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Salinity is a well-recognised factor that shapes population dynamics and community structure through direct and indirect effects. The responses of Moina micrura (Cladocera) to slight salinity increases (up to 6.0) were evaluated through laboratory manipulative experiments and field observations at Cabiúnas lagoon (Rio de Janeiro, Brazil). From the field data, a positive relationship between M. micrura density and salinity was observed, whereas the opposite was found for other co-occurring microcrustaceans. Life-table observations from laboratory experiments showed better performance of individuals exposed to control conditions (no salt addition) and 1.0 and 2.0 of salinity. The difference between field and laboratory results may be explained considering possible direct and indirect effects of salinity on M. micrura populations. Salinity may have direct negative physiological effects in individuals. However, under natural conditions, M. micrura might benefit from reduced invertebrate predation and interspecific competition. This balance between negative and positive effects of salinity is probably more positive under natural conditions, explaining the success of M. micrura in Cabiúnas lagoon under mild salinity conditions. Although salinity negatively affects many species living in freshwater systems, slight increases may be beneficial for some species under natural conditions.

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Element cycling in aquatic systems is driven chiefly by planktonic processes, and the structure of the planktonic food web determines the efficiency of carbon transfer through trophic levels. However, few studies have comprehensively evaluated all planktonic food-web components in tropical regions. The aim of this study was to unravel the top-down controls (metazooplankton community structure), bottom-up controls (resource availability), and hydrologic (water residence time) and physical (temperature) variables that affect different components of the microbial food web (MFW) carbon stock in tropical reservoirs, through structural equation models (SEM). We conducted a field study in four deep Brazilian reservoirs (Balbina, Tucuruí, Três Marias, and Funil) with different trophic states (oligo-, meso-, and eutrophic). We found evidence of a high contribution of the MFW (up to 50% of total planktonic carbon), especially in the less-eutrophic reservoirs (Balbina and Tucuruí). Bottom-up and top-down effects assessed through SEM indicated negative interactions between soluble reactive phosphorus and phototrophic picoplankton (PPP), dissolved inorganic nitrogen, and heterotrophic nanoflagellates (HNF). Copepods positively affected ciliates, and cladocerans positively affected heterotrophic bacteria (HB) and PPP. Higher copepod/cladoceran ratios and an indirect positive effect of copepods on HB might strengthen HB-HNF coupling. We also found low values for the degree of uncoupling (D) and a low HNF/HB ratio compared with literature data (mostly from temperate regions). This study demonstrates the importance of evaluating the whole size spectrum (including microbial compartments) of the different planktonic compartments, in order to capture the complex carbon dynamics of tropical aquatic ecosystems.