THF CARIBBEAM AREA. 



be greater in areas only moderately rich in plants at any given ti-ne than in others much 

 richer but not utilized. The fluctuations in plankton distribution may be a reflection of 

 the relative intensity of grazing rather than control through nutrients or physical con- 

 ditions of the waters. 



Hhen diatoms sink below the level of the photosynthetic zone, they must be considered 

 as lost to the reproducing population. Although diatoms are especially adapted to a float- 

 ing existence by means of suspensory organs and oil globules, these cxily serve to retard sink- 

 ing and cannot always overcome the constant pull of gravity. In connection with the velocity 

 of sinking, temperature is very important since the viscosity of the water is inversely pro- 

 portional to the temperature and the rate of sinking inversely proportional to the viscosity. 

 When, due to transportation or water masses vertically in the water column, portions of the 

 phytoplankton population are included, a direct loss of reproductive capacity is caused if the 

 organisms are taken out of the photosynthetic zone. 



Indirect factors involving productivity are- many and have complicated relationships. 

 Temperature, for instance, modifies the rate of metabolism while at the same time it reg- 

 ulates the viscosity. The vertical temperature gradient tends to reduce active turbulence. 

 On \-jie other hand, turbulence, upwelling, or convection currents are essential to the return 

 of nutrients to the photosynthetic layer. Other indirect factors are water movements of 

 various kinds, stability of waters in the photosynthetic zone, discharge of rivers, meteor- 

 ological conditions, bathymetric conditions, and geographic positions. 



Hydrogen-ion concentration, salinity, water temperature, available radiant energy, oxygen 

 and carbon dioxide concentration, and suitable phytoplankton species are available in almost 

 optimal conditions in the Caribbean area. These then are not the limiting factors of the 

 productivity level. It appears that the distribution and concentration of the nutrient salts — 

 phosphates and nitrates — in the water column holds the key to the problem. 



Nutrient Salts ; — Dissimilarities in productivity between known sea areas do not appear to 

 be caused so much by differences in the absolute amounts of nutrient salts but rather in their 

 distribution within the water column. In arctic and temperate seas, the upper surface layers 

 are replenished periodically by annual overturns of the water layers brought about by compli- 

 cated physical changes. In the tropical seas, however, iriiile tiiere are great stores of these 

 nutrients to be found in the lower and middle water layers, there are no annual overturns to 

 make them available to life in the photosynthetic zone, 1/ They are relatively useless unless 

 in the zone of light penetratiai. Tropical waters are characteristically stable, and there is 

 usually little interchanges of waters from layer to layer. 



Under certain conditions, masses of nutrient rich waters may be brought to the surface and 

 affect the productivity of an area tremendously. These conditions are summarized as follows: 



Oceanic currents ; These water movements may carry nutrient salts, depending on the origin 

 of the movement, the regions traversed by the flow, the distance covered, character of the 

 stream, and other factors, 



Upwelling ; Movements of intennediate or bottom waters rich in nutrients to the surface 

 occur under certain conditions of wind, shelf contour and currents, and this has a tremendous 

 effect on plankton production since it provides constant replacement of nutrients in the pho- 

 tosynthetic zone. 



Rivers and land outwash: Great quantities of dissolved salts, in addition to silt are 

 added by these means. They becoaie available to increase the productivity sometimes at con- 

 siderable distances from the area of discharge. Since sunlight penetration is necessary for 

 photosynthesis, the river mouths are not necessarily plankton rich, for there is often great 

 turbidity. 



1/ The upper surface zones extending to depths of about 30 fathoms in which there is 

 sufficient sunlight for photosynthetic reactions. 



