Calizza E et al. (2018), Time- and depth-wise trophic niche shifts in An...

ECB-ART-46201

PLoS One 2018 Jan 01;133:e0194796. doi: 10.1371/journal.pone.0194796.

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Time- and depth-wise trophic niche shifts in Antarctic benthos.

Calizza E , Careddu G , Sporta Caputi S , Rossi L , Costantini ML .

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Climate change is expected to affect resource-consumer interactions underlying stability in polar food webs. Polar benthic organisms have adapted to the marked seasonality characterising their habitats by concentrating foraging and reproductive activity in summer months, when inputs from sympagic and pelagic producers increase. While this enables the persistence of biodiverse food webs, the mechanisms underlying changes in resource use and nutrient transfer are poorly understood. Thus, our understanding of how temporal and spatial variations in the supply of resources may affect food web structure and functioning is limited. By means of C and N isotopic analyses of two key Antarctic benthic consumers (Adamussium colbecki, Bivalvia, and Sterechinus neumayeri, Echinoidea) and Bayesian mixing models, we describe changes in trophic niche and nutrient transfer across trophic levels associated with the long- and short-term diet and body size of specimens sampled in midsummer in both shallow and deep waters. Samplings occurred soon after the sea-ice broke up at Tethys Bay, an area characterised by extreme seasonality in sea-ice coverage and productivity in the Ross Sea. In the long term, the trophic niche was broader and variation between specimens was greater, with intermediate-size specimens generally consuming a higher number of resources than small and large specimens. The coupling of energy channels in the food web was consequently more direct than in the short term. Sediment and benthic algae were more frequently consumed in the long term, before the sea-ice broke up, while consumers specialised on sympagic algae and plankton in the short term. Regardless of the time scale, sympagic algae were more frequently consumed in shallow waters, while plankton was more frequently consumed in deep waters. Our results suggest a strong temporal relationship between resource availability and the trophic niche of benthic consumers in Antarctica. Potential climate-driven changes in the timing and quality of nutrient inputs may have profound implications for the structure of polar food webs and the persistence of their constituent species, which have adapted their trophic niches to a highly predictable schedule of resource inputs.

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Genes referenced: LOC100887844 LOC115925415 LOC586781

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	Fig 1. Isotopic signatures of food sources.Mean (±S.E.) δ13C and δ15N values of basal food sources at Tethys Bay (Ross Sea, Antarctica). Coarse (C), fine (F) and ultra-fine (UF) refer to different size fractions of organic matter in sediment (Sed). Resource guilds are indicated with colours, i.e. blue: sympagic resources; green: pelagic resources; brown: organic matter in sediments; purple: benthic primary producers.
	Fig 2. Isotopic niches of consumers.Isotopic distribution of A. colbecki (ADA) and S. neumayeri (STE) in shallow (15–25 m depth) and deep (50–150 m depth) waters in the long term (Tissue, based on analysis of soft tissues) and short term (Gut, based on analysis of gut contents). Polygons enclosing isotopic data represent the total isotopic area (TA) occupied by each population, while ellipses are the standard ellipse areas (SEAc) encompassing the core (around 40%) of each population.
	Fig 3. Trophic links between consumers and resources.Resource-consumer food webs depicting the diet composition of the bivalve A. colbecki and the echinoderm S. neumayeri in shallow (15–25 m depth) and deep (50–150 m depth) waters in the long term (Tissue: black arrows, based on analysis of soft tissues) and short term (Gut: red arrows, based on analysis of gut contents). Arrow width is proportional to the interaction strength between consumers and resources. Resource guilds are grouped by colour, i.e. brown: organic matter in sediment, including coarse (C), fine (F) and ultra-fine (UF) sediment (Sed.) fractions; purple: benthic primary producers, including the red algae Iridea cordata (I. cord.) and epiphytic diatoms (Epi.); green: plankton, including phytoplankton and zooplankton; blue: sympagic algae, including diatoms (Diat.) and filamentous algae (Fil.) growing at the interface between sea-ice and water (Int.) or within the ice core (IC).
	Fig 4. Body size and diet.Dietary change with specimen size in shallow (15–25 m depth) and deep (50–150 m depth) waters in the long term (Tissue, based on analysis of soft tissues) and short term (Gut, based on analysis of gut contents) (mixSIAR, R software, version 2.15.2). (a) = A. colbecki; (b) = S. neumayeri. The software predicts the contribution of resources over a wider size range than that of the original samples; the red shaded areas indicate the size range actually observed for the specimens analysed (note differences on the horizontal axes between panels). Each coloured line indicates a given resource. A: interface sympagic diatoms, B: I. cordata, C: phytoplankton, D: ultra-fine sediment, E: zooplankton, F: ice-core diatoms, G: epiphytic benthic diatoms, H: ice-core filamentous sympagic algae, I: interface filamentous sympagic algae, J: coarse sediment, K: fine sediment.

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