Ciliates are the Dominant Grazers on Pico- and Nanoplankton in a Shallow, Naturally Highly Eutrophic Lake

Abstract

Abundance and biomass of the microbial loop members [bacteria, heterotrophic nanoflagellates (HNF), and ciliates] were seasonally measured in the naturally eutrophic and shallow (2.8 mean depth) Lake Võrtsjärv, which has a large open surface area (average 270 km2) and highly turbid water (Secchi depth <1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-microm-diameter particles were measured to estimate pico- and nanoplankton grazing (filter feeding) by micro- and metazooplankton. Among grazers, HNF had a low abundance (<50 cells mL-1) and, due to their low specific filtering rates, they only grazed a minor fraction of the bacterioplankton (<or=4.2% of total grazing). Ciliates were relatively abundant (<or=158 cells mL-1) and, considering their high specific feeding rates, were able to graze more than 100% of the bacterial biomass production in the open part of the lake, whereas the average daily grazing accounted for 9.3% of the bacterial standing stock. Ciliates were potentially important grazers of nanoplanktonic organisms (on average, approximately 20% of the standing stock of 3-microm-size particles was grazed daily). Metazooplankton grazed a minor part of the bacterioplankton, accounting for only 0.1% of standing stock of bacteria. Grazing on nanoplankton (3-6 microm) by metazooplankton was higher (0.4% of standing stock). The hypothesis is proposed that ciliates dominate due to a lack of top-down regulation by predators, and HNF have a low abundance due to strong grazing pressure by ciliates.

Contributions of autochthonous and allochthonous sources to dissolved organic matter in a large, shallow, eutrophic lake with a highly calcareous catchment

“…found that, unlike several other eutrophic and shallow lakes, the contribution of macrophytes to the Lake Võ rtsjä rv C budget was low (only 5%). Carbon biomass estimates from Zingel et al (2007) showed that filamentous phytoplankton and bacteria were the most important reservoirs of the living carbon biomass in Võ rtsjä rv. Against our expectations, d 13 C signatures of Au-DOM in Võ rtsjä rv were lower than those of the allochthonous riverine DOM.…”

The trophic role and impact of plankton ciliates in the microbial web structure of a tropical polymictic lake dominated by filamentous cyanobacteria

“…These findings may generalize to an almost cosmopolitan distribution throughout these aquatic systems (Pfister et al, 2002). Several of the observed morphotypes, such Uronema, Cyclidium, Rimostrombidium (Šimek et al, 1995;Zingel et al, 2007), Mesodinium, Colpoda (Foissner et al, 1999;Zingel and Nõges, 2010), and Halteria (Šimek et al, 1995), graze on picoplankton, these may include nanociliates which play an important role in energy and matter transfer through the microbial web because of the high metabolic rates associated with their small size.…”

“…Physical interference by trichomes has been proposed as a mechanism that reduces ciliate predation, and it is an important process because top-down control by predatory fish likely has a strong effect on the microbial web of tropical lakes (Havens et al, 2007). Zingel et al (2007) determined that ciliates are the dominant microbial grazers at a temperate lake that is also dominated by filamentous cyanobacteria because top-down control by fish predators did not occur under these conditions. The dominant cyanobacteria at Catemaco produce toxins (Berry and Lind, 2010) that have unknown effects on other components of the microbial web, and the effect of these toxins on the ciliate assemblages is also unknown.…”

“…The relative importance of ciliates appears to be higher in polymictic lakes where these cyanobacteria are abundant than in other temperate lakes (Zingel et al, 2007). Moreover, the ciliate importance to the food web increases with higher primary productivity (Sherr and Sherr, 2002), although it remains to be shown if this relationship holds true for tropical lakes, such as Catemaco.…”

“…Moreover, the ciliate importance to the food web increases with higher primary productivity (Sherr and Sherr, 2002), although it remains to be shown if this relationship holds true for tropical lakes, such as Catemaco. The nature of lacustrine trophic microbial web components and their interactions have been studied for temperate lakes with different trophic statuses (Straškrabová et al, 1999;Pfister et al, 2002;Auer et al, 2004;Chróst et al, 2009;Agasild et al, 2013;Van Wichelen et al, 2013), including eutrophic temperate lakes affected by cyanobacterial blooms (Moustaka-Gouni et al, 2006;Zingel et al, 2007;Zingel and Nõges, 2010), shallow soda lakes of Africa (Burian et al, 2013), relatively shallow eutrophic lakes, deep oligotrophic tropical lakes of Eastern Africa (Yasindi and Taylor, 2003), deep athalassohaline lakes of North America Peštová et al, 2008), and shallow subtropical lakes with differing trophic statuses (Beaver and Crisman, 1989;Beaver and Crisman, 1990;Gomes and Godinho, 2003;MachadoVelho et al, 2005;Havens et al, 2007;Bagatini et al, 2013;Machado-Velho et al, 2013). These studies have demonstrated that microbial food webs, particularly the composition, abundance, biomass and diversity of ciliates, present specific responses to the geographic, climatic and ecological features of different lakes and reservoirs.…”