THE CARIBBEAN ARE^ 



Phytoplankton production is dependent upon radiant energy for reproduction and growth. 

 With carbon dioxide and water, plants are able, by virtue of the chlorophyll or other con- 

 tained pigments, to utilize the radiant energy of the sun to manufacture organic con^jounds. 

 This process furnishes a store of energy in the plant which becomes a source of chemical 

 energy for life processes, not only for the plant itself, but indirectly for other links in 

 the food chain. An important by-product is oxygen and this has great importance in the 

 respiration of organisms and in the oxidation of inorganic and organic substances in the sea. 

 Since oxygen is always used in the respiration of the algae, the free supply dissolved in the 

 water is drawn v^on for metabolic processes during periods of low-light intensity or ■»rtien the 

 plants have sunk to a depth at which the light intensity is below the minimum required for 

 oxygen production by photosynthesis to balance oxygen consumption by respiraticxi. The depth 

 at which light intensity is just sufficient to bring about this balance is known as the 

 compensation depth. 



The factors of food supply for phytoplanktonic organisms are dependent on the dissolved 

 gasses and salts. 



Carbon dioxide is used directly in photosynthesis and, although it is of tremendous 

 ioportance, it cannot be considered a limiting factor since there are practically always 

 ample stocks available in the sea. 



Dissolved nutrient salts, particularly those of nitrogen and phosphorus, have a great 

 limiting or controlling effect on phytoplankton production. Iron, magnesium, and other 

 elements may also have some influence. Nitrates (also nitrites aind ammonia) and phosphates 

 are usually regarded as indicators of the probable productive capacity. When concentrations 

 of these salts are found in sufficient amounts above the compensation depth, the phytoplankton 

 flourishes; otherwise, it does not. Since phytoplankton is the basis of life in the sea, it 

 is an essential in fish production. 



Silicates which are important in the formation of sUiceous sheila of diatoms and certain 

 other organisms appear to be important but not controlling items. 



There are also certain other accessory growth factors, the nature of which is little 

 understood but which appear to be necessary to diaton growth. These factors seeca. to be both 

 seasonal and regional in occurrence. 



The metabolism of the phytoplankton is influenced by three in?5ortant factors and these 

 are ten^erature, salinity, and hydrogen-ion concentration. Temperature is important because 

 the metabolic rate is accelerated by rises in ten^serature , According to van't Hoff 'a law, 

 this increase is two to three times for each 10-degree rise in tenqserature within favorable 

 limits. Increases in metabolism include greater reproductive action and shorter life spans 

 but a greater number of generations. Salinity affects the osmotic relationship between the 

 organism and its surrtsundings. Coastal forms are usually quite tolerant of salinity shifts 

 and are known as eurytialine forms while oceanic forms, known as stenohaline types, are not. 

 Salinity variations seem to affect the types of species found rather than the fertility in 

 the area. 



Hydrogen-ion concentration is generally quite stabilized and, therefore, its range of 

 variatiois is so small that it has only a very moderate influence, if any, as a limiting 

 factor for phytoplankton. 



The population density of phytoplaiikton is controlled by the factors affecting the re- 

 production of the organisms. Since these animals increase by simple cell divisions, under 

 optimal nutritional and metabolic conditions, the popnilations would build up a geometric 

 progression unless disturbed by limiting factors. 



Since phytoplankton is the main pasturage of the sea, it is drawn upon by herbivorous 

 animals, usually planktonic, which live within or periodically invade the photosynthetic 

 zone. Copepods are probably the chief diatom users but other forms also utilize diatoms as 

 food. With consumption at certain levels, it is possible that the actual production might 



