Ecologia do fitoplâncton de um sistema rio-reservatório do semiárido brasileiro : uma abordagem funcional.

Abstract

River damming for reservoir construction is fairly common in semi-arid regions, where water availability is limited, and constitutes a physical and chemical discontinuity affecting the ecological attributes of biological communities (Serial Descontinuity Concept theory, SDC). Modification in flow conditions, and consequently increase in residence time and matter sedimentation towards the dam region are among the main reservoir impacts in rivers. This variation may cause the development of distinct regions exhibiting different trophic and hydrodynamic characteristics, which may vary temporally, adversely influencing the establishment of biological communities. In this study, we evaluated longitudinal (lotic region upstream, transition, lacustrine region and lotic region downstream) and temporal (months without rainfalls and rainy months) variation in the attributes of the phytoplankton community (biovolume, trait abundance and functional diversity) in a river-reservoir system in Brazilian semi-arid as a function of environmental variables. Six monthly samplings were performed from October 2014 to March 2015 in eight sites. Two approaches were used: (1) spatial and temporal variation in the biovolume of phytoplankton morphology-based functional groups (MBFG) and (2) spatial and temporal variation in functional diversity and the trait abundance distribution in the studied system. A spatial gradient was recorded in which the lotic region upstream presented greater turbidity and had a greater predominance of MBFG V; The reservoir region was stratified and presented higher values of chlorophyll a and larger phytoplankton biovolume; MBFG VIII was dominant. The lotic region downstream presented higher water flow and phosphate concentration and was dominated by MBFGs VI, V and IV. The most important functional traits were aerotopes, brown pigments and flagella. The functional richness was higher in the reservoir, functional equitability did not varied according to regions and functional diversity (Rao) was lower in the reservoir. No significant temporal gradients were verified, probably due to the strong spatial gradients and pluviometric irregularities of the study period. The results indicate a discontinuity caused by the reservoir but the patterns observed in the functional diversity indices differ partially from those expected in relation to the SDC theory; this can be partialy explained by the intermittent characteristics of the semi-arid rivers, which cause a temporal discontinuity in these environments.