The role of microorganisms in a planktonic food web of a floodplain lake.

Food webs include complex ecological interactions that define the flow of matter and energy, and are fundamental in understanding the functioning of an ecosystem. Temporal variations in the densities of communities belonging to the planktonic food web (i.e., microbial: bacteria, flagellate, and ciliate; and grazing: zooplankton and phytoplankton) were investigated, aiming to clarify the interactions between these organisms and the dynamics of the planktonic food web in a floodplain lake. We hypothesized that hydrological pulse determines the path of matter and energy flow through the planktonic food web of this floodplain lake. Data were collected monthly from March 2007 to February 2008 at three different sites in Guaraná Lake (Mato Grosso do Sul State, Brazil). The path analysis provided evidence that the dynamics of the planktonic food web was strongly influenced by the hydrological pulse. The high-water period favored interactions among the organisms of the microbial loop, rather than their relationships with zooplankton and phytoplankton. Therefore, in this period, the strong interaction among the organisms of the grazing food chain suggests that the microbial loop functions as a sink of matter and energy. In turn, in the low-water period, higher primary productivity appeared to favor different interactions between the components of the grazing food chain and microorganisms, which would function as a link to the higher trophic levels.

Planktonic heterotrophic nanoflagellate abundance along a trophic gradient in subtropical reservoirs: influence of a bottom-up or top-down control mechanism? / Abundância de nanoflagelados heterotróficos ao longo de um gradiente trófico em reservatórios tropicais: influência do mecanismo de controle bottom-up ou top-down?

The aim of the current study was to assess the variability in heterotrophic nanoflagellate abundance attributable to variables responsible for the influence of bottom-up and top-down control mechanisms in the plankton food web along a trophic gradient in subtropical reservoirs (Paraná State, Brazil). In particular, we hypothesised that food resources were the main determinant for the abundance of heterotrophic nanoflagellates (HNF) along a trophic gradient. Results showed that HNF communities were weakly influenced by the bottom-up mechanism. Moreover, there was evidence of a slight influence from a top-down control mechanism on this community. Therefore, the results of the present study did not corroborate the hypothesis initially proposed, and further studies are needed to elucidate the complex outcomes of trophic gradients on planktonic HNF communities, including other predictor variables related to abiotic interactions, such as morphometrics, hydrology, priorities uses and climate conditions.

O objetivo do presente estudo foi investigar a variabilidade da abundância de nanoflagelados heterotróficos (NFH) planctônicos atribuída a variáveis responsáveis pela influencia dos mecanismos de controle bottom-up e top-down na cadeia alimentar planctônica ao longo de um gradiente trófico em reservatórios subtropicais (Estado do Paraná, Brasil). Nós avaliamos a hipótese de que a abundância de NFH aumenta ao longo de um gradiente trófico principalmente devido ao efeito bottom-up. Os resultados mostraram importantes relações presa-predador entre NFH e bactéria e fioplâncton. Entretanto, a ausência de um padrão claro de aumento da abundância de NFH em direção aos reservatórios eutróficos sugere que o controle bottom-up não foi relacionado ao gradiente trófico. Além disso, apesar de significante, um fraco efeito bottom-up e um sinal do mecanismo de controle top-down sugerem que o recurso alimentar não foi o principal fator regulador da dinâmica de NFH nos reservatórios estudados. Assim, os resultados do presente estudo não corroboraram a hipótese inicialmente proposta e futuros estudos são necessários para elucidar a complexidade da influencia de gradientes tróficos em NFH, incluindo outras variáveis preditoras relacionadas às interações abióticas como morfometria, hidrologia, prioridade de uso e condições climáticas.