Fasting or feeding: A planktonic food web under lake ice

Abstract

Zooplankton can spend winter actively under the ice cover of lakes. However, dietary resources under lake ice are both quantitatively and qualitatively limited, and feeding might not be energetically rewarding for most zooplankton species. Many zooplankters are expected to fast throughout the winter, exhausting their previously accumulated fat storage. We hypothesised that only a fraction of the actively overwintering zooplankton contributes to an active food web under lake ice, leading to few trophic linkages within the planktonic community. Zooplankton habitats and feeding were investigated under the ice of Lake Onego. Zooplankton habitats and migrations were studied by coupling zooplankton sampling around the clock to measurements of particle movement using an acoustic Doppler current profiler. Secondly, fatty acid‐specific stable isotope compositions were used to determine whether and which zooplankton fatty acids ultimately came from the assimilation of under‐ice seston. The algal biomass was low under ice and mostly dominated by large diatoms. Copepods dominated the zooplankton community. Species present as late copepodite and adult instars were confined to the deeper layers, while nauplii occupied the surface layer. Diel vertical migration by Cyclops was the most tangible observation of persistent feeding under the ice. Previously accumulated fat storage represented most of zooplankton fatty acids, with few, yet detectable, fatty acids recently acquired by feeding under the ice. Although some zooplankton taxa maintained feeding activity under the ice of Lake Onego, the food source available beneath the ice was not sufficiently rewarding to leave an isotopic imprint upon the dominant fatty acids of bulk zooplankton. The seston fatty acids that were passed on to zooplankton from feeding under ice were not provided by diatoms, although they made up most of the phytoplankton biovolume. Instead, the zooplankton food web was supported by mixotrophic phytoplankton (i.e. cryptophytes and chrysophytes) that represented <5% of the under‐ice biovolume. Consequently, the planktonic food web under the ice of Lake Onego had few trophic linkages, and thereby low connectance. Environmental conditions under the ice of Lake Onego do not depart significantly from those observed in lakes of similar latitudes. Therefore, low connectance food webs could be relatively common under ice for lakes above 60° latitude. Our methodological approach is applicable to other lakes and could thus disclose the variability of under‐ice food webs. This would provide a much more complete picture of annual dynamics of food webs in lakes that ice over.

Winter Limnology: How do Hydrodynamics and Biogeochemistry Shape Ecosystems Under Ice?

“…ic metabolism (Figure 8). Small crustacean zooplankton that store less resources and need more frequent foraging, such as cladocerans or copepod nauplii (Grosbois et al, 2017;Larsson & Wathne, 2006) could, on the other hand, be expected to exhibit a vertical migration pattern following the depth distribution of their phytoplankton prey (Perga et al, 2020b) (Table 1; Q19). When primary productivity is limited by low irradiance in winter, the role of organisms in the classical food web is expected to decline (Hampton et al, 2017) (Table 1; Q15).…”

“…Adaptations to be viable and competitive under the ice include acclimatization to low irradiance (Neale & Priscu, 1995) and motility to maintain access to light (Forsström et al., 2007; Pithart, 1997; Vehmaa & Salonen, 2009), increase food encounters, and to avoid predation (Franks, 1992; Özkundakci et al., 2016). Strategies may vary with morphological stages of the life cycle (Jewson & Granin, 2015; Perga et al., 2020b). Motile taxa can dominate phytoplankton communities in weakly stratified water columns (Özkundakci et al., 2016; Rue et al., 2020), form vertical layers under ice (Kiili et al., 2009; Vehmaa & Salonen, 2009; Vincent, 1981), or perform (daily) vertical migrations between the photic zone and a nutrient‐rich refuge layer (Pithart, 1997; Pettersson, 1985) (Figure 6).…”

Under‐ice plankton community response to snow removal experiment in bog lake

“…Zooplankton communities under lake ice are typically dominated by copepods and rotifers (Blank et al 2009; Perga et al 2021). However, we did not observe a single copepod in the under‐ice samples from South Sparkling Bog, even when nauplii and adult copepods were present in fall and spring zooplankton samples.…”

“…Though many copepods actively overwinter, many employ diapause strategies. In general, adult and late‐stage juveniles remain in the deeper waters near the sediments, while nauplii are distributed nearer to the surface and water–ice interface (Perga et al 2021). It is unlikely that our sampling protocol was unable to capture deep zooplankton, as a lack of oxygen in the water column below 2 m renders most of the water column uninhabitable.…”

The Lake Ice Continuum Concept: Influence of Winter Conditions on Energy and Ecosystem Dynamics

“…Increasing width of wedges suggests an increase in response variable and wedge color corresponds to the shift along the continuum of winter conditions. of secondary production and foraging success are expected to increase the number and strength of trophic linkages within food webs (Figures 2.3a and 2.3c;Perga et al, 2020).…”