Abstract The steady-state is a situation of little variability of species dominance and total biomass over time. Maintenance of cyanobacteria are often observed in tropical and eutrophic ecosystems and can cause imbalances in aquatic ecosystem. Bayeasian networks allow the construction of simpls models that summarizes a large amount of variables and can predict the probability of occurrence of a given event. Studies considering Bayeasian networks built from environmental data to predict the occurrence of steady-state in aquatic ecosystems are scarce. This study aims to propose a Bayeasian network model to assess the occurrence, composition and duration of cyanobacteria steady-state in a tropical and eutrophic ecosystem. It was hypothesized long lasting steady-state events, composed by filamentous cyanobacteria species and directly influenced by eutrophication and drought. Our model showed steady-state lasting between 3 and 17 weeks with the monodominance or co-dominance of filamentous species, mainly Raphidiopsis raciborskii and Planktothrix agardhii. These evens occurred frequently under drought and high turbidity, however higher nutrients concentrations did not increase the probability steady-state occurrence or longer duration. The proposed model appears as a tool to assess the effects of future warming on steady-state occurrence and it can be a useful to more traditional process-based models for reservoirs.

Este estudo investigou a qualidade da água em um reservatório eutrófico urbano no Nordeste do Brasil, considerando a influência da sazonalidade. Mensalmente, amostras foram coletadas na sub-superfície do reservatório. As seguintes variáveis abióticas foram analisadas: temperatura, pH, oxigênio dissolvido, cor aparente, turbidez, condutividade, flúor, nitrogênio total, cloretos, sólidos totais dissolvidos, dureza total, ferro, cobre, manganês, alumínio, clorofila-a e feofitina. Coliformes totais e termotolerantes foram analisados de acordo com APHA (2012). A densidade de cianobactérias foi quantificada através de sua biomassa. Os dados foram analisados por meio de análise de variância (one-way ANOVA) e teste de correlação de Pearson. Elevados valores médios de biomassa de fitoplâncton (26.3mm3.L–1) ocorreram na estação seca, com destaque para Planktothrix agardhiie Geiterinema amphibium, que ocorreram em 100% das amostras. Elevado índice de estado trófico foi detectado ao longo do ano. Sazonalidade exerceu alguma influência em variáveis tanto bióticas como abióticas, ocasionando alterações na qualidade da água do reservatório.

This study investigated the water quality in an urban eutrophic reservoir in Northeastern Brazil, considering the influence of seasonality. Monthly, samples were collected in the sub-surface reservoir. The following abiotic variables were analyzed: temperature, pH, dissolved oxygen, apparent color, turbidity, conductivity, fluoride, total nitrogen, chlorides, total dissolved solids, total hardness, iron, copper, manganese, aluminum, chlorophyll-a and phaeophytin. Total and thermotolerant coliforms were analyzed according to APHA (2012). Cyanobacteria density was quantified through its biomass. The data were analyzed using one- way ANOVA and Pearson correlation test. Higher values mean phytoplankton biomass (26.3mm3.L–1) occurred in the dry season, especially Planktothrix agardhiiand Geiterinema amphibium, which occurred in 100% of samples. High trophic state index was detected throughout the year. Seasonality exerted some influence on both biotic and abiotc variables, leading to changes in water quality of the reservoir.