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Omnivory does not prevent trophic cascades in pelagic food webs


José Luiz Attayde, Departamento de Botânica, Ecologia e Zoologia, Universidade Federal do Rio Grande do Norte, Natal, CEP 59072-970, RN, Brazil.


1. Strong trophic cascades have been well documented in pelagic food webs of temperate lakes. In contrast, the limited available evidence suggests that strong cascades are less typical in tropical lakes.

2. To measure the effects of omnivorous tilapia on planktonic communities and water transparency of a small man-made tropical lake, we performed a 5-week in situ enclosure experiment with five densities of fish randomly allocated to 20 enclosures. Zooplankton and Phytoplankton biomasses as well as water transparency were measured weekly.

3. Results show that omnivorous tilapia significantly decreased the abundance of large Cladocerans, increased the abundance of small algae (greatest axial linear dimension <50 μm) and decreased water transparency as predicted by trophic cascade theory.

4. Therefore, omnivory was not a sufficient factor to prevent a trophic cascade in this pelagic community, although the cascade effect was weaker than reported from many north temperate, nutrient-rich lakes.