Fatores ambientais que direcionam as respostas ecológicas da comunidade fitoplanctônica em reservatórios de uma região semiárida tropical brasileira

Abstract

The intrinsic climatic characteristics of the Brazilian Semiarid Region (BSR) promote a scenario of vulnerability to water scarcity, which throughout the century encouraged the creation of artificial reservoirs to guarantee water security, especially in periods of drought. The multiple uses of water are one of the factors that can lead to an increase in the load of nutrients in the reservoirs, which, associated with the high evaporation rates in the region and the high residence time, makes these systems more susceptible to eutrophication, contributing to the deterioration of water quality. Such conditions favor the excessive growth of the phytoplankton community, with the dominance of toxic cyanobacteria, resulting in significant risks to biodiversity and ecosystem functioning. Thus, the functional approach of the phytoplankton community can be useful to establish relationships between species characteristics and ecological processes that occur in the aquatic system. Thus, the present thesis aims to evaluate the environmental factors that influence water quality and the phytoplankton community in seven reservoirs with different trophic levels, in the semi-arid region of Ceará, Northeastern Brazil. Surface water samples (n = 56) were collected quarterly over two years (February 2018 to November 2019) in the lacustrine zone, where the catchment for human supply is located. In each sampling, hydroclimatic variables, physical-chemical variables of the water, and biological components were investigated. The limnological variables were strongly influenced by the seasonal effect (between the rainy and dry seasons) and the results were variable among the reservoirs, indicating that intrinsic responses must be considered. In addition to precipitation, the morphometric characteristics (size and depth) and the land use of the reservoirs were important factors in determining the limnological characteristics. With the reduction in the volume of water, there was an increase in the concentrations of nutrients (phosphorus and nitrogen) and algae biomass (chlorophyll-a) in the reservoirs, regardless of the period of drought or rain. Eutrophication intensified during the rainy season, probably due to the loading of allochthonous material from the basin. The relatively low concentrations of nutrients associated with good water quality conditions in the Tatajuba and Cachoeira reservoirs and moderate concentrations in Prazeres, Olho D'Água, and Rosário seem to be limiting phytoplankton growth and, consequently, primary production in these systems. On the other hand, the Lima Campos and Ubaldinho reservoirs showed the highest concentrations of nutrients, chlorophyll-a, and turbidity regardless of weather conditions, showing that the sources are continuous and the water quality of these reservoirs is more threatened. A total of 159 phytoplanktonic taxa were identified in the investigated reservoirs, distributed mainly in Green algae (77 spp.), Cyanobacteria (52 spp.), and Diatoms (15 spp.), but also Euglenophyceae (eight spp.), Cryptophyceae (three spp.), Xanthophyceae (three spp.) and Dinophyceae (one spp.). The identified taxa were included in 23 functional groups, of which 13 (F, H1, J, K, Lm, Lo, M, MP, N, P, S1, Sn, and W1) were considered reservoir descriptors and contributed > 70% of the total biovolume. In turn, a decrease in richness and diversity was observed towards the higher trophic gradient (eg Lima Campos and Ubaldinho), while the opposite occurred for phytoplankton biovolume. The functional groups were representative of the trophic state of the investigated reservoirs and showed signs of eutrophication in oligo to mesotrophic conditions. Light availability and nutrient concentration were the main factors that determined the environmental threshold for dominance and abundance of functional groups. This approach proved to be a robust tool to distinguish the water quality of reservoirs according to their trophic state. The higher biomass and dominance of cyanobacteria in Lima Campos and Ubaldinho were probably driven by conditions of high turbidity, stable water column, and eutrophication. In these reservoirs, specific taxa of cyanobacteria represented > 98% of the phytoplanktonic biomass and were influenced by different abiotic factors. For example, total nitrogen and phosphorus were positively associated with Microcystis aeruginosa and nitrate with Dolichospermum solitarium, while pH and nitrite were positively associated with Raphidiopsis raciborskii and Planktolyngbya limnetica. Microcystins were detected throughout the study in both reservoirs, with 100% of the samples from Lima Campos above the limit established by Brazilian legislation (> 1 µg L-1) and 50% of the samples from Ubaldinho. Thus, our findings show the need for mitigation measures and better water management in the studied reservoirs, avoiding the deterioration of water quality and compromising its uses since potential cyanotoxin producers are becoming more frequent due to adaptive capacities within the current scenario of global climate changes.