Fasting or feeding: Planktonic food web under lake ice

Abstract

1. Zooplankton can spend winter actively under the ice cover of lakes. Yet, dietary resources under the 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 hypothesized that only a fraction of the actively overwintering zooplankton takes part to an active food web under lake ice, leading to few trophic linkages within the planktonic community.
2. Zooplankton habitats and feeding were investigated under the ice of the large lake Onego. Zooplankton habitats and migrations were studied by coupling zooplankton samplings round the clock to measurements of particlemovement by an acoustic Doppler current profiler. Second, 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. 
3. The algal biomass was low under the ice and largely dominated by large diatoms. Copepods dominated the zooplankton community. Species present at late copepodite and adult instars were confined to the deeper layers, while nauplii occupied the surface layer. The diel vertical migration of 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 that had been acquired by recent feeding under ice. 
4. We provide evidence that although some zooplankton taxa maintained some feeding activities under the ice of Lake Onego, the dietary source was not sufficiently rewarding to leave an isotopic imprint upon the dominant fatty acids of bulk zooplankton. The seston fatty acids that got passed on to zooplankton from feeding under the ice were not provided by the diatom biomass, despite dominant. Instead, the zooplankton food web was supported by phytoplankton taxa (i.e. cryptophytes and chrysophytes) that represented < 5% of the under ice biovolume. As consequence, the planktonic food web under the ice had few trophic linkages, and thereby a low connectance. 

Datasets

1. FA_Onego : Fatty acid composition of POM (particulate organic matter=seston) and bulk zooplankton under the ice of Lake Onego, March 2017

Compound=fatty acids, POM_mean: mean concentration of the given FA in seston (n=5), POM_sd: standard deviation of the concentration of the given FA in seston (n=5), ZPk_mean: mean concentration of the given FA in bulk zooplankton (n=6), Zpk_sd: standard deviation of the concentration of the given FA in bulk zooplankton (n=6)

2. FAd13C_Onego : carbon isotope composition (d13C) of specific fatty acids in POM (particulate organic matter=seston) and bulk zooplankton under the ice of Lake Onego, March 2017

Compound=fatty acids, POM_mean: mean d13C value of the given FA in seston (n=5), POM_sd: standard deviation of the d13C value of the given FA in seston (n=5), ZPk_mean: mean d13C value of the given FA in bulk zooplankton (n=6), Zpk_sd: standard deviation of the d13C value of the given FA in bulk zooplankton (n=6)

3. Zpkdensity_Onego: density (number of individuals per squared m=ind.m-2 ) of zooplankton taxa for the 3 depth layers (0-5m, 5-10m and 10-27m) measured every 3 hours for 36h under the ice of lake Onego in March 2017.