l6] VEGETATIONAL TYPES OF SEAS 519 



for plant growth and photosynthesis — in particular carbon, nitrogen, 

 and phosphorus. Before all comes the question of available stock. 



Sometimes the turbidity of the water is such that the compensation 

 point {see p. 512) is reached within 5 to 10 metres of the surface, and 

 at times the water is so depleted of certain elements, in particular 

 combined nitrogen and phosphorus (which are taken up by the living 

 organisms), that further development is practically dependent upon 

 replenishment through death and decay, or importation. Paucity 

 of certain metallic elements, particularly iron, also seems to be prone 

 to limit phytoplanktonic growth. Thus fertility may be controlled 

 by the availability of inorganic nutrients, and marine productivity 

 in turn limited by the rate of supply of organic foods — as has been 

 strikingly demonstrated by the extraordinarily increased yield of 

 ' manured ' arms of the sea. Here inorganic nutrients were 

 added, that led to an increase in organic food in the form of phyto- 

 plankton, which in turn benefited the Fish. 



As in fresh waters, replenishment of nutrients by vertical mixing 

 currents takes place in the sea most rapidly in autumn and winter 

 with the cooling and sinking of surface waters, so that in spring 

 there is a relatively large supply available, and the phytoplanktonic 

 population can ' bloom ' luxuriantly. It seems that in some of the 

 most actively productive systems of sea-water, almost every atom 

 of phosphorus and nitrogen must be reassimilated several times each 

 year to permit the synthesis of the total organic material observed. 

 The most lastingly productive waters are commonly those where 

 there is continual upwelling from the depths, such as occurs at the 

 boundaries of oceanic currents. 



Where conditions fluctuate, the phytoplanktonic crop can develop 

 within a few days of their becoming favourable, whereas a much 

 longer period is required for the development of zooplankton. 

 Nevertheless, owing to animal grazing, to sinking below the lowest 

 level at which they can photosynthesize effectively, to horizontal 

 transport, and to ' indirect factors ' such as changes of temperature 

 (quite apart from available nutrients), phytoplanktonic populations 

 tend to vary markedly and often rapidly. Especially is even mild 

 grazing often extremely effective in reducing the population, so that 

 it has been calculated that if only one plant out of every ten in a 

 developing population is eaten, in six divisions 100 plants will pro- 

 duce but 3,400 individuals instead of 6,400 — although a mere 413 

 have been consumed. 



The simple cell division of most phytoplanktonic organisms is a 



