Abundance and size structure of planktonic protist communities in a Neotropical floodplain: effects of top-down and bottom-up controls

Bianca Ramos Meira, Felipe Rafael de Oliveira, Fernando Miranda Lansac-Tôha, Bianca Trevizan Segovia, Paulo Roberto Bressan Buosi, Luzia Cleide Rodrigues, Susicley Jati, Fábio Amodêo Lansac-Tôha, Luiz Felipe Machado Velho


Aim: We aimed to assess the influence of bottom-up and top-down control mechanisms on the abundance and size structure of  protist communities (heterotrophic flagellates and ciliates). We formulated the following hypothesis: bottom-up control mechanisms, related to the availability of resources in the environment, are responsible for structuring the abundance of these communities, whereas top-down control mechanisms, related to predation effects, determine the size pattern of these organisms. Methods: Samples for planktonic organisms were taken in 20 shallow lakes belonging to three different subsystems of the upper Paraná River floodplain (Paraná, Baía and Ivinheima rivers). We evaluated linear regression models to select the best model which predicts the patterns observed according to Akaike Information Criterion. Results: The best models selected to explain the abundance of heterotrophic flagellates included negative relations with picophytoplankton and positive with rotifers, while for their size structure, negative relationships were found with heterotrophic bacteria, ciliates and rotifers. In relation to the ciliates, their densities were positively related to the rotifers and picophytoplankton and negatively with the heterotrophic bacteria. On the other hand, for the biovolume, the best models selected strong negative relations with the microcrustaceans, in addition to relations with the different fractions of the phytoplankton. Conclusion: For both flagellates and ciliates, their abundance is being mainly regulated by a bottom up control mechanism, whereas for the size structure the results showed that both food resources and predators were important, indicating that bottom-up and top-down mechanisms act simultaneously in determining the size of these microorganisms.


Protist; Plankton; Freshwater; Size structure; Shallow lakes; Microbial food web

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