Efeito das macrófitas como habitat na interação entre fitoplâncton-zooplâncton na presença de peixes planctívoros em reservatórios tropicais com diferentes estados tróficos

Abstract

The present thesis aimed to analyze the trophic interaction between phytoplankton, zooplankton and fish in the presence and absence of floating and submerged macrophytes in two tropical reservoirs of public supply, Cursaí and Cajueiro, located in Northeastern Brazil. The thesis was structured in three manuscripts. Quarterly samplings were carried out between November/2018 and November/2019 to collection and analysis of phytoplankton, chlorophyll-a, floating and submerged macrophytes, nutrients and other abiotic variables for writing the first manuscript, in addition to zooplankton for writing the second manuscript. The samples were carried out in nine sampling points in each reservoir: three without macrophytes, three in floating macrophyte banks and three in submerged macrophyte banks. In addition to field monitoring, an in situ experiment was carried out at the Cursaí reservoir for the writing of the third manuscript, using data from phytoplankton, zooplankton, invertebrate predators (Chaoborus), fish (Astyanax lacustris Lütken, 1875) and abiotic variables. The top-down and bottom-up control promoted by the fish A. lacustris and the effects of the morphology of floating and submerged macrophytes on the trophic interaction were analyzed. The experiment lasted ten days and consisted of four treatments (n = 3): a control (C) without fish, and treatments with the addition of fish (+F), fish + floating macrophyte (+FFM) and fish + submerged macrophyte (+FSM). In treatments with macrophytes, artificial plants were added as a refuge for zooplankton and Chaoborus. In the first manuscript, phytoplankton was classified based on morphological type, resource acquisition, and behavioral characteristics. The effect of macrophytes on the taxonomic and functional diversity of phytoplankton was evaluated using the indices of taxonomic diversity (richness, equitability and Shannon index) and functional diversity (richness - FRic, evenness - FEve and divergence - FDiv). Macrophytes exerted positive effects on taxonomic richness, Shannon index and FDiv in Cajueiro, and negative effects on taxonomic evenness and EFve of phytoplankton in Cursaí. Phytoplankton functional diversity was negatively influenced by water transparency and positively by total dissolved solids, pH and nitrate in sites with macrophytes. In the second manuscript, phytoplankton was classified according to type of life and size, and zooplankton based on type of feeding strategies. The main question of the work was: do macrophytes favor nutrient-phytoplankton and phytoplankton-zooplankton interaction? The results showed that macrophytes favored the growth of small and large algae and unicellular, colonial and filamentous species, as well as favored the nutrient-phytoplankton interaction. In open water areas (without macrophytes), flagellated phytoplankton were positively influenced by nitrate and inorganic phosphate, while unicellular, colonial, filamentous, small, medium and large phytoplankton were positively influenced by total phosphorus and nitrite. Colonial phytoplankton, mainly represented by the cyanobacteria Coelomoron tropicale P.A.C.Senna, A.C.Peres & Komárek, was positively related to zooplankton in areas with macrophytes, and flagellated phytoplankton was negatively related to open water areas. In the third manuscript, it was observed that A. lacustris had a top-down effect on zooplankton and a bottom-up effect on phytoplankton, and Chaoborus interfered in the functioning of food webs. Submerged macrophytes were efficient as a refuge for copepods and Chaoborus, while floating macrophytes favored an increase in the biomass of cyanobacteria, desmids and diatoms. Floating and submerged macrophytes played different roles in the interactions between aquatic communities and Astyanax lacustris was a trophic network modifier in the studied reservoirs, as well as Chaoborus which increased the trophic levels to four in the Cursaí reservoir. The results found in the present study contributed to the understanding of the role of floating and submerged macrophytes in the trophic interactions of tropical reservoirs. In addition, the results can help the development of strategies for biomanipulation actions to maintain water quality in tropical public supply reservoirs.