Why predation is not a controlling factor of phytoplankton in a Neotropical shallow lake: a morpho-functional perspective

Abstract

This work explores the reasons why predation plays a minor role in structuring phytoplankton composition in a Neotropical shallow lake. Phytoplankton, zooplankton, and fish were sampled from a shallow lake over the course of a year, and the stomach contents of 80 individuals of the dominant omnivorous–planktivorous fish species were analyzed. The field study was complemented with a 5-day microcosm experiment in which the predation effects of micro, meso, and macrozooplankton were measured. Stomach content analysis revealed that fish predation was high, and primarily comprised by Cladocera (Ivlev’s index >0.75). Both meso and macrozooplankton fractions are able to feed on colonial cyanobacteria, silica cell-wall organisms (<35 µm), and mixotrophic flagellate (>35 µm). Macrozooplankton, however, can feed on single cells, mucilaginous colonies, non-mucilaginous colonies, and silica cell-wall organisms (>35 µm) (P < 0.05 in all cases). Our results support the idea that absence of predation on phytoplankton is mainly mediated by fish predation on zooplankton and morpho-functional characteristics of algae which prevent zooplankton predation; showing fish a lack of direct predation effect on phytoplankton.