Abstract
The Barents Sea is a hotspot for environmental change due to its rapid warming, and information on dietary preferences of zooplankton is crucial to better understand the impacts of these changes on food-web dynamics. We combined lipid-based trophic marker approaches, namely analysis of fatty acids (FAs), highly branched isoprenoids (HBIs) and sterols, to compare late summer (August) and early winter (November/December) feeding of key Barents Sea zooplankters; the copepods Calanus glacialis, C. hyperboreus and C. finmarchicus and the amphipods Themisto libellula and T. abyssorum. Based on FAs, copepods showed a stronger reliance on a diatom-based diet. Phytosterols, produced mainly by diatoms, declined from summer to winter in C. glacialis and C. hyperboreus, indicating the strong direct linkage of their feeding to primary production. By contrast, C. finmarchicus showed evidence of year-round feeding, indicated by the higher winter carnivory FA ratios of 18:1(n-9)/18:1(n-7) than its larger congeners. This, plus differences in seasonal lipid dynamics, suggests varied overwintering strategies among the copepods; namely diapause in C. glacialis and C. hyperboreus and continued feeding activity in C. finmarchicus. Based on the absence of sea ice algae-associated HBIs (IP25 and IPSO25) in the three copepod species during both seasons, their carbon sources were likely primarily of pelagic origin. In both amphipods, increased FA carnivory ratios during winter indicated that they relied strongly on heterotrophic prey during the polar night. Both amphipod species contained sea ice algae-derived HBIs, present in broadly similar concentrations between species and seasons. Our results indicate that sea ice-derived carbon forms a supplementary food rather than a crucial dietary component for these two amphipod species in summer and winter, with carnivory potentially providing them with a degree of resilience to the rapid decline in Barents Sea (winter) sea-ice extent and thickness. The weak trophic link of both zooplankton taxa to sea ice-derived carbon in our study likely reflects the low abundance and quality of ice-associated carbon during late summer and the inaccessibility of algae trapped inside the ice during winter.
Highlights
In Arctic marine ecosystems, primary and secondary production, and subsequently its availability to higher trophic levels, are subject to a strong seasonality (Wassmann and Slagstad, 1993; Weydmann et al, 2013)
The relative proportions of the neutral lipids, mainly wax esters (WEs) and triacylglycerols (TAGs), during summer and winter exceeded the proportions of the polar lipids, mainly phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) during summer and winter
During some periods of the year, sea ice algae can serve as a valuable carbon source for ice-associated grazers and subsequently their predators, both in the Arctic (Wang et al, 2015, 2016; Kohlbach et al, 2016, 2019) and the Antarctic marine environment (Kohlbach et al, 2017, 2018; Bernard et al, 2018; Schmidt et al, 2018)
Summary
In Arctic marine ecosystems, primary and secondary production, and subsequently its availability to higher trophic levels, are subject to a strong seasonality (Wassmann and Slagstad, 1993; Weydmann et al, 2013). Zooplankton in the Barents Sea are well adapted to interannual and seasonal environmental changes and fluctuations in food supply, from a low sea-ice cover or open water conditions and high incident irradiance during the summer months ranging to a consolidated sea-ice cover and extremely low light levels during the winter period (Conover and Huntley, 1991; Hagen, 1999; Bandara et al, 2016). Sea ice-associated (sympagic) primary production can provide an alternative or supplemental source of carbon for some zooplankton taxa, early in the season (Søreide et al, 2008; Gradinger, 2009) and during summer (Scott et al, 1999; Assmy et al, 2013; Kohlbach et al, 2016, 2021; Brown et al, 2017). The reduced expansion of winter sea ice and continuing thinning of the ice cover has consequences for winter-active species that depend on the sympagic habitat for shelter and foraging (Hop et al, 2000; Poltermann, 2001; Werner and Gradinger, 2002; Werner and Auel, 2005), but little is known about the impact on foodweb interactions
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