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Abstract Nearshore (littoral) habitats of clear lakes with high water quality are increasingly experiencing unexplained proliferations of filamentous algae that grow on submerged surfaces. These filamentous algal blooms (FABs) are sometimes associated with nutrient pollution in groundwater, but complex changes in climate, nutrient transport, lake hydrodynamics, and food web structure may also facilitate this emerging threat to clear lakes. A coordinated effort among members of the public, managers, and scientists is needed to document the occurrence of FABs, to standardize methods for measuring their severity, to adapt existing data collection networks to include nearshore habitats, and to mitigate and reverse this profound structural change in lake ecosystems. Current models of lake eutrophication do not explain this littoral greening. However, a cohesive response to it is essential for protecting some of the world's most valued lakes and the flora, fauna, and ecosystem services they sustain. 

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. baikalensisindividuals 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. baikalensisis 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 ofE. baikalensisinitiated 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.

 

Climate warming impacts ecosystems through multiple interacting pathways, including via direct thermal responses of individual taxa and the combined responses of closely interacting species. In this study, we examined how warming and infection by an oomycete parasite (Saprolegnia) affect the dominant zooplankter of Russia's Lake Baikal, the endemic copepodEpischurella baikalensis. We used a combination of laboratory experiments, long‐term monitoring data, and population modeling. Experiments showed a large difference in the thermal optima of host and parasite, with strong negative effects of warm temperatures onE. baikalensissurvival and reproduction and a negative effect ofSaprolegniainfection on survival.Saprolegniainfection had an unexpected positive effect onE. baikalensisreproductive output, which may be consistent with fecundity compensation by females exposed to the parasite. Long‐term monitoring data suggested thatSaprolegniainfections were most common during the warmest periods of the year. Population models, parameterized with experimental and literature data, correctly predicted the timing ofSaprolegniaepizootics, but overestimated the negative effect of warming onE. baikalensispopulations. Models suggest that diel vertical migration may allowE. baikalensisto escape the negative effects of increasing temperatures and parasitism and enableE. baikalensisto persist in the face of moderate warming of Lake Baikal. Our results contribute to understanding of how warming and parasitism interact to affect the pelagic ecosystems of cold lakes and oceans and how the consequences of these interacting stressors can vary seasonally, spatially, and interannually.