William Senior

Seasonal distribution of nutrients and primary productivity on the eastern continental shelf of Venezuela as influenced by the Orinoco River

Nitrogenous nutrients, dissolved silicate, and salinity were measured in surface waters and shallow hydrocasts along similar cruise tracks during the spring (dry season) and fall (wet season) of 1988. Both cruises transected the eastern Caribbean, transited the Gulf of Paria, ran parallel to the Orinoco Delta and into the main channel of the Orinoco River. Trends in primary productivity were also measured by daily carbon 14 incubations. In both seasons, samples covered the range from highly oligotrophic and transparent to highly productive and rich in biogenic and abiogenic particulate matter. Most of the Orinoco outflow appears to turn N to NW and remains in shallow waters off Venezuela and surrounding Trinidad, permitting benthic regeneration of river-borne nutrients. However, the role of the Orinoco and associated low-salinity coastal waters in fertilizing large areas of the eastern Caribbean basin, as suggested by satellite imagery, can be approximated crudely from the nutrient composition at Boca de Dragon, which is representative of the nutrient status of these waters as they flow into deeper Caribbean waters. Additional nutrients may be supplied to the area primarily from Amazon-derived water entering the Caribbean Basin further north, with some coastal upwelling along the continental shelf in the dry season.

Growth of the tropical scallop, Euvola (Pecten) ziczac, in bottom and suspended culture in the Golfo de Cariaco, Venezuela

Abstract We compared the growth of the scallop Euvola (Pecten) ziczac (L.) in three situations which potentially could be used for commercial culture, in cages maintained in suspension, in cages on the bottom and in cages partly buried in a sediment bottom. The latter permitted the scallops to bury themselves as in their natural habitat. Throughout the 7-month study, growth, as measured by shell length and muscle mass, was by far superior for scallops in the partly buried cages. Possible explanations for this are (1) that the scallops are stressed by enclosures which prevent them from burying themselves and (2) that organic material at the sediment/water interface is an important food resource and E. ziczac has better access to this when it buries itself flush with the bottom. The timing of gonadal growth and spawning varied markedly among treatments. Some spawnings coincided with temperature increases but others did not. Differences between scallops in suspension compared to those in bottom treatments suggested that reproduction is as much controlled by conditions in the immediate environment of the scallops as by large-scale environmental factors. Survival was highest for the scallops maintained in partly buried cages.