Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Accueil du site → Doctorat → Allemagne → 2018 → The role of water level fluctuations in the promotion of phytoplankton and macrophyte pioneer species in a tropical reservoir in the Brazilian semiarid

Technische Universität Berlin (2018)

The role of water level fluctuations in the promotion of phytoplankton and macrophyte pioneer species in a tropical reservoir in the Brazilian semiarid

Maciel Barros Lima, Débora

Titre : The role of water level fluctuations in the promotion of phytoplankton and macrophyte pioneer species in a tropical reservoir in the Brazilian semiarid

Die Rolle von Wasserspiegelschwankungen bei der Entwicklung von Phytoplankton- und Makrophyten-Pionierarten in einem tropischen Stausee im semiariden Brasilien

Auteur : Maciel Barros Lima, Débora

Université de soutenance : Technische Universität Berlin

Grade : Doktor der Naturwissenschaften - Dr. rer. nat. 2018

Résumé
In the semiarid northeast Brazil, water paucity instigates multiple uses of reservoirs, such as hydropower production, drinking water supply, water supply for livestock and for irrigation, fisheries, aquaculture and leisure activities. As consequence of high anthropogenic pressure, most reservoirs in this region are fragile and eutrophicated systems. Blooms of algae and macrophytes are commonly registered, impacting water quality, water supply, biodiversity, among others. Knowledge on limnic processes underlying phytoplankton and macrophyte blooms in semiarid reservoirs is still scarce but necessary for restoration of such deteriorated waterbodies. In Itaparica reservoir, São Francisco River, Northeast Brazil, intense land and water uses (e.g. nutrient load from watershed and surrounding irrigation agriculture, water level fluctuations due to hydropower production) exert high pressure on aquatic functions and blooms of toxic cyanobacteria (such as Cylindrospermopsis raciborskii, Microcystis sp.) and macrophytes (Egeria densa) have been reported. Under this context, the present work aimed firstly at determining the ecological processes driving cyanobacteria and E. densa blooms considering climate, hydrology and morphometric characteristics of the reservoir. Such understanding was essential for the second aim of this study, which consisted in defining appropriate management measures to avoid massive development of such species. In the main stream of Itaparica reservoir, phytoplankton abundance is controlled mainly by high water discharge rates ( 2000 m³ s-1) applied for hydropower production. This is particularly important during warm rain season, when high loads of organic matter enter the reservoir and together with stratification of the water column, maintain algae density at moderate levels (max. 4.4 x 104 Org. mL-1). Internal fertilization plays also a crucial role in this reservoir, as in the end of rain season isothermal conditions (in combination with water level rise and residence time increase), promoted high diatoms development (max. 6.6 x 105 Org. mL-1). During dry season, nutrients availability gradually subsided and concurrently phytoplankton density (max. 8.2 x 103 Org. mL-1 in the end of dry season). High water flow rates also seem to prevent cyanobacteria development, as their density was not prevalent in the main stream of Itaparica reservoir. In a drought scenario, higher water retention times increases the risk of cyanobacteria blooms, considering that C. raciborskii presence was steady throughout the study period (2007-2010). In the hydrodynamically isolated Icó-Mandantes bay, the cyanobacteria C. raciborskii developed more frequently, with extended effects. Here, the risk of cyanobacteria blooms seems to be regulated by additional drivers. In this shallow bay, the macrophyte Egeria densa is widely distributed along the bay down to 6-8 m water depth in dense mats. E. densa biomass is a significant nutrients storage in the system. P availability for phytoplankton is affected by the macrophyte growth regime related to water level fluctuations (WLF). Egeria densa massive stands result from traits like fast growth rate, light limitation tolerance and high affinity for water N, P and DIC, which allow a fast and outcompeting growth when water level rises. On the contrary, water level drawdown leads to E. densa stands decay, and nutrients availability for algae and cyanobacteria. C. raciborskii, diazotrophic and well adapted to P pulses, successfully develops in such conditions. Hence, WLF alternately provide opportunity windows for the pioneer species E. densa and C. raciborskii to develop. Enhanced nutrient load by desiccated sediments rewetting comprise a surplus of at least 50 % and 170 % of P and N, respectively, in comparison to reservoir water. Moreover, the seasonal decay of littoral communities decreases biodiversity, which further foment outbreaks of pioneer species. Considering the value of water in semiarid northeast Brazil, oligotrophication of this reservoir is very important for maintaining water quality. For that, constant water level, buffer biotopes construction and Egeria densa harvesting were the main three measures proposed in this work to improve ecosystem services of Itaparica reservoir.

Présentation

Version intégrale (4,85 Mb)

Page publiée le 6 janvier 2019