64 



REDFIELD, KETCHUM AND RICHARDS 



[CHAP. 2 



Thomsen (1931) and is discussed by Riley (1951) in terms of the circulation 

 pattern. The accumulation of nutrients in the deep water is limited by the ex- 

 change through the Strait of Gibraltar. A steady-state condition exists when 

 the income of nutrients in the upper layers of the strait equals the outgo in 

 the deeper layers. Accumulation in the deeper layers can only develop to the 

 point where this condition is met, since any further increase in the concentra- 

 tion in the outgoing water will lead to a net loss of nutrients from the sea as a 

 whole. 



Fig. 13 shows that, within the Mediterranean, the concentration of phosphate 



Fig. 13. Distribution of phosphorus in Black Sea, Mediterranean and off-lying Atlantic 

 Ocean (diagramatic). Arrows indicate direction of currents in the Bosphorus and 

 Strait of Gibraltar. Contours, phosphate phosphorus in mg atoms/m 3 . Depths in 

 meters. 



decreases progressively in both the sub -surface and deep water as one goes 

 eastward in the direction of flow of the surface layers. This is the relation to be 

 expected in an anti-estuarine circulation. 



The circulation of the Black Sea is similar in character to that of a fjord. It 

 receives the discharge of the great rivers of eastern Europe of which the excess 

 escapes through the Bosphorus as a strong surface current. Beneath this flow a 

 counter current transports more saline water from the Mediterranean into the 

 Black Sea. Since the Bosphorus is less than 100 m deep and the deeper part is 

 occupied by an inflowing current, the deep water of the basin cannot escape 

 except by mixing upward into the surface layers. Nutrients accumulated in 

 such deep water re-enter the biochemical cycle when brought to the surface 

 and are returned by sinking as organized matter to the depths (see Caspers, 

 1957). 



The estuarine circulation of the Bosphorus thus produces a trap which 

 hinders the loss of nutrients from the Black Sea and promotes their accumula- 

 tion. In contrast, water from the depths of the Mediterranean may escape 

 through the Strait of Gibraltar without re-entering the euphotic zone. 



A factor which must contribute to the greater accumulation of regenerated 

 nutrients in the Black Sea, as contrasted with the Mediterranean, is the stability 

 of the water column. In the Black Sea the accessions of fresh water at the surface 

 produce strong density gradients in the upper layers which retard the diffusion 

 of deep water into the levels at which escape across the sill is possible. In con- 

 trast, the deeper water of the Mediterranean is homogeneous up to depths well 



