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



61 



is twice the quantity found in its offshore source. The total quantity of phos- 

 phorus in the water column increases from 1 1 mg atoms/m 2 at the shallow sill 

 which divides the Gulf to 16 mg atoms/m 2 near the head of the Gulf. The 

 oxygen is reduced in the deep water in the proportions expected from the 

 JO/ZlP ratio (Redfield, 1955). 



Fig. 11. Distribution of total phosphorus and oxygen in a section along axis of Gulf of 

 Venezuela. Contour interval: Total phosphorus 0.2 mg atom/m 3 ; oxygen 0.5 ml/1. 

 Depths in meters. (After Redfield, 1955.) 



The amount of accumulation varies greatly in different estuaries. It may be 

 expected to increase with the rate of production of organic matter and with 

 the length of the basin, to decrease with turbulence and the velocity of flow, 

 and to vary with the relative depth and velocity of the surface and deep layers. 

 Because of the variation in the accession of fresh water with the seasons, 

 steady-state conditions are rarely observed. In shallow estuaries subject to 

 strong tides, vertical advection is so strong that no differential distribution of 

 nutrients can develop. Accumulation may occur, however, in small shallow 

 estuaries if they are sufficiently isolated from tidal effects, as in the case of 

 Great Pond, Falmouth, Massachusetts, studied by Hulburt (1956). 



If the velocity of the currents is rapid the deep layer may be replaced before 

 any notable accumulation can develop. This appears to be the case in the Strait 

 of Juan de Fuca, where a well-developed counter current exists. In spite of the 

 high productivity of the surface layer, no clear gradient of phosphate develops 

 in the deeper layer along its length. 



Because of the annual variation in the accessions of fresh water, and the in- 

 fluence of climate on the hydrography and the production of organic matter, 

 steady-state conditions rarely exist in estuaries. The balance of the factors 

 leading to accumulation consequently varies with the seasons. In Long Island 

 Sound it is estimated that the countercurrent mechanism causes an accumula- 

 tion of phosphorus to the order of 0.7-1.5 mg atoms/m 3 in summer while 

 comparable losses occur in the winter (Riley and Conover, 1956). Similar annual 

 fluctuations occur in the nitrate exchange (Harris, 1959). 



A striking example of the effect of seasonal variation in the influence of the 



