Eutrophication boosts the competitive advantage of invasive gibel carp over endangered crucian carp

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Abstract
Anthropogenic eutrophication poses a significant threat to freshwater environments globally. It also influences the population dynamics of invasive and native species, yet the cumulative effects of eutrophication and invasive species on native organisms are not well understood. We used invasive gibel carp (Carassius gibelio Bloch, 1782) and native crucian carp (Carassius carassius Linnaeus, 1758) as model taxa to investigate how eutrophication influences their interspecific competition. Carassius carassius is being outcompeted by invasive C. gibelio across most of its native range, and eutrophication may play a role in the competitive displacement of C. carassius. We explored how varying eutrophication levels influence growth and survival of both species in laboratory and mesocosm experiments, where we exposed them to different feeding rations and nutrient levels, respectively. We hypothesized that (1) C. gibelio benefits more from increased nutrient levels than C. carassius—which favors invasive C. gibelio over native C. carassius in interspecific competition—and that (2) the growth and survival of C. carassius deteriorate under interspecific competition with C. gibelio compared to intraspecific competition. Our experiments revealed that excess nutrient availability was more beneficial to the invasive C. gibelio. Overall, C. gibelio individuals grew more than C. carassius, and the difference between the two species increased with nutrient levels in both experiments, especially in the weight of smaller individuals. Moreover, interspecific competition led to lower survival in C. carassius but not in C. gibelio in the mesocosm experiment. This study shows that eutrophication may modulate interspecific competition, and cultural eutrophication of freshwater habitats may enhance the success of invasive species. Graphical abstract

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