Florações Algais no estado de Pernambuco: causas, consequências e controle

Abstract

This thesis aims to understand the regulatory factors of algal blooms in tropical reservoirs, considering the morphofunctional characteristics of the species, in addition to defining the causes and consequences on phytoplankton and zooplankton and proposing strategies to control these blooms. This study was performed in 10 public water supply reservoirs with different climatic and eutrophication conditions in Pernambuco state, Brazil. Samples were collected quarterly for one year in each environment (n = 42) to understand the role of limnological, climatic, and biotic variables on the intensity of blooms and their effects on planktonic communities. All reservoirs showed cyanobacterial blooms classified as level 1 (biomass >0.2 and <10 mg L-1) or level 2 (biomass >10 mg L-1) of World Health Organization alert. With that, data were distributed in five articles. The first article aimed to evaluate the influence of physicochemical variables and trophic interactions with zooplankton on cyanobacterial blooms. The partial canonical correspondence analysis revealed that blooms were driven mainly by chemical variables, followed by physical and zooplankton variables. To achieve the objective, a structural equation model was built, which demonstrated that the cyanobacterial biomass was mainly favored by the total dissolved phosphorus and the presence of cyclopoid copepods. On the other hand, solar radiation, air temperature, mixing zone, salinity, calanoid copepods, and cladocerans explained the biomass of cyanobacterial morphotypes. In the second article, the objective was to understand the dynamics of the functional groups of Reynolds phytoplankton to establish the relationships between the driving factors of its biomass and distribution. Habitat templates and assembly rules for 20 functional groups were created, integrating data on group preferences and tolerances to 10 environmental gradients. Additive generalized models revealed that salinity, trophic state, reactive soluble phosphorus, water transparency, relative biomass of herbivorous zooplankton, and dissolved inorganic nitrogen were the main predictors of functional group biomass. The third article aimed to understand how algal blooms intensify the deterioration of water quality, alter the diversity and structure of phytoplankton and zooplankton, and affect ecosystem functioning. To that, data were compared to environments without blooms. In this study, cyanobacterial and mixed blooms (cyanobacteria, dinoflagellates, chlorophytes, and diatoms) were associated with an intense deterioration of water quality, reduced richness, and zooplankton resource use efficiency, while these blooms increased phytoplankton species richness and resource use efficiency. On the other hand, blooms of dinoflagellates and chlorophytes had less impact on the environment. The other two articles aimed to analyze the potential control of cyanobacterial blooms through the biomanipulation of submerged macrophytes and large zooplankton (cladocerans), in addition to verifying the impacts of these additions on phytoplankton and zooplankton structures. After 10 days of the experiment, all treatments with macrophyte additions reduced up to 85% of the total biomass of cyanobacteria and up to 99% of filamentous cyanobacteria, even with nutrient additions. Furthermore, macrophytes reduced the biomass of cyanobacteria and green algae, while zooplankton consumed diatoms and phytoflagellates. In this sense, macrophytes were the main driver of the diversity, composition, and structure of native phytoplankton and zooplankton. Therefore, the introduction of submerged macrophytes with allelopathic potential proved to be an efficient technique to control tropical cyanobacterial blooms. In general, this thesis contributes to the understanding of the role of climatic variables, eutrophication, salinity, and trophic interactions on the dominance of several phytoplankton groups, in addition to verifying how submerged macrophytes and large zooplankton can control blooms. The results of this thesis highlight the potential risk of increasing harmful algal blooms in tropical reservoirs, especially in the Brazilian semiarid region, considering the forecasts of increased temperature, prolonged droughts, eutrophication, and salinity in water bodies, with serious impacts on biodiversity and ecosystem functioning. Therefore, mitigation measures need to be adopted to improve the efficiency and conservation of water resources in drinking water reservoirs with blooms, ensuring the supply of potable water.