INDICATOR SPECIES 



This gradually merges into the cold deep water below the North Atlantic 

 Drift and there occur also: 



Saqitta iiuwroccvhala I 



. J> arrow- worms 



Saqitta zctcsios J 



Gactaiius pilcattis copepod 



Aiiialopc'iuwiis clcqaiis decapod 



SpiratcUa lielicoides mollusc 



With information of this type we can, as an example, trace the inflow 

 and mixture of Atlantic water into the North Sea from the north. The 

 currents and their immense significance in the biology of the seas have already 

 been briefly dealt with in Chapter 8 : west of the British Isles we fnid the main 

 mass of Atlantic water with the cosmopolitan oceanic plankton includmg the 

 indicator species mentioned under 3 above. Just beyond the edge of the shelf 

 there is the Lusitanian Stream; some years it is hardly present at all but in 

 other years quite marked. Where it is present we find the indicator species 

 under 4 above mixed with the others, not replacing them, hi an ordinary 

 year these can be clearly traced west of Ireland and Scotland and continuing 

 close to the edge of the shelf and so into the Faroe-Shetland Channel. The 

 plankton selected for the photographs produced as Plate XXXIX were 

 taken from this mixed oceanic water mass. The indicator species gradually 

 die out ('/; route according to their tolerance and usually only the hardiest 

 members will reach near Orkney or Shetland. In years when the water 

 movement is unusually strong, e.g. in 1953 and 1954, they can be found in 

 the North Sea itself. As the stream flows so close to the edge of the shelf it 

 gradually upwells into the shallower water to mix along the north coastal 

 area of Scotland towards Orkney. It is important to realize that the total 

 volume of water in this Lusitanian stream is very small indeeci compared with 

 the main water mass, even west of Ireland, but it can be traced because of its 

 plankton long after its physical characteristics have disappeared. 



The main Atlantic inflow passes through the Faroe-Shetland Channel, 

 taking with it the more cosmopolitan oceanic species, but again mixing on 

 the way so that the less tolerant species die out and are replaced by those 

 species which need the mixed conditions. A mixed water species, such as 

 Sagitta ekgaus or Mctridia liiceiis, found off the west Atlantic coasts thus in- 

 dicates a mixture of coastal water with the Atlantic, but the same species in 

 the North Sea, Irish Sea or other shelf waters indicates the mixing of oceanic 

 water. 



Because the origin of the plankton species is linked with their breeding, 

 the indicators tend to come in groups at the same time each year, giving a 



145 



