Some chemical and hydrographic observations along the north coast of South America—I. Cabo Tres Puntas to Curaçao, including the Cariaco Trench and the Gulf of Cariaco

Abstract

Chemical and hydrographic observations were made on five north-south sections along the Caribbean coast of Venezuela in October and November, 1958. In addition, 15 stations were occupied in the Gulf of Cariaco. Chemical determinations included dissolved oxygen, total and inorganic phosphorus, nitrate, silicate and, in the anaeric waters of the Gulf of Cariaco and the Cariaco Trench, hydrogen sulphide.Except for some low-salinity near-surface water, these waters frrm a single mass, on the basis of the temperature-salinity diagram, in agreement with the findings of Parr (1937). Minimu salinities are found at 700–800 m and can be attributed to Antarctic Intermediate sources. Maximum salinities are in the upper 200 m, at depths which increase to the north.Near the coats, isopleths of salinity, temperature, density and the nutrient ions generally slope upward toward the south, particularly in the eastern sections. This is interpreted as evidence of upwelling along this coast, a phenomenon which appears to be the only important source of nutrients in the photic zone, and which accounts for the high biological productivity of the region.The Gulf of Cariaco was filled with nearly isohaline water having the same temperature correlations as the Caribbean water outside at the time of the observations. Upwelling at the eastern end of the Gulf is clearly evident from the temperature, oxygen and nutrient distributions. The water deeper than about 54 m is isolated from, and is colder than, water at equal depths outside. Past observations indicate that this deep water may be renewed some (and possibly all) years, in February. These deep waters are isolated, vertically, by a sharp temperature gradient, as stagnant, and contain sulphides. The upwelling in the east results in high biological productivity in the region, which is evident from the rich flora and fauna.An estimate of the productivity of the region has been made from the fixation of phosphate-phosphorous which upwells near the coast, north of Puerto La Cruz. The calculation indicates a production of 0·14 gm C/m2/day, a figur which is consistent with estimates made in the region by other methods. An estimate made in the Gulf of Cariaco, also on the basis of the fixation of inorganic phosphate, indicated a daily production of 2·8 to 5·7 gm C/m2.