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Accueil du site → Doctorat → Allemagne → Aquatic ecosystem functions and oligotrophication potential of the Itaparica reservoir, Sao Francisco river, in the semi-arid Northeast Brazil

Technische Universität Berlin (2017)

Aquatic ecosystem functions and oligotrophication potential of the Itaparica reservoir, Sao Francisco river, in the semi-arid Northeast Brazil

Selge Florian

Titre : Aquatic ecosystem functions and oligotrophication potential of the Itaparica reservoir, Sao Francisco river, in the semi-arid Northeast Brazil

Auteur : Selge Florian

Université de soutenance : Technische Universität Berlin

Grade : Doctor 2017

Water reservoirs are globally an important tool for water management with multiple uses. Often reservoirs induce an economic prosperity in the region due to reliable water accessibility for irrigation agriculture, hydropower generation, water supply, fisheries, transport, recreation, and flood and pollution control. However, often an inacceptable price had to be paid for reservoir construction regarding the various socio-economic and environmental impacts by hydrological regime alteration. One example is the hydropower generating Itaparica reservoir located within the São Francisco river in the semi-arid Northeast Brazil. For the promotion of sustainable ecosystem services at this reservoir the binational INNOVATE-project (INterplay among the multiple uses of water reservoirs via inNOVative coupling of substance cycles in Aquatic and Terrestrial Ecosystems) was initiated, in which frame this study about water quality and aquatic ecosystem functions is conducted. In this region, characterized by the Caatinga biome, water is a very limited resource and frequent occurrence of droughts hampers economic development. Furthermore, future socio-economic and climate trends will enhance disturbances and aggravate conservation efforts. Rainfall is spatially and temporally erratic and 80 % of the annual rainfall occurs within 15 days ; however, it shows a high inter- and intra-annually variability. Hence, large numbers of water storage systems, such as reservoirs, cisterns and subsurface dams were constructed for water supply during the dry season. Although their dimensions are small, the vast abundance creates cumulated hydrological watershed effects. The reduction of discharge by artificial retention in the Pajeú river watershed leads to a reduced first flush and therewith to reduced nutrient and suspended loadings to principal water reservoirs or rivers. Water quality of the Itaparica reservoir is driven seasonally and is characterized as oligo-mesotrophic during the dry and eutrophic during the rain period in the São Francisco river watershed. The applied multivariate statistical approach with hierarchical cluster analysis and principal component analysis could show that water quality of the Itaparica reservoir is (i) mainly dominated by processes induced by rainfall in the closer region where erosion and soil leaching contributes to the seasonal variation, (ii) rainfall in the upper watershed and (iii) by seasonal water level changes (up to 5 m), inducing a flood-drying cycle at littoral areas (up to 27 % of reservoir surface), which alters mineralization, nutrient turn-over, wave erosion, macrophyte distribution and resuspension processes. In detail, in the Icó-Mandantes bay water temperature is high (23.7 – 31.3 °C) and water column stability can be described by atelomixis. Phosphorus is mostly limited due to the high binding capacity of clay, high Fe/P ratio in sediments, and enhanced P uptake by the abundant macrophyte Egeria densa. However, after occasional heavy rainfall nutrient concentrations are significantly increased, followed by strong development of Cylindrospermopsis raciborskii. The critical TP concentration is estimated for the rain period to 25 µg L-1 characterized by an enhanced P-use efficiency, resulting in a carrying capacity of 0.475 g m-2 year-1 P for the Icó-Mandantes bay. In the dry period no correlation of Chl a with TP and TN was observed, indicating another limiting factor, such as CO2 and light. The annual P-load of natural and anthropogenic sources is exceeding the estimated carrying capacity by factor 2.9 in 2013, with major contributions from sub-basin export (44 %) and seasonally desiccated and mineralized macrophytes (40 %). During the extended low water period water clarification processes were strongly decelerated compared to the regular water level change in the bay. In summary, Icó-Mandantes bays’ water quality is in short-term mainly affected by strong rainfall events in the sub-basin with increased nutrient and sediment loads followed by severe eutrophication processes and on long-term by seasonal water level changes and slow exchange and dilution processes with the main stream.

Présentation (WorldCat)

Page publiée le 1er novembre 2017