TOE CARIBBEAI'J AREA 



Chemistr-j'- ; — The concentration and distribution of certain salts, principa]J.y phosphates 

 and nitrates, in sea waters have a great effect on the productivity. 



The vertical distribution Of phosphates and nitrates in the Caribbean, in general, is 

 about the same as found in the deep water of the Atlantic and elsewhere. The zone of maximum 

 phosphate and nitrate concentration falls between 300 and 500 fathoms, usually at about 400. 

 It is associated with the most extreme part of the thennocline and in the Caribbean with the 

 isotherm of 7 degrees C. This region is, also, the level of minimum salinity and of the 

 maximum vertical stability of the water. From. 4.00 to 750 fathoms, the phosphates and nitrates 

 diminish and below this the water is almost completely homogeneous. 



At the surface, thei-e is a central area of very low phosphate concentration (less than 

 .10 milligram atoms per liter — Rakestraw and Smith, 1937) and this suggests an inflow of water 

 from the Sargasso Sea to the northeast. Higher surface concentrations occur to the south and 

 west and east (from ,10 to .50 milligram atans per liter). 



At 200 fa-Uioms, the entire northern half of the Caribbean (north of a line from Cape 

 Gracias a Dios to Martinique) has a phosphate concentration of from .50 to 1.25 milligram 

 atoms per liter, and this is also tme in an intrusion from the eastward between St. Vincent 

 and Trinidad and reaching a point off central Venezuela. Off central sind wertem Venezuela 

 and eastern Colombia is an extensive area where phosphates run over 2.00 milligram atoms per 

 liter. In general, the Caribbean area at all levels is rather richer by the active regener- 

 ation of the phosphates by organic development in the Caribbean and in the Cayman Seas. 



Nitrate values at the surface are very lov* throughout the Caribbean but the Cayman area 

 seeius to have a slightly higher concentration than the Caribbean proper. The northwestern 

 third runs more than 15 milligram atoms of nitrate per liter. Ibe central third runs from 10 

 to 15 milligram atoms, and the southern third less than 10 milligram atoms of nitrate per 

 liter. 



The maximum nitrate concentration occurs at 400 fathoms and this extends all over the 

 southern and western parts of the area. 



As is the case of the phosphates, the nitrates are higher in the Caribbean than just 

 outside. It is evident that nitrate regeneration occurs in the Caribbean, 



There is a close relation between the hydrogen-ion concentration (pH) and nitrate and 

 phosphate, since the generation of the latter substances results from the oxidation of organic 

 matter. This is accompanied by the formation of carbon dioxide and an increase in acidity 

 (lowering of pH is the result). The surface waters of the Caribbean range from 8.10 to 8.2 

 except for the southeastern quarter where they are higher than 8.2. 



The lowest pH values occur at 400 fathoms and this might be expected since phosphate 

 and nitrate are both at maximum value at this point. 



The oxygen content of the surface waters is about 4.50 milligrams per liter throughout 

 the Caribbean. This drops to around 3.00 at 250 fathoms depth and then increases to 5.50 

 in the deep-water basins. 



Productivity ; — ^As a general rule, resident fish populations in tropical seas are not 

 large enough to justify intensive commercial fisheries. There are, however, certain exceptions 

 to this generalization, and these are caused by certain well-known factors. 



The primary food supply of the sea is produced by the living microscopic plant life 

 (phytoplankton) and the abundance and distribution of phytoplankton is affected by the 

 chemistry, physics, and biology of its environment. Phytoplankton includes all the passively- 

 floating sea plants and is coo^josed chiefly of diatoms and secondarily of dinof lagellates . 

 Two broad general groupings of the phytoplankton are the coastal or neritic species and the 

 oceanic species. Many of the neritic forms are able to form spores, by which means they are 

 able to survive during unfavorable periods and to germinate new crops when conditicxis become 

 favorable . 



