Efeitos do represamento sobre a estrutura e dinâmica da comunidade zooplanctônica.

Abstract

The river impoundment for reservoir creation is considered an anthropogenic activity of vast impact for biological communities. The present study was elaborated in two chapters and aimed to obtain greater information about the alterations in the structure and dynamics of zooplankton community during the reservoirs filling, since the ecological importance of this community in reservoirs and due to the scarcity of information about these changes in this community. The first chapter was elaborated from data obtained in one subtropical reservoir (Santa Clara reservoir, Jordão river, Paraná State) and predicted that: (i) the species richness and specific diversity of zooplankton groups are higher in the beginning of the filling phase, due to the presence of non planktonic species; (ii) the abundance of the groups is greater in the end of the same phase, in function of the development and predominance of large planktonic populations due to the food resources availability and the reduction of current flow and (iii) the alterations in the species richness, abundance, specific diversity are more remarkable in the beginning of the reservoir filling, due to the changes in the physical, chemical and biological features of the water. We identified 108 rotifer species, 32 cladoceran species and 16 copepod species. There was an increasing trend in the species richness of all groups during the filling, and the higher values were observed at the end of the period. Rotifers compounded the group with the highest alteration in species composition, followed by cladocerans and copepods. Greater abundances of all groups were registered at the end of the study period, but the peak of abundance of rotifers was higher and previous than those recorded for cladocerans and copepods (cyclopoids and calanoids). In general, the variations in the values of specific diversity were clear to all zooplankton groups and were mainly related to evenness results. As also observed for specific diversity, the evenness values of all groups presented remarkable variations. The pattern of species distribution was distinct among the sampling days, and allowed to observe a temporal gradient, when some species of rotifers (Proales sordida and Trichocerca capuccina) and cladocerans (Disparalona dadayi and Kurzia latissima) presented a preferential distribution in the beginning of the study; and other species of rotifers (Ascomorpha saltans, Gastropus hyptopus, Hexarthra mira) and one calanoid species (Notodiaptomus spinuliferus) characterized the last days of the same period, and the copepods occurred only from the middle of the period. The multiple regression analysis evidenced the importance of several food resources (biomass of bacteria, heterotrophic flagellate and phytoplankton) on the variation of zooplankton abundance during the filling of Santa Clara reservoir. community, and the food availability is the main factor driving the abundance variation during this period. The second chapter was prepared with data obtained during the filling of Santa Clara reservoir (Jordão river, Paraná State) and Corumbá reservoir (Corumbá river, Goiás State), predicting that there is a pattern of temporal coherence in the alterations of species composition present in both reservoirs, due to the reduction in the current flow resulting of impoundment process. We registered 125 taxa in Santa Clara and 79 in Corumbá, and 54 were common to both reservoirs. The rotifers were the group that most contributed to the species richness during the filling of both environments, and this contribution presented a slight reduction along the time. Cladocerans and calanoid copepods presented a higher contribution in Corumbá, and cyclopoid copepods, in Santa Clara. The highest alteration in species composition of all groups was verified in Santa Clara reservoir. Considering the abundance, in Santa Clara, rotifers dominated the community for almost the whole study period, whereas the microcrustaceans, especially cladocerans and cyclopoid copepods contributed to the abundance mainly at the end of the filling. In the Corumbá Reservoir, rotifers and cyclopoid copepods alternated in the dominance of the community, and the cladocerans were more important almost at the end of the filling phase. The results from Procrustes analysis and Mantel's partial test suggest that the temporal variation of zooplankton composition presents a pattern of temporal coherence between the reservoirs, i.e., the changes in the species composition along the time were similar. Some species were more related to the beginning, middle or the end of the filling phase in both environments. Disparalona dadayi, Euchlanis dilatata, Brachionus falcatus, Filinia longiseta, F. opoliensis, Conochilus coenobasis, Epiphanes macrourus (rotifers) and Moina minuta (cladoceran) were the most coherent species in both environments, emphasizing the coherent variation result of species composition in the two studied reservoirs. An evaluation of temporal variation of taxa number in the reservoirs distinguishing each zooplankton group did not suggest significant divergences. In Santa Clara, the species richness of the three groups decreased in the beginning of the filling phase, and afterwards increased. In Corumbá, the rotifer species richness decreased soon in the beginning and, afterwards, increased, while the species richness of cladocerans and copepods did not present The obtained results suggested that the zooplankton communities, present during the filling phase of Santa Clara and Corumbá reservoirs, responded similarly, at least regarding the changes in the species composition, to some intrinsic environmental factor or to a set of factors. Since the current flow is the one of the principal environmental factors affecting this community in reservoirs, we suppose that this factor is the responsible for the coherent variation in species composition in the present study. From the results obtained in these two studies, it was possible to observe that the reservoir creation causes remarkable changes in the structure and dynamics of zooplankton, emphasizing the importance of food resource availability for the variation of zooplankton abundance during the reservoirs filling. Furthermore, the communities present during this period in different reservoir responded similarly, at least regarding the changes in the species composition, to some environmental factor, such as the current flow, since this hydrological variable is of paramount importance for the establishment and development of this community in reservoirs.