THE CARIBBEAJJ AREA 



Overturns and vertical mixing: The presence or absence of a perpetual thermocline 

 indicates the possibility of overturns of water layers. A perpetual thermocline indicates 

 great stability in the water masses and that there is practically no mixing. Consequently, 

 there is no periodic replenishments of nutrients. The lack of a perpetual thermocline in- 

 dicates that such mixing does occur and indicates a better chance for productivity. 



Oceanic Currents ; — The waters of the Caribbean are derived mainly from the North Equator- 

 ial Current (Drift), the Sargasso Region, and froin the South Equatorial Current. "Hie effect 

 of the northeast and southeast trade winds is to form two great drift currents, setting in a 

 westerly direction across the Atlantic from Africa toward the American continent. At tiines, 

 their combined width covers upward of 50 degrees of latitude. These are known as the North 

 and South Equatorial Currents or Drifts, according as they arise from the trade winds of the 

 northern or southern hemisphere. The South Equatorial Current is the more extensive, origi- 

 nating off Africa south of the Guinea coast. It begins its flow with an average velocity of 

 .6 knot and maintains a general westward set. The portion near the Equator later acquires a 

 northward flowing component. The rate of drift steadily increases until, on arriving off the 

 South American coast, a rate of 2.5 knots is not uncommion. At Cape San Roque (Brazil) the 

 current splits, the Main or Equatorial branch flowing along the Guiana coast, while the other 

 portion is deflected to the southward. 



The North Equatorial Current originates to the northwai^i of the Cape Verde Islands and 

 sets across the ocean in a due west dir«ction in about latitude 15 degi^es north and with a 

 velocity of about 0.7 knot. An Equatorial Counter Current is found between the North and 

 South Equatorial Currents. It sets to the eastward, being propelled by the southwest mon- 

 soon. This wind prevails in an area of varying extent north of the Equator and extending 

 some distsuice westward from the western tip of Africa. The extent and force of the counter 

 current varies. In July and August its effect is apparent from longitude 50 degrees W. to 

 the Gulf of Guinea while in November and December its influence is slight and prevails for 

 only a limited distance from the African coast. The North and South Equatorial Currents 

 unite in about mid-Atlantic to the westward of the region of the counter current and a large 

 part of the combined stream flows into the Caribbean Sea through the passages between the 

 Windward Islands, between Trinidad anc^ Tobago, between Trinidad and Grenada, and to a lesser 

 extent through the Gulf of Paria. After passing into the Caribbean, the currents pass west- 

 ward and then northwestward, finally reaching the Yucatan Channel. A massive eddy is formed 

 in the Costa Rlca-Panama-Colombia bight and another round the island of Jamaica. 



A complicated system of eddies and counter currents occurs between Cuba cind the Cayman 

 Islands, and the Cuban Counter Current runs eastward along the south coast of Cuba and 

 Hispaniola, ending at Mona Passage. Another counter current passes southward along the 

 Yucatan, British Honduras, Uiskito Coast, swings eastward along the coast of Panama and 

 Colombia, and joins the eddy movemoit. Increments of water are, also, observed from the 

 northward through the Windward and Mona Passages. 



During the long journey of the currents from the Eastern Atlantic across the equatorial 

 regions to the South American coast, the nutrients in the surface layers become exhausted. 

 Dead animal and plant tissues drop out of the warmer surface zone, but no replacement of 

 nutrient salts from below is possible because of the stability of the water layers as indicated 

 by the perpetual themoclijie. Phosphates run about .3 mg. of P2O5 per cubic meter, and there 

 are practically no nutrients present, the only significant amounts of salt are in the organic 

 foim in the living tissues of animals, and in the very meager phytoplankton. When these are 

 liberated through the death of an organism, they are reutilized almost immediately and, thus, 

 have a very transient existence. 



As the current sweeps along the Brazilian and Guiana coasts, it picks up and transports 

 vast quantities of nutrient salts from the effluents of the great rivers of this region — the 

 Amazon, Uaroni, Courantine, Essequibo, and Orinoco. With the increased amounts of available 

 nutrients, the water becomes murky and oftentimes pea-green in color with phytoplankton. 

 Great schools of pelagic fish feed along the margins of the current. A small part of the 

 current sweeps through the serpent's Mouth into the Gulf of Paria and picks up additional 

 nutrients from the streams emptying into the Gulf. Ttxe main current flows into the Caribbean 

 through the channel between Trinidad and Tobago and other branches may enter through other 

 passages in the Tlindwards. The effects of the increased fertility is sometimes noticeable as 



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