THE DISTRIBUTION OF PHOSPHATE IN THE SEA 



INTRODUCTION 



Phosphate and inorganic nitrogen compounds are 

 regarded by many oceanographers as factors regulating 

 and limiting the production of lite in the sea. For ex- 

 ample, in the North Sea, Atkins (1930 and earlier pa- 

 pers), Harvey (1926, 1928a) and Cooper (1933) found that 

 during the summer either the phosphate or the nitrate 

 content may be reduced to a point at which it cannot be 

 detected by exceedingly sensitive methods. On the other 

 hand, Moberg (1928) reports that the water along the 

 coast of Southern California always contains a sufficient 

 quantity of phosphate to support plant growth although 

 the nitrate may be entirely lacking in the upper 15 me- 

 ters or more during several months in the summer. 



The concentration of both substances in sea water is 

 exceedingly small and varies considerably, not only with 

 the season, but with depth and locality as well. Because 

 of this variability and because of their importance to the 

 life in the sea, the determination of these substances, 



has been included in most recent oceanographic pro- 

 grams concerned with the study of biological conditions 

 in the sea. This is especially true for phosphate for 

 which this method is more satisfactory than for nitrates. 



It is not always practicable to include in the same 

 program the determination of all the plant -nutrient sub- 

 stances that are present in sea water in sufficiently 

 small quantities to be possible limiting factors of the 

 growth of plants. It has been found, however, that as a 

 rule all these substances parallel each other in their 

 distribution. Consequently a reasonably clear picture of 

 the biological fertility of sea water can usually be ob- 

 tained by determining only one of these substances. 



The parallelism in the distribution of various plant 

 nutrients can readily be understood since Redfield (1934) 

 has pointed out that these substances occur dissolved in 

 sea water in the same proportion as in the whole plank- 

 ton. 



VERTICAL DISTRIBUTION 



In the illuminated water layers near the surface 

 where adequate light permits an active growth of phyto- 

 plankton, the phosphate content is less than at lower 

 levels. The phosphates and other nutrient ions are 

 taken up from the water in the synthesis of plant sub- 

 stances. This drain on the nutrient ions may continue 

 until their concentration becomes so low that they can 

 no longer be utilized and plant activity is curtailed. If 

 small quantities of nutrients are being supplied from 

 greater depths by convective currents, however, or by 

 decomposition of organic material within the photosyn- 

 thetic zone, a slow growth of plants may occur, regu- 

 lated by the amount of nutrients available. In this way 

 a vegetable population may act to maintain a continuous 

 depletion of nutrients in the upper water layers. This 

 apparently occurs in certain tropical and subtropical 

 latitudes where the additions of nutrients to the sur- 

 face layer are small and where other factors control- 

 ling the growth of plants, such as light and tempera- 

 ture, are favorable. Thus, at many Carnegie stations 

 the surface water contained 5 mg PO4 per cubic meter 

 or less. Where the physical factors are not so favor- 

 able for plant growth, or where convection or other ver- 

 tical water movements carry larger quantities of nutri- 

 ents to the photic zone (as in high latitudes particularly 

 in the winter), the amount of nutrients brought into the 

 upper layers may exceed that utilized by plants and as 

 a consequence there will be an increase in the quantity 

 of phosphate in these layers. At the Carnegie stations 

 south of the Aleutian Islands, surface water containing 

 more than 100 mg PO4 per cubic meter was found in the 

 summer. 



The depth to which photosynthesis takes place varies 

 with the latitude, the season of the year, the turbidity of 

 the water, the density of the plankton population, and 

 probably other factors. In Oslo Fiord, Gaarder and 

 Gran (1927) found that at 10 meters photosynthesis and 



respiration balanced each other, whereas Marshall and 

 Orr (1927) found this condition at a depth of 20 to 30 me- 

 ters north of Scotland in the summer. In lower latitudes 

 and farther from the coast suitable conditions for photo- 

 synthesis would undoubtedly be found at considerably 

 greater depths. It should be realized, of course, that 

 even at depths where respiration is more rapid than 

 photosynthesis, phosphates and other inorganic sub- 

 stances are consumed by plants. It is possible that all 

 depths at which plants are found in good condition should 

 be included in the photosynthetic zone,. According to 

 Steuer (1910) phytoplankton has been found at a depth of 

 400 meters but it is improbable that active growth oc- 

 curs at this depth. In most localities it is probable that 

 no great quantities of phosphates are consumed below a 

 depth of 75 to 100 meters. The water, however, may be 

 deficient in phosphate to much greater depths. 



The depth to which very low concentrations of phos- 

 phate occur is greatly influenced by the depth of the 

 convection layer. Throughout the convection layer the 

 concentration of phosphate is more or less uniform, ow- 

 ing to the thorough mixing of the water. If the lower 

 limit of the convection layer is below the photosynthetic 

 zone, uniformly low concentrations of nutrients will ob- 

 tain at greater depths than it it is above this zone. Thus, 

 less than 10 mg PO4 per cubic meter were found at 

 many stations in the North Pacific to depths greater 

 than 200 meters. It is probable that this was not owing 

 to active growth at that depth but rather to the unstable 

 condition of the water in the upper 200 or more meters, 

 which permitted a thorough mixing of the water down to 

 these depths. This allowed the plants nearer the surface 

 to maintain low concentrations of nutrients to depths 

 much below the photosynthetic zone. It is also possible 

 that in areas of sinking, water of low phosphate content 

 may be carried to greater depths than otherwise would 

 be the case. 



