Analyse des param�tres hydrobiologiques dans la remont�e de Cabo Frio (Br�sil)

Abstract

South Atlantic Central Waters (SACW) upwell close to Cabo Frio (Brazil, Rio de Janeiro) shore. The resultant typical hydrobiological conditions were studied at an anchorage station over an annual cycle from February 1973–February 1974. Multivariate analyses of hydrological and planktonic data revealed the structure of the ecosystem and the factorial relations governing its dynamics. Alternation, superposition or mixing of the water masses of different origin (Brazil current, coastal, SACW) constitute the most important factors responsible for the great hydrobiological variability observed in the study area. Spasmodic changes in wind direction and force are superimposed on a seasonal trend of more frequent upwelling in summer than during the rest of the year. The deep water is characterized by temperatures of <18°C, nitrate contents of 10 μg-at l-1 and by organic matter mainly composed of detritic elements from the shelf. Temperature variations together with nutrient contents (NO3 or PO4) reflect variations in primary biomass at the surface but not at 50 m depth, where detrital matter precluded valid measurements. Water of the Brazilian Current (salinity ≧36.0‰) frequently mixes with deep water of the thermal front, or with coastal water (<35.0‰) which invades the area when south-west winds prevail. This lower-salinity water is rich in seston particles. During the study period, primary biomass was relatively low due to eutrophication. We observed less than 3 μg l-1 chlorophyll and 106 phytoplankton cells per litre: the phytoplankton populations were highly diversified, indicating an advanced degree of complexity and evolution of this ecosystem. The observed formation of a thermocline constitutes an important inducing factor for an algal bloom. Simultaneous phyto- and zooplankton maxima would induce an increased grazing rate by herbivorous zooplankton which would also partly explain the relatively low level of primary biomass. Zooplankton is as abundant here as in other great upwelling regions: 100 organisms l-1 and 200 mg organic matter m-3.