Abstract
Abstract: 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 the upper Paraná River floodplain. 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 abundance and positive with rotifers abundance, while for their size structure, negative relationships were found with heterotrophic bacteria, ciliates and rotifers biovolumes. In relation to the ciliates, their abundances were positively related to the rotifers and picophytoplankton abundances and negatively with the heterotrophic bacteria abundance. On the other hand, for the size structure, the best models selected strong negative relations with the microcrustaceans biovolumes, 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.
Highlights
The understanding of the ecosystem functioning occurs mostly through the knowledge of species interactions within its food webs, through which the energy and matter flow (Pomeroy, 1974)
The components of microbial food webs can be strongly affected by both bottom-up control – related to resource availability in the environment (Gasol et al, 1995; Šimek et al, 2003) – and top-down control, which is related to predation effects (Šimek et al, 1997; Auer et al, 2004)
Some studies found a strong negative impact of rotifer predation on ciliates (Lischke et al, 2016; Weisse & Frahm, 2002), the positive relationships evidenced in our study suggest that they are both benefiting from the same resource, in this case, heterotrophic bacteria and/or picophytoplankton, or, alternatively, that they are being controlled by the same predators, such as microcrustaceans (Li et al, 2016)
Summary
The understanding of the ecosystem functioning occurs mostly through the knowledge of species interactions within its food webs, through which the energy and matter flow (Pomeroy, 1974). Since the concepts of microbial loop (Azam et al, 1983) and microbial food web (Sherr & Sherr, 1988), studies approaching the interactions among its components were recognized as a key stone for understanding the food web structure, due to its crucial role in nutrient cycling, biomass accumulation, and carbon flow (Weisse, 2002). If the abundance and biomass of lower trophic levels remain unchanged, the top‐down control will certainly be visible in the size structure of organisms, which is known as “partial trophic cascade” (Sommer, 2008)
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