Calanoid copepod grazing affects plankton size structure and composition in a deep, large lake

Abstract

Cultural oligotrophication is expected to shift lake zooplankton to become dominated by calanoid copepods. Hence, understanding the influence of calanoids on the taxonomic and size structure of the lower plankton food web is crucial for predicting the effects of oligotrophication on energy fluxes in these systems. We studied the effect of an omnivorous calanoid, Eudiaptomus gracilis, on the lower planktonic food web using an in situ incubation approach in large and deep Lake Constance. We show that E. gracilis significantly reduced ciliate, phytoplankton, rotifer, but increased bacteria biovolume. Highest clearance rates were observed for ciliates whose biovolume declines may have caused a release of predation pressure on bacteria. E. gracilis grazing shifted the size structure of the phytoplankton community by reducing mean phytoplankton cell size (directional selection) and simultaneously increasing cell size variance (disruptive selection). Ciliate cell sizes experienced a similar selective regime in one of the experiments, whereas in the other two experiments, no change of size structure was detected. Results suggest strong influences of E. gracilis grazing on the lower plankton food web and a significant shift in phytoplankton size structure. For evaluating detailed effects of omnivorous consumers on plankton size structure, cascading interactions need to be considered.

Relationships of total phosphorus and chlorophyll in lakes worldwide

“…This is because of processes such as elevated nutrient levels due to sediment resuspension (Qin et al 2020; Zhang et al 2020) and shifts between alternative stable states (Beisner et al 2003), which are more prevalent in shallow lakes. The TP–Chl a relationship is also mediated by zooplankton communities heavily grazing on algal populations (Fonseca Da Silva et al 2019; Kunzmann et al 2019; Pomati et al 2020), high salinity levels limiting P bioavailability (Evans and Prepas 1997; McGowan et al 2020), lower algal utilization of P in acidic conditions (Jansson et al 1986; Gerson et al 2016), variability in dissolved organic carbon (DOC) concentrations changing water clarity and algal productivity (Carpenter et al 1998; Brothers et al 2014), and landscape characteristics, such as pasture and wetlands (Wagner et al 2011; Filstrup et al 2014). The TP–Chl a relationship is thus complex and predicting patterns over time or space requires exploration of these co‐occurring mechanisms.…”

Zooplankton in northern lakes show taxon‐specific responses in fatty acids across climate‐productivity and ecosystem size gradients

“…These findings suggest that copepods have a strong ability to regulate FA composition irrespective of changes in environmental conditions and seston composition, either by selective feeding on available PUFA‐rich food sources (DeMott 1989; Sterner 1989) and/or by internal FA metabolism (Ravet et al 2010). Copepod grazers are also able to feed on microfauna in the pelagic (e.g., ciliates and protozoans) (Karlsson et al 2007; Kunzmann et al 2019) and on meiofauna in benthic habitats in winter (Muschiol et al 2008), when they spend their copepodite stages in surficial sediments (Goedkoop and Johnson 1996). Trophic upgrading by microfauna and meiofauna, that is, the conversion of dietary precursor FA to LC‐PUFA, may, therefore, contribute to PUFA‐enrichment of food resources for copepods (Martin‐Creuzburg et al 2005).…”

Depth-dependent dynamics of protist communities as an integral part of spring succession in a freshwater reservoir

“…The simultaneous establishment of a stable population of Eudiaptomus gracilis probably did not contribute much to the rotifers' top-down control, but this copepod successfully replaced rotifers and daphnids as a powerful algal grazer in the later phase [96]. E. gracilis was also shown to exhibit strong influence on the lower food web organization due to a high clearance rate of ciliates [97]. The drop in ciliate densities, including high proportions of bacterivorous or omnivorous species, most likely resulted in a short-term increase in bacterial numbers (Figure 1d, f).…”