Abstract
Determining food web architecture and its seasonal cycles is a precondition for making predictions about Antarctic marine biodiversity under varying climate change scenarios. However, few scientific data concerning Antarctic food web structure, the species playing key roles in web stability and the community responses to changes in sea-ice dynamics are available. Based on C and N stable isotope analysis, we describe Antarctic benthic food webs and the diet of species occurring in shallow waters (Tethys Bay, Ross Sea) before and after seasonal sea-ice break-up. We hypothesized that the increased availability of primary producers (sympagic algae) following sea-ice break-up affects the diet of species and thus food web architecture. Basal resources had distinct isotopic signatures that did not change after sea-ice break-up, enabling a robust description of consumer diets based on Bayesian mixing models. Sympagic algae had the highest δ13C (∼−14‰) and red macroalgae the lowest (∼−37‰). Consumer isotopic niches and signatures changed after sea-ice break-up, reflecting the values of sympagic algae. Differences in food web topology were also observed. The number of taxa and the number of links per taxon were higher before the thaw than after it. After sea-ice break-up, sympagic inputs allowed consumers to specialize on abundant resources at lower trophic levels. Foraging optimization by consumers led to a simpler food web, with lower potential competition and shorter food chains. However, basal resources and Antarctic species such as the bivalve Adamussium colbecki and the sea-urchin Sterechinus neumayeri were central and highly connected both before and after the sea-ice break-up, thus playing key roles in interconnecting species and compartments in the web. Any disturbance affecting these species is expected to have cascading effects on the entire food web. The seasonal break-up of sea ice in Antarctica ensures the availability of resources that are limiting for coastal communities for the rest of the year. Identification of species playing a key role in regulating food web structure in relation to seasonal sea-ice dynamics, which are expected to change with global warming, is central to understanding how these communities will respond to climate change.
Highlights
The persistence of biodiversity in seasonally variable environments is linked to the ability of populations to adapt to variable and/or temporally constrained resource inputs (McMeans et al, 2015)
Since the number of species in shallow-water Antarctic benthic communities is subject to natural seasonal variation (Gutt and Piepenburg, 2003; Thrush and Cummings, 2011; Clark et al, 2013; Ingels et al, 2020), to avoid any bias arising from species number in the interpretation of niche and food web metrics, we considered taxa common to both study periods
Other Antarctic key species such as the predaceous sea star Odontaster meridionalis and the anemone Urticinopsis antarctica occupied a central position in the isotopic space, with intermediate δ13C and high δ15N values, indicating consumption of materials mainly of animal origin, both BEFORE and AFTER (Table 1 and Figure 2)
Summary
The persistence of biodiversity in seasonally variable environments is linked to the ability of populations to adapt to variable and/or temporally constrained resource inputs (McMeans et al, 2015). Sea-ice shrinking during spring, together with sea-ice break-up in summer, allows a massive increase in ice-bound primary production (sympagic production), which is subsequently released into the water column, fuelling both pelagic and benthic communities (Pusceddu et al, 1999; Lizotte, 2001; Leu et al, 2015) Under these conditions, sympagic algae represent the main direct (via vertical flux) and indirect (after sedimentation) food input for a large part of Antarctic marine diversity (Lizotte, 2001; Michel et al, 2019; Rossi et al, 2019)
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