Potential drivers of seasonal shifts in fish omnivory in a subtropical stream
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The trophic structure of fish assemblages often varies seasonally, following the changes in food availability and supposedly water temperature. To unveil potential drivers of trophic shifts, we studied changes in fish trophic structure at both whole-assemblage and species levels at contrasting food availability and water temperatures in a subtropical stream. We analysed the diet of the most abundant omnivorous species (Bryconamericus iheringii) monthly along the year, searching for relationships with environmental variables changing seasonally (i.e. temperature and water level) and with fish reproductive stage. We ran a single-species food choice field experiment with fixed animal and vegetal food availability in contrasting seasons to test food availability as driver of diet shifts. At the assemblage level, we found a higher consumption of vegetal during summer, reflecting the increased proportion of vegetal in the diet of omnivores, which dominated the assemblage. At the species level, the enhanced vegetal consumption was related to increases in temperature and reduction in water level. Moreover, fish selected for vegetal during summer and for animal food in winter under experimental conditions. Our findings support the role of temperature driving food web dynamics by increasing fish herbivory towards warmer scenarios, with potential strong implications for whole-assemblage trophic structure.
KeywordsFish diet Stream seasonality Temperature effect Food availability Stream food webs Fish herbivory
This research was funded by the Uruguayan Agency for Research and Innovation (ANII), project ANII_FCE 2009_1_2530 and partially by ANII_ FCE 2009 2749. MM was supported by the L´Oréal-UNESCO national award for Women in Science and ANII -SNB (to IGB) (Uruguay). MM and EJ were also supported by Danish FNU (Funds for research of Nature and Universe) grant (16-7745) and the MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the 7th EU Framework Programme, Theme 6 (Environment including Climate Change), Contract No.: 603378 (http://www.mars-project.eu), the Research Council for Nature and Universe (272-08-0406), and CIRCE (Center for Research into Complexity in Ecology). IGB, GG, MM and FTM were supported by SNI-ANII (National System of Researchers) (Uruguay). We thank Anne Mette Poulsen for manuscript editing; Carlos Iglesias, Claudia Fosalba, Natalie Corrales and Cinthia Rojas for field and lab assistance; and also landowner Oscar Laturre for supporting the field work.
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