SECT. 4] GEOGEAPHIC VARIATIONS IN PRODUCTIVITY 351 



pure sea-water is taken as 0.04. By comparing coefficients calculated by this 

 means with those observed in Long Island Sound, Riley concluded that two- 

 thirds of the light extinction of that region is caused by non-living particulate 

 matter in suspension. The same method indicated that no more than half the 

 turbidity of continental-shelf waters off the eastern United States is attributable 

 to living phytoplankton (Ryther, unpublished data). 



Looking at organic production as the utilization of solar radiation then, we 

 see the contrast between the clear tropical seas on the one hand, where the 

 organisms are so scarce that the water absorbs most of the energy, and the more 

 turbid northern seas and coastal regions on the other, where the organisms must 

 compete with their own excreta, exuvia and remains and with terrigenous and 

 sedimentary materials for the available light. Though the causes are quite 

 different, the end result in both situations is the utilization of an almost equally 

 small fraction of the light energy which penetrates the sea surface. This is one 

 reason why organic production, when considered over long periods of time, 

 does not appear to differ greatly between temperate and tropical seas and 

 between oceanic, coastal and estuarine environments — a hypothesis, first 

 advanced by Riley (1941), which will be explored below. 



4. Nutrients 



The availability of nutrients is the other environmental factor which critically 

 limits organic production in the ocean as a whole. This, however, must be 

 considered in relation to the previously discussed factor, illumination, for it is 

 only the nutrients available in the upper, euphotic layers which are pertinent 

 to the discussion. 



A brief comparison of the situation in the ocean with that on land will serve 

 to illustrate the relative infertility of the former. Nitrogen will be used as an 

 example in this discussion and it will be assumed that other nutrients are 

 present relative to nitrogen in amounts roughly proportional to their con- 

 centrations in plant tissue, an assumption which has some validity at least for 

 the ocean (e.g. Redfield, 1934). Rich fertile soil contains some 5% organic 

 humus and as much as 0.5% nitrogen. This and the accompanying nutrients in 

 a cubic meter of rich soil, together with atmospherically-supplied carbon, 

 hydrogen and oxygen, can support a crop of some 50 kg of dry organic matter, 

 an amount equivalent to more than 200 tons per acre of soil 3 ft deep. 



Under optimal conditions plants are capable of converting the solar energy 

 falling on a square meter of surface to an organic yield of the order of 10 g/day 

 in excess of their own metabolic requirements (Ryther, 1959), an annual produc- 

 tion of several kilograms per square meter. If terrestrial plants can sink their 

 roots into 3 ft of rich soil, they have access, then, to enough nutrients to grow 

 at their maximum potential rate for periods of several to many years. Thus 

 forests, representing the accumulation of decades of organic production, are 

 possible. 



There are no comparable forests of plankton in the sea. The richest ocean 



