INDICATOR SPECIES 



origin of the water naass containing them Russell coined the term 'plankton 

 indicator species'. 



Most species, including the plankton indicators, arc much more tolerant 

 as individuals than as a race because, although they can live — according to 

 their degree of tolerance — in a somewhat changed habitat they can only 

 breed within much closer limits. This means that the dispersal of water over 

 a fairly wide area, accompanied by physical changes, can still be linked with 

 one or more closely defmed origins. 



By using a sequence of species with a range of tolerance to change, 

 followed by those suited to the new conditions, an even wider dispersal can 

 be traced. As an example we can later try to trace the sources of origin and the 

 mixing of the different water masses comprising the North Sea, but before 

 doing so we must first obtain some information about the species themselves. 



The arrow-worms, the Chaetognatha (p. 59), are one of the most useful 

 groups. There arc several species found in the area in the neighbourhood of 

 the British Isles, each readily distinguished with a little practice. Sagitta 

 setosa (Plate XXXVI top) likes a moderately low salinity, not high enough to 

 let it live in the open ocean, and not low enough to let it live in the Baltic. It 

 is found in North Sea water, in the Irish Sea, in estuaries like the Gironde, in 

 parts of the Mediterranean, the Black Sea and near Labrador. Sagitta elegans 

 (Plate XXXVI bottom) likes a rather higher salinity but this must be linked 

 with a mixture of oceanic and coastal water so that it is rare where oceanic 

 water does not reach, and it is not often found in the open ocean. It is thus 

 found in the mixed waters of the North Sea, off the west coast of the British 

 Isles, Norway, Faroe, Iceland and the American and Canadian coasts. Sagitta 

 scrratodciitata likes a high salinity but it will not thrive in very deep water. 

 Sagitta lyra prefers it warm and moderately deep. Sagitta iiiaxiiiia likes it cold, 

 and it does not matter whether it is the cold near the surface in the Arctic or 

 the cold in the deep underlying water much farther south. Sagitta niacro- 

 ccphala and 5. zetesios are only found in deep water. 



The copepods (p. 65) can be similarly used, and there are vast numbers 

 of species compared with the arrow- worms. To name a mere selection, we 

 have Lahidoccra wollastoiii in the North Sea uninfluenced by oceanic inflow, 

 Mctridia lucciis in the mixed water, Calafiiis liypcrhoiriis in the cold water, 

 Parcucliaeta harhata in the cold and deep, and Plctironiaiiima abdotninalis in the 

 warmer but fairly deep water. There are many other groups also, so that 

 when thinking along these lines the plankton must be considered as a com- 

 munity of many species of similar tastes living together. They say that one 

 swallow does not make a summer: neither does one Sagitta indicate a water 

 mass, but if accompanied by the others of the same community the inference 

 is proportionately stronger. 



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