SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 63 



water is partially renewed regularly by advection and diffusion. In others 

 seasonal variations in turbulence, stability and productivity lead to some 

 renewal of the upper layers of the deep water each winter. The bottom waters, 

 however, are renewed only periodically when sea-water crosses the sill in suffi- 

 cient quantity to displace the stagnant water from the bottom upward. Strom 

 suggests that the most general cause of renewal is the gradual warming of the 

 deep water to the point where its density becomes less than that of the water 

 entering over the sill. In the relatively long intervals between renewal, the 

 effects of stagnation gradually develop. In two basins with varying degrees of 

 stagnation, the quantity of H2S present in the bottom water increased at the 

 rate of 1.0-1.5 ml/year. 



b. The anti-estuarine circulation 



In regions where rainfall is small, evaporation from the sea surface may 

 exceed precipitation and the additions of fresh water from rivers. Under these 

 conditions a reversal of the currents characteristic of the estuarine circulation 

 may be expected, since sea-water must flow in to replace water lost by evapora- 

 tion from the surface, while water concentrated by evaporation will sink and 

 flow out along the bottom. We know of no case where estuaries of this type 

 have been well enough described to show whether the current system actually 

 reduces the nutrient content of the water, as might be expected. However, one 

 large sea, the Mediterranean, is well known to be low in nutrients and is charac- 

 terized by an anti-estuarine circulation. 



c. The Mediterranean and Black Seas 



These seas together illustrate how greatly the characteristics of the circula- 

 tion can influence the distribution of the biochemically important elements in 

 adjacent bodies of water. The Mediterranean is the most impoverished large 

 body of water known, while in the Black Sea nutrients have accumulated to an 

 extreme degree. 



In the Mediterranean evaporation exceeds the accession of fresh water to 

 such an extent that the salinity is increased by about 4%. The dynamics of the 

 situation are such that a much larger volume of water flows in through the Strait 

 of Gibraltar than is required to replace the loss by evaporation. The excess 

 escapes through the Strait as a counter current moving seaward below the in- 

 flowing surface layer. Fig. 13 shows the situation diagrammatically. The 

 nutrient content of Mediterranean water, as indicated by the phosphate 

 content, is very much lower than that of the off-lying Atlantic. This is due 

 initially to the fact that water enters the sea across a shallow sill and is thus 

 skimmed from the surface layers of the ocean which are already greatly de- 

 pleted in nutrients. This fact alone does not explain why a great accumulation 

 of nutrients does not occur in the deep water, as it does in many other basins 

 isolated by shallow sills. 



The impoverished condition of the Mediterranean was first described by 



