Efeito de fatores abióticos e interações bióticas nas respostas ecofisiológicas das principais espécies fitoplanctônicas de um reservatório naturalmente eutrofizado

Abstract

In the aquatic environment, the planktonic community is primarily responsible for the transfer of energy to the other trophic levels, since phytoplankton (autotrophic) forms the basis of the food webs while zooplankton is the main link between producers and consumers. However, the interaction between phytoplankton and zooplankton depends on the physiological, morphological and behavioral traits of both communities. Among the several existing approaches to the study of planktonic communities (taxonomic, functional diversity and molecular), the use of functional groups and functional traits has been shown to be effective due to the possibility of a better understanding of the role of species in the aquatic ecosystem, as well as the link between them. For the phytoplankton community, the approach of morphollogically based functional groups (MBFG) has been widely recognized for its facilitation in modeling the dynamics of phytoplankton communities in different environmental scenarios. On the other hand, the use of functional traits for the zooplankton community has been shown to be able to relate more clearly the ecological requirements of zooplankton and its interaction with phytoplankton. At first, this thesis evaluated the temporal and vertical dynamics of the phytoplankton and zooplankton communities of a shallow tropical reservoir (Camorim Reservoir) based on the functional traits of the species, aiming to clarify the main drivers of its structuring (chapter I). In this chapter, we observed that biomass and phytoplankton diversity were linked to environmental variables, mainly in the warmer/rainy season, where the thermal stratification of the water column led to the emergence of filamentous species with aerotopes (MBFG III) and the high precipitation values were decisive for the reduction of total biomass. Phytoplankton species with body size ranging from medium to large, without specialized traits (MBFG IV), single-cell (MBFG V) and with silica exoskeleton (MBFG VI) were the main contributors to phytoplankton biomass. Cyclopoid copepod and rotifers were the main contributors to zooplankton biomass. The approach based on functional traits showed that the zooplankton community was dominated by small body size, raptorial, omnivorous and sexually reproductive. The functional traits observed in phytoplankton and zooplankton suggests a weak relationship between these communities and, consequently, an inefficient top-down control. In a second step, laboratory experiments, combining different light and temperature conditions, were carried out in order to deepen the knowledge about some ecophysiological parameters (growth, morphology, saxitoxins production, neutral lipid content and photosynthetic capacity) of two dominant phytoplanktonic species in the Camorim Reservoir, Raphidiopsis raciborskii (cyanobacteria) (chapter II) and Aulacoseira ambigua (diatom) (chapter III), and also to seek results that could explain their occurrences in the studied reservoir, as well as in the other aquatic ecosystems. In Chapter II, Raphidiopsis raciborskii showed the highest growth rates under high light intensities, combined at temperatures above 15°C, depending on the strain. This species reduced the production of saxitoxins and cellular quota of saxitoxins under high light intensity at 30°C. Therefore, the increase in temperature combined with sufficient light will lead to an increase in the biomass of R. raciborskii, but blooms may become less toxic in tropical regions. In chapter III, Aulacoseira ambigua was sensitive to high light intensity since it was unable to grow. Its photosynthetic capacity was reduced and there was an increase in the content of neutral lipids. A. ambigua was able to change its morphology under different combinations of light and temperature. Thus, our results demonstrated that the two phytoplankton species are capable of occurring and co-existing in the Camorim Reservoir, as well as in several freshwater ecosystems, due to the different light and temperature requirements and the wide phenotypic plasticity exhibited. In addition, the success of R. raciborskii and A. ambigua in several aquatic ecosystems may be linked to the functional traits that reduce herbivory pressure (e.g., toxins, presence of silica, respectively).