Chapter 5 



Ecological Systems and Transport 



55 



sea which has been found by Worthington 

 (1954) in the North Atlantic, provides one 

 means of computing the total quantity of or- 

 ganic matter required. Worthington observed 

 a decrease of 0.3 milliliters of oxygen per liter 

 in thirty years at depths between 2500 meters 

 and the bottom. In the Atlantic Ocean this 

 corresponds to an average thickness of 1500 

 meters and the total quantity of organic matter 

 required to produce this change in oxygen is 

 equivalent to the decomposition of 8 grams of 

 organic carbon per square meter per year in 

 this layer. This quantity of organic matter is 

 nearly 15 per cent of the annual mean produc- 

 tion according to Steemann Nielsen (1954) 

 and from 1.4 to 7 per cent of the mean sug- 

 gested by Riley (1944). Part of the secular 

 change in oxygen may have been produced by 

 eddy diffusion into the oxygen minimum layer, 

 which would reduce the quantity of organic 

 carbon reaching greater depths. 



The effects of gravity may be accentuated 

 when the surface currents are opposed to the 

 currents in the deeper layers. This type of 

 circulation pattern is very common in estuaries, 

 on continental shelves, and in those areas where 

 offshore winds produce upwelling of the deeper 

 waters. In all of these cases the nutrient rich 

 deep water is carried inshore in a sub-surface 

 drift, and brought to the surface by upwelling 

 or vertical mixing. The nutrients are assimilated 

 by the plankton in the surface layers and are 

 carried offshore in the surface current. "When 

 the organisms sink, they again reach the on- 

 shore sub-surface current where they decompose 

 liberating more nutrients into water which is 

 already relatively rich. Thus the elements in- 

 volved in biological processes follow a different 

 cycle from the circulation of the water and 

 this cycle leads to an accumulation of elements 

 greater than can be found in either of the source 

 waters (Strom, 1936; Hulburt, In press). 



Nutrient elements are commonly concentrated 

 by this type of mechanism in fjords. Where the 

 deepest water is relatively stagnant and isolated 

 from the intermediate and surface layers, con- 

 siderable concentrations of organic derivatives 

 can be developed. In the Norwegian fjords 

 with a relatively shallow sill, for example, 

 anaerobic conditions may be produced in the 

 bottom water and the nutrients are five to ten 

 times as concentrated as in either of the source 

 waters (Strom, 1936). In the Black Sea the 



deep water is isolated from the surface by a 

 strong density gradient and its average age has 

 been estimated at 2500-5000 years (Sverdrup, 

 Johnson and Fleming, 1942, p. 651). Very 

 large accumulations of organic derivatives are 

 found in this deep water. (Gololobov, 1949.) 



Opposed currents can, however, work in the 

 opposite way and lead to a decrease in the 

 concentration of elements involved in the bio- 

 logical cycle. The classic example of this type 

 of circulation is the Mediterranean, where the 

 nutrients available for plant growth are less 

 than half of the concentration available in the 

 adjacent parts of the Atlantic. In the Mediter- 

 ranean the supply comes from the surface wa- 

 ters of the North Atlantic which are already 

 impoverished by plant growth. Since evapora- 

 tion exceeds precipitation in the Mediterranean 

 the water becomes more saline, sinks and is 

 lost as a deep outflow over the sill at Gibraltar 

 (Thomsen, 1931). The accumulation of ele- 

 ments in sinking organisms transfers these ele- 

 ments from the inflowing surface water to the 

 outflowing deep water. They are eventually 

 lost from the Mediterranean. A similar process 

 apparently applies to the entire North Atlantic. 

 There is a large inflow of South Atlantic sur- 

 face water which contains low concentrations 

 of elements involved in the ecological cycle. 

 The outflow from the North Atlantic required 

 to balance the water budget occurs at depths 

 and this water contains considerable quantities 

 of the elements which had been returned to the 

 water (Sverdrup et al., 1942). 



In summary the various peculiarities of dis- 

 tribution which can be attributed to gravitational 

 effects on the ecological cycle are therefore 

 (1) the accumulation of elements at inter- 

 mediate depths as a result of sinking and de- 

 composition, (2) the concentration of elements 

 in areas of opposed flow where the deep water 

 is brought to the surface by upwelling or ver- 

 tical mixing and (3) the impoverishment of 

 areas where the supply of water is from the 

 surface and the loss from greater depths. 



In addition to the passive gravitational effects 

 on organisms, animal plankton forms exhibit 

 vertical migrations. A considerable literature 

 has developed in this field over the last ten 

 years, but the effects of these vertical migrations 

 on the distribution of elements has not been 

 studied directly and must be inferred from our 

 knowledge of the ecological system. 



