Planktonic food web structure and potential carbon flow in the Lower River Rhine with a focus on the role of protozoans

Abstract

Studies during the last two decades have revealed the importance of planktonic micro-organisms in the aquatic matter flux. However, studies on the planktonic food web structure have mostly concentrated on lentic waters. In the present study the biomass partitioning of the major plankton groups (bacteria, algae, heterotrophic flagellates (HF), ciliates and metazoans) in a large river (Lower River Rhine) has been analysed for a complete annual cycle and calculations on potential carbon flow within the planktonic food web were conducted for the four seasons. The plankton biomass was dominated by phytoplankton followed by bacterioplankton. The zooplankton was dominated by HF, contributing more than 65% of the total zooplankton biomass in all seasons. A significant part of the HF biomass was contributed by large flagellates (>20 mm). In accordance with the dominance of the HF and their high growth rates, this group contributed the largest part of the planktonic matter turnover within the zooplankton. The calculations suggest that the HF were able to consume between 11 and 65% of the seasonal mean bacterial production and that the HF consumed a larger amount of phytoplankton than both ciliates and metazoans. According to these calculations, only a small part of the total potential production of the phytoplankton as well as that of the total zooplankton was consumed by planktonic predators. Since the plankton abundance did not generally increase during the downstream passage, the present data suggest that a large part of the plankton production is lost by other means. The role of benthic predators has been discussed in this context.

Competition and protist predation are important regulators of riverine bacterial community composition and size distribution

“…The alpha-Proteobacteria do not normally represent the highest proportion of bacteria in freshwater systems (Methe & Zehr 1999;Gl€ ockner et al 2000;Klammer et al 2002;Lemke et al 2009). However, in line with our findings, filaments forming alpha-Proteobacteria are often found under high grazing pressure (J€ urgens et al 1999;Simek et al 1999;Weitere et al 2005;Thelaus et al 2008). Regarding the highly diverse group of the Actinobacteria, it has been claimed that some of the most abundant freshwater Actinobacteria would not suffer size changes induced by protists digestion, because of their cell wall structure (Sekar et al 2003;Tarao et al 2009;Simek et al 2013) or their constantly small cell size (Hahn et al 2003;Salcher 2013).…”

“…Thus, a more comprehensive understanding of the bacteria-protist interaction is one of the central issues in carbon transfer in aquatic systems and, more generally, in freshwater ecology. Among the bacterivorous protists, heterotrophic nanoflagellates (HNFs) are widely considered to be the main grazers of bacteria in freshwaters (J€ urgens et al 1997;Arndt et al 2000;Jacquet et al 2005;Weitere et al 2005;Bong & Lee 2011). HNFs' grazing can impact both the standing stock and taxonomic structure of bacterial communities, as well as size and morphology of each bacterial phylogenetic group Fazi et al 2008).…”

Effects of temperature increase on the grazing of planktonic bacteria by biofilm-dwelling consumers

“…The Rhine carries a well-established planktonic community, which is predominantly composed of unicellular organisms, i.e. bacteria, algae and heterotrophic protists (Weitere et al 2005b). The vast majority of planktonic bacteria are suspended as single cells (Weitere & Arndt 2002).…”

“…However, being relatively small-sized, this group of ciliates only contributed a small proportion to the total biofilmassociated grazer biovolume and was thus not expected to significantly reduce plankton from the water column. The large proportion of biofilm-dwelling planktivorous ciliates probably reflects the high availability of suspended prey (bacteria and microalgae) during early summer (Weitere et al 2005b).…”

Long‐Term Dynamics of Microbial Biofilm Communities of the River Rhine with Special References to Ciliates

“…Laboratory experiments have shown that biofilm communities are not only composed of benthivorous but also of planktivorous organisms, which may have a considerable effect on riverine plankton communities (MONAGHAN et al, 2001;WEITERE et al, 2005;KATHOL et al, 2009). During our present study, we exposed glass slides in a large river system (River Rhine) to ensure a continuous development of the biofilm community over a period of 15 months.…”