Avaliação ecológica da produtividade natural do estuário do rio dos bons sinais, na perspectiva do desenvolvimento sustentável da aquacultura de camarão marinho

Abstract

The study was carried out in the estuary of the Bons Sinais river in Quelimane, in the province of Zambézia. The work began in September 2000 and ended in March 2001 and consisted of the assessment of natural productivity from the perspective of sustainable development of marine shrimp Aquaculture in Mozambique through the analysis of some abiotic and biotic parameters. The first part consisted of the monthly measurement of hydrological parameters (t°C, pH, S%o and electrical conductivity) in the field using a HORIBA multiparameter probe. Water samples were taken at different fixed stations along the estuary, both from the surface and the bottom during the high and low tide peaks of a spring tide each month, to assess total matter, organic matter and mineral matter in suspension. Phytoplankton abundance was evaluated by determining the biomass chlorophyll a and the phaeophytin a level. A second part consisted of measuring and harvesting the same parameters once every three months during the diurnal spring tide cycle to assess the cyclical influence of the tide on these parameters. The results show that the seasonal variations of the parameters that qualify the water in the Bons Sinais estuary are relevant in comparison with spatial variations from upstream to downstream or even variations in depth, that is, at a given moment, it is not expected that the quality river water is very different from place to place, except for local variability. During the dry season both MES and MO presented high values ​​compared to the wet season. This superiority was also observed between surface and bottom values. An increasing gradient of suspended matter and organic matter was recorded from the mouth to the upstream. Chlorophyllin concentration during the dry season and at the beginning of the wet season had an increasing trophic gradient (oligotrophic to eutrophic) from downstream to upstream passing at the end of the wet season, to an oligotrophic and horizontally homogeneous ecosystem. Tidal cycles showed an impact on seasonal variations both from the trophic point of view of the system and also from the physiological quality of the phytoplankton cells. In fact, during the dry season a small percentage of the chlorophyll “a” is degraded to pheophytin “a”. On the contrary, in the wet season, plankton populations enter into senescence. The mesotrophic or oligotrophic waters observed from November to February are of good trophic condition for an aquaculture but require a contribution of nutrients, while the eutrophic aquatic environment observed in September/October of strong natural productivity and trophic quality indicates that the system of culture only needs a little nutritional reinforcement. (TRADUÇÃO NOSSA)