Distribuição espacial das comunidades fitoplanctônicas em ambientes temporarios em uma região no semiarido nordesino

Abstract

Rock pools are temporary aquatic ecosystems formed by the accumulation of rainwater in depressions in rocky outcrops. They are common in many regions worldwide. These ecosystems play an important ecological and socio-economic role, particularly during periods of water scarcity in arid and semi-arid regions. Due to their seasonal and unstable conditions, rock pools exhibit high spatial and temporal physical, chemical and biological variability, particularly with regard to phytoplankton, whose structure is strongly influenced by the dynamics of climate, hydrology, nutrients and light availability. This study aimed to analyse the spatial and temporal distribution of phytoplankton functional groups, and to estimate the functional redundancy metrics of phytoplankton communities in temporary rock pools in a semiarid region of Brazil. Eleven rock pools were sampled in two areas: one urban (n = 7) and one rural (n = 4). Water samples were collected monthly between June 2016 and June 2017. During the study period, the hydrological cycle was divided into three phases: Phase I: June–November 2016 (prolonged drought); Phase II: December 2016–March 2017 (months of highest precipitation); and Phase III: April–June 2017 (second period of precipitation followed by drought). In situ samples were collected at the deepest point in the water column of the tanks (20 cm). The analysed physical-chemical variables were water transparency, euphotic zone, pH, water temperature (°C), electrical conductivity (μS cm−1), total dissolved solids (TDS; mg L−1) and total phosphorus (μg L−1). For phytoplankton, the following were determined: biovolume (mm3 L−1), taxonomic and functional composition, functional diversity and redundancy indices, and functional richness (FRic), evenness (FEve) and dispersion (FDis). ANOVA analysis (p < 0.05) was applied to the data, and the Tukey's honest significant post hoc test was applied to compare the means and identify the significance of the groups. The results showed the influence of seasonality and spatiality on phytoplankton groups in the tanks. Kruk's classification revealed the predominance of phytoplankton functional groups IV and V in the Pocinhos area, and groups IV, V, and I in the Fazenda de Pedras area. For Reynolds' classification, tanks in the Pocinho area showed a higher representation of functional groups W1, W2, J, and P, particularly in phase 1. In contrast, the predominant functional phytoplankton groups in Parque das Pedras were W2, Lo, and J. The functional indices were influenced by seasonality due to its effect on light and phosphorus availability. The highest functional redundancy in phase 1 indicates an overlap of ecological functions, possibly due to the concentration of nutrients and organisms in small habitat areas. Functional richness and uniformity were higher in phase 2 and functional dispersion was higher in phase 3, which is associated with greater water volumes and light availability.