Efeito da sazonalidade na regulação bottom-up e top-down do fitoplâncton em reservatórios semiáridos tropicais

• Ano de defesa: 2026
• Autor(a) principal: Cesar, Maria Eduarda Dantas
• Orientador(a): Saveriano, Juliana dos Santos
• Banca de defesa: Costa, Mariana Rodrigues Amaral , Santos, Felipe Antônio dos
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Estadual da Paraíba; 24004014011P9
• Departamento: Centro de Ciências Biológicas e da Saúde - CCBS
• País: BR
• Palavras-chave em Português: Comunidade fitoplanctônica; Dinâmica sazonal; Interações tróficas; Nutrientes; Zooplâncton
• Área do conhecimento CNPq: CIENCIAS BIOLOGICAS
• Link de acesso: https://repositorio.uepb.edu.br/handle/123456789/77110

The phytoplankton community in aquatic ecosystems is strongly influenced by bottom-up and top-down regulations; however, the influence of seasonality on these regulatory mechanisms, especially in tropical semi-arid regions, is still poorly understood. This study aims to evaluate the effect of seasonality on bottom-up (nutrients) and top-down (zooplankton grazing) regulations on the phytoplankton community in tropical semi-arid reservoirs. We hypothesize that (i) bottom-up regulation is relatively more important in the dry season, associated with higher nutrient concentrations, and (ii) top-down regulation will be more effective on smaller phytoplankton groups without morphological or chemical defenses (e.g., cryptophytes, green algae), while groups with grazing resistance characteristics (e.g., cyanobacteria/toxic organisms, large dinoflagellates) will be primarily structured by bottom-up regulation. and that (iii) herbivorous filter feeders, especially medium and large-sized ones, exert the strongest direct control over palatable phytoplankton, while predators and omnivores act predominantly indirectly, via trophic cascades. The study was conducted in 8 reservoirs in Paraíba, Northeast Brazil, during the dry (December/21, November/22 and December/23) and rainy (May/22, June/23 and August/24) seasons. Samples were collected at three sampling locations (lentic, transition and transition) and three depths (surface, 1% and bottom). To investigate the relationships and relative importance of nutrients and zooplankton (biomass and functional groups) on phytoplankton groups during the dry and rainy seasons, piecewise SEM structural equation modeling and partial redundancy analysis (pRDA) were used, respectively. Our results showed that seasonality modulated phytoplankton community biomass, with a greater influence from top-down regulation, mainly in the dry season, as well as for all phytoplankton groups. In the dry season, total phosphorus had a positive effect on Cyanobacteria, while ammonia had a negative effect on Cyanobacteria, Bacillariophyceae, and Euglenophyceae; in the rainy season, total phosphorus was positive for Cyanobacteria and negative for "green algae," and nitrogen forms and the NID/FRS ratio negatively affected most groups. As for zooplankton, herbivorous filter feeders had both positive and negative effects on phytoplankton groups, mainly on Cyanobacteria, "green algae," Bacillariophyceae, and Dinophyceae, while omnivorous raptors acted on Cryptophyceae, Euglenophyceae, Cyanobacteria, and green algae, and carnivorous predators influenced Bacillariophyceae, Cynobacteria, and Euglenophyceae. We conclude that seasonality acts as a modulator of bottom-up and top-down regulations on phytoplankton. The complexity of these patterns reinforces the challenges inherent in understanding the combined effects of multiple processes in aquatic ecosystems, particularly in freshwater systems subject to eutrophication. Future studies incorporating functional group-based approaches may broaden the understanding of these interactions, since functional characteristics influence the responses of the phytoplankton community to environmental and trophic pressures.