Comunidade zooplanctônica de cinco reservatórios tropicais do sistema FURNAS S.A. (Brasil): abundância e biomassa em carbono

Abstract

Gases generation, as contributor to the greenhouse, inside reservoirs it is one of the possible impacts caused for these systems to environment. Inside reservoirs, the zooplankton community had constituted one source of carbon so that this community would be also under influence of other physical, chemical and biological factors associated with morphometric features and age of these ecosystems. The present study aimed to characterize the zooplankton community and to verify its temporal and spacial distribution of abundance and biomass (carbon content), in reservoirs of FURNAS S.A. SYSTEM, being one cascading reservoirs (Furnas Reservoir, Mascarenhas de Morais Reservoir and LCB de Carvalho Reservoir), one reservoir in different ages (Manso Reservoir) and one eutrophic reservoir (Funil Reservoir). This study was accomplished during three different hydrological periods (before rain, after rain and dry season) in sampling stations located in riverine zone, transition zone and lacustrine zone. In relation to cascading reservoirs, the temporal distribution of richness and abundance presented the same pattern, where the values presented a reduction in upstream-downstream direction according with morphometric features and retention time of each reservoir. The biomass values were more correlated to the size of the zooplankton community´s organisms and to feeding availability offered to the zooplankton. For the Principal Components Analysis, the highest biomasses, in all cascading reservoirs, were correlated to the high temperatures, to bacteria biomass and to phytoplankton biomass. The first analyses of zooplankton community accomplished in Manso occurred almost in four years after its filling. After three years, new samplings were done in the same stations of the previous analyses. Manso´s taxa richness was little influenced by age factor. On the other hand, the decreasing of retention time in the second year of study contributed to the decreasing of the richness. As well as the richness, the abundance was also associated to the retention time and in addition to this factor, the high feeding availability, in the first year of study, promoted a high density in this period. It was not verified any pattern of biomass’ temporal distribution among the two years of studies, so the highest values always were found in the dry season. The lowest values were found before rain in the first year and after rain in the second year. In relation to abundance and biomass, only the lower values of both coincided in the first year. As much abundance as biomass values coincided in all hydrological periods. The abundance distribution along the longitudinal axis of the reservoir in the two years of studies seemed to be under influence of the hydrodinamics effects and nutrients input from the areas under tributaries influence. The biomass distribution presented itself equal in the two years of studies so the highest biomasses were found in transition zone in the rain absence periods while the highest biomasses were found in lacustrine zone in after rain. The statistic analysis evidenced differences in the average biomasses among the two years of studies, with the highest values being found in the first year. In this case, others biotic and abiotic factors in addition to factors associated to the operational regime at the reservoir seem to be influencing in major intensity this community than ageing process inside this environment. Funil Reservoir was classified as eutrophic so that richness seemed to be more associated to the sampling effort and its dendritic shape than its trofic state. Highest abundance values were correlated to the feeding availability and to a reduction in the cyanobacteria presence in the after rain period. Moreover, the reservoir also presented the highest retention time in the after rain period. The highest densities were given by copepods (Calanoida and Cyclopoida). These organisms seemed more adapted to the Funil Reservoir than rotifers and cladocerans. The biomass values coincided with the density values only in the after rain season and the highest values were found in this period. All zooplanktonic groups presented their highest biomasses in the after rain season. In spite of the before rain period presented the smallest densities, it was the second in biomass. On the other hand, even the dry season being the second in abundance values, it presented the smallest biomass. The longitudinal distribution of abundance and biomass were strongly influenced by hydrodinamics so that the regions under river influence presented smallest densities and biomasses. Besides biomass values were well correlated with the density values, so in all periods, density and biomass had presented their highest values, medium values and smallest values in the same region. Statistic analysis of the biomass distribution showed a high correlation between it and feeding availability as well as the decreasing in cyanobacteria abundance and biomass. Moreover, temperature and residence time factors were also favourable to this community so that they contribute for an increase in the biomass of these organisms as a whole. The zooplankton community contributed with significative biomass values in all environments studied so that this community showed to be a important contributor source to the reservoirs’ carbon reserve. On the other hand, little information is known about biomass (carbon content) as a whole in different tropical environments and reservoirs.