Microbial Food Webs in an Artificially Divided Acidic Bog Lake

Abstract

The microbial loop of a naturally acidic bog lake, GroSe Fuchskuhle (Northeastern Germany), that had been artificially divided into 4 basins, was investigated. In the northeast (NE) and southwest (SW) basins, which differ strongly in chemistry and primary production, we conducted intensive studies of the main carbon fluxes through microbial food webs. In the less acidic, NE basin, much higher phytoplankton as well as bacterial biomass and production were found in parallel with negligible numbers of larger zooplankters. Weakly top-down controlled populations of protists were characterized by an exceptionally low numerical proportion of heterotrophic nanoflagellates (HNF) to ciliates (-1.5-3.5). The ciliate community was dominated by a scuticociliate, Cyclidium sp. (>95% of total ciliates), with an estimated grazing rate equal to 4 6 4 0 % of heterotrophic bacterial production. In contrast, in the more humic, SW basin, both phyto-and bacterioplankton dynamics seemed to be top-down controlled by abundant populations of small fine-filter feeding cladocerans, Cerioduphnia quadrungula and Diuphanosomu bruchyurum. Consequently, ciliates disappeared from the food web structure of the SW basin, HNF dropped to negligible numbers and bacteria showed very uniform morphology, dominated by small cocci or short rods. Our investigations have shown that the division of the lake into separate compartments can lead to very different microbial food web structures with extreme species compositions.

Members of a Readily Enriched β-Proteobacterial Clade Are Common in Surface Waters of a Humic Lake

“…During the summer months the lake forms a steep oxycline, and the two studied compartments differ in several fundamental parameters, e.g., water chemistry, primary productivity, the structure of the phytoplankton community, and the structure of the microbial food web (4,17,45,49). In a previous study, the more acidified, humic-acid-richer SW compartment was inhabited by large cladocerans, which effectively suppressed protistan bacterial predators (49). Bacterial mortality in the more productive NE compartment was caused mainly by a small oligotrich ciliate, Cyclidium sp., which is rather untypical for freshwater systems.…”

“…1). Since the division, the compartments have diverged in their physical and chemical parameters, in microbial activity (2,4), and in the structure of the microbial food web (17,49). For this study two of the compartments were selected: the SW compartment, which has an acidotrophic humic character and a higher influx of allochthonous DOC, and the NE compartment, which has a more mesotrophic character.…”

“…This lake is divided into artificial compartments with contrasting limnological properties and plankton community structure (17,24,49). Enrich-ment experiments with inocula from two of these compartments were performed with dilutions of ambient water from the more acidic, humic-rich compartment.…”

Trophic roles of heterotrophic nanoflagellates and ciliates among planktonic organisms in a hypereutrophic pond

“…Thus, the decrease in protozoan densities may have been due to Microcystis toxin. Simek & Straskrabova (1992) and Simek et al (1998) noted that the genus Cyclidium became dominant when the abundance of large or floc-forming phytoplankters was high in a mesoeutrophic reservoir and a dystrophic lake, respectively. However, we could not find any coupled oscillations between abundance of Cyclidium spp.…”

“…Thus, not only HNF but also ciliates are important bacteria consumers in Furuike Pond. There is partitioning of bacterial food between HNF, ciliates and cladocerans (Sanders et al 1989, Simek et al 1990, 1998, Simek & Straskrabova 1992, Nakano et al 1998, and consumption of bacteria by ciliates occasionally becomes the largest in total consumption of bacteria in mesoeutrophic (Simek et al 1990, Simek & Straskrabova 1992 and dystrophic (Simek et al 1998) lakes. However, we still have limited information about bacterial consumption by protozoa in hypereutrophic lakes.…”