Abstract
An experimental study using mesocosms was conducted in the main shallow lake of a temperate wetland (Otamendi Natural Reserve, Argentina) to analyse the impact of zooplankton on phytoplankton and the microbial assemblages. The lake is characterised by the presence of a fluctuating cover of floating macrophytes, whose shading effects shape the phytoplakton community and the ecosystem functioning, which was absent during the study period. The experiment was run in situ using polyethylene bags, comparing treatments with and without zooplankton. The cascade effect of zooplankton on phytoplankton and the lower levels of the microbial food web (ciliates, heterotrophic nanoflagellates (HNF) and picoplankton) were analysed. A significant zooplankton grazing on the nano-phytoplankton fraction (3–30 μm) was observed. Conversely, large algae (filamentous cyanobacteria, colonial chlorophytes and large diatoms) increased in all mesocosms until day 10, suggesting that they were not actively grazed by zooplankton during this period. However, from day 10 until day 17 this fraction decreased in the enclosures with mesozooplankton, probably due to an increase in the abundance of large herbivores. The results of the experiment would also indicate a trophic cascade effect on the lower levels of the microbial community. In the treatment where zooplankton was removed, the abundance of ciliates followed the same increasing pattern as the abundance of HNF, but with a time lag in its response. In the enclosures without zooplankton, HNF remained relatively constant throughout the experiment, whereas ciliates strongly decrease during the last week. Total picoplankton abundance increased in the enclosures with mesozooplankton, thus supporting the existence of a four-link trophic cascade (copepods–microzooplankton–HNF–picoplankton). Zooplankton composition changed significantly from the beginning until the end of the experiment; cyclopoid nauplii and rotifers were notoriously dominant at t 0, whereas 10 days later the community showed a more equitable proportion of cyclopoids, calanoids, nauplii, cladocerans and rotifers.
Highlights
Zooplankton grazing usually provokes a decrease in phytoplankton biomass; because of the effect of the selective feeding, some inedible algae may increase their abundances in a lake during active grazing phases, since they can take advantage of the availability of nutrients when the competition pressure with other algae diminishes (Queimalinos, Modenutti, & Balseiro, 1998)
Most of the studies conducted in the aquatic systems of the Otamendi Reserve were focused on the biodiversity and ecology of the phytoplankton community
The phytoplanktonic communities of this wetland are characterised by their high biodiversity
Summary
The effects of zooplankton grazing on the structure of prey assemblages, and its impact at the lower levels via trophic cascades were discussed in many papers (Carpenter & Kitchell, 1988; Carpenter, Kitchell, & Hodgson, 1985; Gliwicz, 2002; Pace, Cole, & Carpenter, 1998; Schnetzer & Caron, 2005; Sommer et al, 2001).Zooplankton grazing usually provokes a decrease in phytoplankton biomass; because of the effect of the selective feeding, some inedible algae may increase their abundances in a lake during active grazing phases, since they can take advantage of the availability of nutrients when the competition pressure with other algae diminishes (Queimalinos, Modenutti, & Balseiro, 1998). Ciliates can contribute to the grazing on picoplankton, HNF are often the main grazers on this fraction (Hahn & Hofle, 2001; Sanders, Porter, Bennett, & DeBiase, 1989). Another important effect of lake zooplankton is the recycling of nutrients, when they are limiting for phytoplankton (Attayde & Hansson, 1999; Balseiro, Modenutti, & Queimalinos, 1997; Carrillo, Reche, Sanchez Castillo, & Cruz-Pizarro, 1995; Queimalinos et al, 1998). In oligotrophic lakes, which are more dependent on internal recycling, zooplankton regulates the availability of nutrients via excretion (Urabe, 1993)
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