic or both, and the 

 limitations may be most pronounced with respect to reproduction, or during 

 the larval stage. Usually the adult vegetative stage is the most tolerant. Ranges 

 of tolerance may be within different salinity or temperature limits. Thus, 

 an animal may be stenohaline to either high or low salinities. Although the 

 coastal animals are usually considered the more tolerant to environmental 

 changes, still many of these have characteristic limits for reproduction, as 

 shown by investigations carried out by Runnstrom (1927) on thermal relations. 

 There are also other factors at play that are less well understood which in- 

 fluence endemic distribution, such as non-conservative chemical elements, 

 inter-specific competition and duration of maximum or minimum temperatures. 



It is to be expected, then, that bodies of water with distinctive characteristics, 

 such as water-masses, will produce environments for distinctive faunas. This is, 

 indeed, what has been found to be the case in many instances where detailed 

 studies have been made. The preference of certain species for North Sea waters 

 and of others for the adjacent oceanic waters provide classical examples 

 (Russell, 1939). Thus, a biological means is provided to aid in the identification 

 and delineation of water-masses or to trace the source, direction and extent of 

 water currents, and the isolations and pathways they may have provided in 

 speciation of the organisms. It is increasingly realized that the main faunistic 

 regions of the high-oceanic pelagic fauna (certain features of which were 

 heralded by Steuer, 1933) are more strongly characterized taxonomically than 

 they were formerly believed to be (cf. Ekman, 1953, p. 319). 



Before discussing representative investigations showing a relationship of 

 species distribution with specific water-masses or currents, it will be useful 

 first to consider some generalizations as to the types of organisms that can best 

 be used as "indicators", and to consider some of their limitations. 



Although the geographic distribution of benthic or bottom-living animals is 

 doubtless often dependent directly upon the type of water and the nature of 



[MS received July, 1960] 381 



