Cyanobacteria consumption by cladocerans: a case study on facilitation

Abstract

Eutrophic systems are stressful for zooplankton species, especially for small organisms (< 1000 μm) that are inefficient in consuming large colonies or filaments of cyanobacteria. Certain mechanisms, however, enable organisms to coexist in spite of the stress related to poor food quality or manageability of the diet. The present work suggests that coprophagy is recurrent behavior in cladocerans that may facilitate the survival and growth of some species in eutrophic systems. We chose three clones of Moina macrocopa that inhabit eutrophic systems. The species selected as possible facilitators were the cladocerans Simocephalus vetulus and Daphnia similis, and the ostracod Heterocypris incongruens, because they are cosmopolitan, have high rates of cyanobacterial filtration and in some cases coexist with Moina. The design used for the demographic experiments of the three clones of M. macrocopa was based on the source of nutrition and consisted of a control diet of Scenedesmus acutus (1 × 106 cells ml), a diet of undigested cyanobacteria (10 × 106 cells ml), and this same diet after digestion by D. similis, S. vetulus or H. incongruens. The excreta of the cladocerans was a deficient diet for Moina, whose populations decreased by more than 1 individual per day. On the contrary, fecal pellets of the ostracod were a diet that allowed Moina to have positive population growth (0.3 day−1). Our results help to explain the high densities achieved by cladocerans in eutrophic tropical water bodies where cyanobacterial blooms are normal.