Diversidade e estrutura planctônica em ecossistemas aquáticos de uma região semiárida tropical: efeitos dos fatores ambientais e espaciais

Abstract

In semi-arid regions, lakes are strongly influenced by the hydrological regime. The long dry periods cause drastic reductions in water volume with a consequent decrease in inflow and outflow rates and changes in the trophic state of lakes. The rainy season is characterized by concentrated precipitation in a few months of the year. Because of decreased connectivity between environments during the dry season, we believe that biological community might be affected mainly by spatial factors during dry seasons (i.e. limitations to dispersion) and by local environmental factors during rainy periods. Furthermore, due to constant and strong seasonal changes, we also believe that the physical-chemical characteristics of water and biological communities may respond to these changes. In this work, we test the hypothesis that rainy periods might increase homogeneity in the planktonic community while dry periods increase beta diversity and correlate the environmental changes between seasons to the composition and structure of the zooplankton community. For this, samples were made in 40 lakes of Rio Grande do Norte state, located at Caatinga, a semi-arid region in Brazil. We performed one sample during the dry season and a sample during the rainy season (First and Second Chapters). Additionally, a long-term study of two lakes (24 months) was realized to test the effects of increases in drought severity over zooplankton community (Third Chapter). Results showed that the 40 lakes were more dissimilar during the dry period, with a higher beta diversity of plankton than during rainy period. Spatial variables (i.e. geographic distances) explained more the variance in zooplankton community than local environmental predictors during the dry season. Higher beta diversity was found especially for larger planktonic organisms (i. e. mesozooplankton) with low dispersion ability. Moreover, in Second Chapter we observed that during the dry season, lakes became more eutrophic, presenting changes in their physical-chemical characteristics, as well as changes in the composition of phytoplankton groups, which were correlated to changes in the structure and composition of the zooplankton community. The long-term monitoring in Third Chapter showed that the two lakes studied became different during time. Despite the eutrophic state of both lakes, they showed a contrasting phytoplankton biomass as drought period was intensified. Like the results of 40 lakes, the increase in environmental heterogeneity among the two lakes led to an increase in beta diversity of zooplankton during the dry period even with the reduction in alpha diversity in one of the lakes. We conclude that the decrease in connectivity between lakes during dry season might lead to an increase in environmental heterogeneity and beta diversity of planktonic communities.