The reasons for the apparent paradox lie in 

 the restriction and limitations which the planktonic e 

 existence impose upon plants with respect to two 

 basic requirements, light and nutrients. We shall 

 consider these separately. 



Though the oceans are some three miles deep, 

 over 95% of their waters are in virtual darkness, 

 uninhabitable by photosynthetic organisms. The 

 light intensity at which photosynthesis balances 

 respiration, the "compensation intensity " , varies 

 somewhat with the species and physiology of the 

 plant, but is of the general order of 100 foot can- 

 dles. This is equivalent to about 1% of full noon 

 sunlight incident to the surface . Below this com- 

 pensation intensity, no growth of plants is possi- 

 ble. Thus the oversimplified but useful concept 

 has come into use of a "euphotic zone", the depth 

 through which plant growth can occur, which has as 

 its lower limit the depth of penetration of 1% of the 

 incident surface radiation . In the clearest ocean 

 water the euphotic zone extends to about 120 

 meters . 



But this clearest ocean water contains no 

 plants, so a euphotic zone of 120 meters is hypo- 

 thetical. Let us introduce an algal population into 

 this clear solution which we will further stipulate 

 to be adequately enriched with all the vital plant 

 nutrients. Immediately, the plants themselves 

 will absorb and scatter the light, and the euphotic 

 zone will become progressively shallower as the 

 population grows . Thus, the phytoplankton , shut- 

 ting out its own light, becomes self-limiting. But 

 as the euphotic zone becomes shallower, a pro- 

 gressively larger fraction of the light is absorbed 

 by the plants , a proportionately smaller fraction by 

 the water itself . Consequently, organic produc- 

 tion increases with a decreasing euphotic zone. 

 However, this relationship holds only as long as 

 the decreasing euphotic zone results from the in- 

 crease of living plants. Not all turbid waters are 

 highly productive, for the turbidity is frequently 

 caused by non-living particulate or dissolved mat- 

 ter. In Long Island Sound, for example, Riley 

 (1956) estimated that two thirds of the incident 

 radiation was absorbed by such material. 



We may conclude, then, that photosynthesis 

 in the ocean is not only restricted to a shallow 

 surface layer, no more than 100 meters deep, but 

 that for the process to proceed at its maximum po- 

 tential rate, the plants must be concentrated in the 

 upper five meters or less of otherwise clear ocean 

 water. Let us now turn to a consideration of the 

 nutrients available to support this production. For 

 a convenient example we shall take nitrogen. 



The highest concentrations of inorganic ni- 

 trogen in the oceans are present as nitrate at in- 

 termediate depths of approximately 1000 meters, 

 and range from about 40 to 60 tig A NO3 - N/l or 

 .56 - .84 gN/m~^ . Water from this depth rarely 

 reaches the surface. The highest known concen- 



trations of nitrogen within the euphotic zone are 

 found in restricted areas where upwelling or diver- 

 gences of water masses bring water from several 

 hundred meters of depth to the surface . Rudd 

 (1930) for example, reports values of about 40 fig 

 A NO3 - N/l in the surface layers of the Antarctic, 

 one of the most fertile oceanic regions. 



In most temperate and northern waters, the 

 surface layers are enriched by winter cooling and 

 mixing to depths of some 300 - 500 meters. The 

 highest concentrations of nitrogen brought to the 

 surface in this way range from 10 to 20 }ig A NO3 - 

 N/l (R . F . Vaccaro, unpublished data for the 

 North Atlantic) . Let us assume that winter mixing 

 has resulted in a surface enrichment of 15 pg AN/1 

 or 210 mg N/m^ . In clear ocean waters with a 100 

 meter euphotic zone, the phytoplankton will have 

 a reservoir of 100 m x 210 mg N/m^ or 21,000 mg 

 of nitrogen per square meter to draw upon . How- 

 ever, as the population grows, it not only depletes 

 this supply, but creates in the process a progres- 

 sively shallower euphotic zone with a correspond- 

 ingly smaller reservoir of nitrogen. By the time the 

 phytoplankton have increased to a population con- 

 taining 10 mg of chlorophyll/m , production is 

 limited to a euphotic zone of about five meters 

 (Riley, 1956) . This water contained an initial 

 amount of only 1050 mg N and, of that, 500 mg 

 were utilized in producing the population (assum- 

 ing the plants to contain about 1% chlorophyll and 

 10% nitrogen) . We may calculate the daily rate of 

 production from the chlorophyll and depth of the 

 euphotic zone for a day of average incident radia- 

 tion , (300 langleys) according to the equations of 

 Ryther and Yentsch (1957) . From these calcula- 

 tions it is estirrBted that 1 .8 grams of organic mat- 

 ter will be produced and about 180 mg nitrogen con- 

 currently consumed within the five meter euphotic 

 zone each day. Thus, after a population equiva- 

 lent to 10 mg chlorophyll/m3 has developed, there 

 is sufficient nitrogen remaining in the resulting 

 shallow euphotic zone to sustain production for 

 less than three days. Clearly in a static situation 

 such as we have pictured, high levels of organic 

 production approaching the potential discussed 

 above can scarcely be attained, much less main- 

 tained . 



The oceans as a whole are not, of course, a 

 static environment , but their surface waters are 

 highly variable in this respect. In certain re- 

 stricted areas hydrographic or meteorological forces 

 bring water from intermediate depths to the surface. 

 This happens when two water masses diverge, as 

 in the equatorial Pacific (Sette , 1955), and most 

 notably along the West Coasts of continents, where 

 prevailing offshore winds produce a surface current 

 which moves seaward, this water being replaced 

 with upwelling, rich, deep water. It is for this 

 reason that the coastal waters off Peru and parts of 

 West Africa are among the most fertile regions of 



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