Trophic coupling of the microbial and the classical food web in Lake Baikal, Siberia

Pelagic copepods often couple the classical and microbial food webs by feeding on microzooplankton (e.g. ciliates) in oligotrophic aquatic systems, and this consumption can trigger trophic cascades within the microbial food web. Consumption of mixotrophic microzooplankton, which are both autotrophic and heterotrophic within the same individual, is of particular interest because of its influence on carbon transfer efficiency within aquatic food webs. In Lake Baikal, Siberia, it is unknown how carbon from a well-developed microbial food web present during summer stratification moves into higher trophic levels within the classical food web. We conducted in situ experiments in August 2015 to test the hypotheses that: (a) the lake's dominant endemic copepod (Epischura baikalensis), previously assumed to be an herbivore feeding on diatoms, connects the microbial and classical food webs by ingesting ciliates; and (b) this feeding initiates top-down effects within the microbial food web. Our results supported these hypotheses. E. baikalensis individuals consumed on average 101-161 ciliates per day, obtaining 96%-98% of their ingested carbon from ciliates and the remainder from small diatoms. Clearly, E. baikalensis is omnivorous, and it is probably channelling more primary production from both the microbial food web and the classical food web of Lake Baikal to higher trophic levels than any other pelagic consumer. Most ciliates consumed were a mixotrophic oligotrich and such taxa are often abundant in summer in other oligotrophic lakes. Consumption of these mixotrophs is likely to boost substantially the transfer efficiency of biomass to higher trophic levels with potential implications for fish production, but this has seldom been investigated in oligotrophic lakes. Feeding of E. baikalensis initiated a three-link predatory cascade which reduced the abundance of ciliates and elevated growth rates of heterotrophic nanoflagellates but did not affect abundance or growth rates of autotrophic picoplankton. This demonstration of a potential trophic cascade in Lake Baikal indicates that investigations at larger spatial-temporal scales are needed to identify the conditions promoting or precluding trophic cascades in this lake.