Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Abstract

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter(-1) d(-1) and decreased thereafter to a level of approximately 124 μg C liter(-1) d(-1). Phytoplankton extracellular release of organic carbon accounted for only 4-9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10-16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40-440 ml(-1)) grazed 3-4% of the bacterial production, while ciliates smaller than 50 μm removed 19-39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.

Modelling ecosystem structure and trophic interactions in a typical cyanobacterial bloom-dominated shallow Lake Dianchi, China

“…Recently, some studies confirmed that cyanobacteria blooms and dead organisms (detritus) could be regarded as important food sources for zooplankton consumers, which eventually supported a significant share of the secondary production of lakes (Tillmanns et al, 2008). Carbon stable-isotope approach and carbon budget method have elucidated that heterotrophic bacteria were an important link between Microcystis and zooplankton (Christoffersen et al, 1990;de Kluijver et al, 2012). These microheterotrophic organisms, including flagellates and small ciliates, feed on bacterioplankton and form a 'microbial loop'.…”

Cyanobacteria as a carbon source for zooplankton in eutrophic Lake Taihu, China, measured by 13C labeling and fatty acid biomarkers

“…Still, there are few studies that showed a direct link from cyanobacteria carbon to zooplankton via bacteria and the microbial food web. Christoffersen et al (1990) studied carbon fluxes in a plankton community during a cyanobacteria bloom, based on carbon budgets and causal relationships. They showed that macrozooplankton (mainly Daphnia) were assimilating carbon when cyanobacteria dominated the phytoplankton community, and that this carbon apparently originated from phytoplankton and thus cyanobacteria.…”

“…They showed that macrozooplankton (mainly Daphnia) were assimilating carbon when cyanobacteria dominated the phytoplankton community, and that this carbon apparently originated from phytoplankton and thus cyanobacteria. Christoffersen et al (1990) also calculated that cyanobacteria were an important carbon source for bacteria and that macrozooplankton were more important bacterivores than were microzooplankton. Work and Havens (2003) showed that macrozooplankton were grazing extensively on cyanobacteria and on bacteria in a eutrophic lake, but the authors did not show a link between cyanobacteria and bacteria, as is shown here with the help of isotope labeling.…”

“…Based on this knowledge, we hypothesize that, in eutrophic systems, cyanobacteria are an important carbon source for zooplankton, but that much more carbon flows via the microbial food web rather than via the herbivory pathway. The importance of the microbial food web as a carbon source for zooplankton during and after a cyanobacteria bloom was previously demonstrated by Christoffersen et al (1990). Their results were based on carbon budgets combined with modeling.…”

Use of Metabolic Inhibitors to Characterize Ecological Interactions in an Estuarine Microbial Food Web

“…However, it is interesting that DCMU did not decrease the abundance of the larger flagellates, which may indicate a capability for switching between bacterivory and algivory in this group. Other studies have demonstrated the predominant contribution of small ciliates to bacterivory in certain environments [9], including a Georgia salt marsh estuary [38]. In July, small flagellate abundance was reduced in the prokaryotic inhibitor treatment, while small ciliate abundance was not affected, suggesting that small flagellates may be the dominant bacterial consumer in the summer.…”