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Competition is a major driving force in freshwaters, especially given the cyclic nature and dynamics of pelagic food webs. Competition is especially important in the initial species assortment during colonization and re-colonization events, which depends strongly on the environmental context. Subtle changes, such as saline intrusion, may disrupt competitive relationships and, thus, influence community composition. Bearing this in mind, our objective was to assess whether low salinity levels (using NaCl as a proxy) alter the competitive outcome (measured as the rate of population biomass increase) of  Daphnia-Simocephalus  experimental microcosms, taking into account interactions with priority effects (sequential species arrival order). With this approach, we aimed to experimentally demonstrate a putative mechanism of differential species sorting in brackish environments or in freshwaters facing secondary salinization. Experiments considered three salinity levels, regarding NaCl added (0.00, 0.75 and 1.50 g L −1 ), crossed with three competition scenarios (no priority, priority of  Daphnia  over  Simocephalus , and vice-versa). At lower NaCl concentrations (0.00 and 0.75 g L −1 ),  Daphnia  was a significantly superior competitor, irrespective of the species inoculation order, suggesting negligible priority effects. However, the strong decrease in  Daphnia  population growth at 1.50 g L −1  alleviated the competitive pressure on  Simocephalus , causing an inversion of the competitive outcome in favour of  Simocephalus . The intensity of this inversion depended on the competition scenario. This salinity-mediated disruption of the competitive outcome demonstrates that subtle environmental changes produce indirect effects in key ecological mechanisms, thus altering community composition, which may lead to serious implications in terms of ecosystem functioning (e.g. lake regime shifts due to reduced grazing) and biodiversity.

Competitive Outcome of Daphnia-Simocephalus Experimental Microcosms: Salinity versus Priority Effects