Stable isotope analysis confirms substantial differences between subtropical and temperate shallow lake food webs
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Differences in trophic web structure in otherwise similar ecosystems as a consequence of direct or indirect effects of ambient temperature differences can lead to changes in ecosystem functioning. Based on nitrogen and carbon stable isotope analysis, we compared the food-web structure in a series of subtropical (Uruguay, 30–35°S) and temperate (Denmark, 55–57°N) shallow lakes. The food-web length was on average one trophic position shorter in the subtropical shallow lakes compared with their temperate counterparts. This may reflect the fact that the large majority of subtropical fish species are omnivores (i.e., feed on more than one trophic level) and have a strong degree of feeding niche overlap. The shapes of the food webs of the subtropical lakes (truncated and trapezoidal) suggest that they are fuelled by a combination of different energy pathways. In contrast, temperate lake food webs tended to be more triangular, likely as a result of more simple pathways with a top predator integrating different carbon sources. The effects of such differences on ecosystem functioning and stability, and the connection with ambient temperature as a major underlying factor, are, however, still incipiently known.
KeywordsFood-web structure Food-web length Omnivory Ecosystem functioning
We are grateful to Anne Mette Poulsen for manuscript editing and to Tinna Christensen for improving the figures. We also thank Frank Landkildehus, Kirsten Landkildehus Thomsen, and Mette E. Bramm in Denmark; and Juan M. Clemente, Claudia Fosalba, Soledad García, Nicolas Vidal, Natalia Barberán, Malvina Masdeu, Mariana Vianna, and Alejandra Kroger in Uruguay, for valuable field assistance. The project was supported by the Ministry of Science, Technology and Innovation of Denmark. EU-WISER and EU-REFRESH, “CLEAR” (a Villum Kann Rasmussen Centre of Excellence project), CRES, CIRCE, and The Research Council for Nature and Universe (272-08-0406 and FNU 16-7745) supported EJ. CI was supported by a PhD Scholarship from Aarhus University-Danish Research Agency. NM was supported by Maestría en Ciencias Ambientales, and NM, MM, and CI were supported by PEDECIBA. NM, MM, FTM, and CI were supported by SNI (ANII) and MM also by ANII-FCE 2009-2749 and the L´Oréal-UNESCO (supported by DICYT) for Women in Science national award. We deeply acknowledge the constructive comments of two anonymous reviewers and the handling editor Katya Kovalenko.
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