Avaliação do plano de ocupação da bacia hidrográfica da Barragem dos Pequenos Libombos e balanço de fósforo

Abstract

This work is another contribution to the knowledge of the ecology of Albufeira dos Pequenos Libombos. Taking into account that the lake, its drainage basin and its tributaries form an ecological unit with mutual influences, this work assesses the level of compliance with the protection strip decreed by the land law in the Albufeira dos Pequenos Libombos drainage basin . It also assesses the external and internal phosphorus load in Albufeira dos Pequenos Libombos. A qualitative assessment of the ecological situation of the banks of the hydrographic basin of the Pequenos Libombos Dam was carried out. To this end, the field work consisted of carrying out surveys and visual observation in the 250 meter strip, the protection strip where, in principle, no activity should be carried out. For the quantification of phosphorus, soil samples were collected from different selected locations within the 250 meter range, in the water column of the main tributary rivers (Calichane and Umbelúzi), in Albufeira and in the sediment of Albufeira. It was found that there is no compliance with the protection strip, there is an occupation of extensive areas for agriculture and in some cases, the use of fertilizers and pesticides on the south bank of Albufeira dos Pequenos Libombos and a population expansion on the south bank of Albufeira that can contribute to the transport of nutrients to the water body. It was also found that agriculture and population settlement are the main non-point sources of phosphorus that can have an impact on the increase of phosphorus in Albufeira. It was found that there is a high concentration of P-orthophosphate in the residential area (2.53 mg of P/100 g of soil) and in the fields both with use and without the use of fertilizers and low concentrations were found in the area without activities human (0.88 mg P/100 g soil). In relation to total-P in the soil, the highest concentration was found in the area without human activities on the north bank of Albufeira (57.18 mg of P/100 g of soil) followed by the residential area on the south bank of Albufeira with (40 .49 mg of P/100 g of soil) and finally the fields with fertilizers also from the south bank of Albufeira with (34.24 mg of P/100 g of soil). The lowest concentrations (14.82 mg of P/100 g of soil) were found in the field without fertilizers on the north bank of Albufeira. There was also an entry of high concentrations of phosphorus (90.30 pg/l and 258.78 pg/l) to the Pequenos Libombos Reservoir through the Umbelúzi River. From the results obtained for phosphorus in the sediment, it was possible to divide the Albufeira area into three zones, having found that the lowest % of phosphorus in the sediment (2.05%) was found at the mouth of the Calichane river and the highest % of phosphorus in the sediment (8.03%) was found at the mouth of the Umbeluzi River and (9.05%) near the dam floodgates. The current trophic state of Albufeira dos Pequenos Libombos is mesotrophic based on the Organization for Economic Cooperation and Development (OECD) model. The need for demarcation and compliance with the safety strip on the banks of the Albufeira dos Pequenos Libombos is highlighted(TRADUÇÃO NOSSA)