What happens if the food web in antarctica is disturbed

ITUs present in the food web. The closeness centrality Cc n of a given node n is defined as the reciprocal of the average shortest path length.

The Cc n of each node is a measure, ranging between 0 and 1, of how fast information spreads from a given node to other nodes in the network The importance of omnivory and intraguild predation were estimated using the foodweb package 78 in R.

The simulated deletion of an ITU is considered as a primary extinction sensu Dunne et al. Lastly, a Mantel test was applied to assess if isotopic signatures of ITUs were predictive of the feeding links they made in the food web, considering both the identity and the relative contribution of each food source to each ITU. The Mantel test, as a permutation test for correlation between two distance or similarity matrices, allowed us to compare multivariate data with different similarity measures Here, the Euclidean distance and the Bray-Curtis dissimilarity were selected to quantify isotopic distances between ITUs and pairwise differences in feeding links respectively 72 , Worm, B.

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Schiaparelli, S. A reassessment of the genus Amphicteis Grube, Polychaeta: Ampharetidae with the description of Amphicteis teresae sp. Italian J. Burns, J. Sea-ice influences key ecological processes in polar ecosystems, such as phytoplankton blooms, physical disturbance of the seabed due to iceberg scouring Gutt and Piepenburg, ; Arrigo, , reproductive cycles, recruitment, and trophic interactions among a wide range of species Constable et al.

Specifically, 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. 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, ; Michel et al. The seasonal availability of this high-quality energy source favors trophic specialization among a large number of taxa in both pelagic and benthic communities, which are specifically adapted to the physical and biological conditions characteristic of polar environments Knox, ; McMahon et al.

In contrast, short but intense phytoplankton blooms occur mostly during open-sea periods Pusceddu et al. The increased availability of basal resources after sea-ice break-up allows consumers to exploit a range of carbon sources in both the water column and on the bottom, reinforcing benthic-pelagic coupling Cattaneo-Vietti et al. The seasonal supply of sympagic algae thus becomes crucial to energy exchanges between ecosystem compartments, also affecting the structure and stability of communities Hobson et al.

Expected changes in seasonal light and sea-ice dynamics due to climate change could directly and indirectly compromise biodiversity persistence Clark et al. These changes, together with ocean warming, could also favor the entry and success of new faster-moving and more generalist species, one of the main causes of biodiversity loss worldwide Aronson et al. This in turn could affect the structure and function of polar communities by altering the interactions between populations and the energy pathways between benthic and pelagic compartments Fossheim et al.

However, failure to consider interactions with all the other species within the community and with the physical environment can lead to ineffective management and conservation measures in the medium and long term Trebilco et al. In the Ross Sea Antarctica , a biodiversity hotspot and the most pristine marine ecosystem on our planet Rossi et al. Expected changes in sea-ice dynamics due to climate change will modify the extent and intensity of algal blooms, with possible effects on benthic food webs and pelagic top predators, thus altering biodiversity architecture Clark et al.

Although our knowledge of biodiversity in the region is increasing Chown et al. This represents a major gap, limiting our ability to conserve biodiversity and anticipate the effects of changes in sea-ice dynamics on Antarctic food webs. Recently, application of stable isotope analysis to Antarctic benthic communities has proved useful for describing space-time variations in nutrient fluxes associated with sea-ice dynamics Michel et al. Studies in the Ross Sea have demonstrated that sympagic production is readily incorporated into benthic biomass following sea-ice break-up Norkko et al.

Accordingly, pulsed sympagic inputs during summer Pusceddu et al. At present, quantitative data on feeding and competition links in Antarctic benthos are extremely scarce, and quantitative descriptions of the complex food web structures and their dependence on sea-ice coverage are even scarcer Rossi et al.

However, key ecological properties including stability, energy flux, vulnerability to biodiversity loss and invasibility are closely related to the complexity of food webs and the distribution of links among species Aronson et al. While challenging, the quantitative analysis of food webs thus represents a necessary step toward understanding the persistence mechanisms of Antarctic communities and improving our ability to conserve Antarctic biodiversity during climate change.

In our study, for the first time we had the opportunity to track seasonal changes at the same site in both the trophic niches of populations and food web structure in the species-rich benthic Antarctic community. By means of C and N stable isotope analysis and Bayesian mixing models, we quantified i the isotopic niche metrics of populations, ii the strength of feeding and competitive interactions, and iii the food web properties of shallow-water benthic communities both before hereafter BEFORE and after hereafter AFTER sea-ice break-up in Tethys Bay, an inlet inside Terra Nova Bay in the Ross Sea.

We sought to understand how the expected increase in resource availability following summer sea-ice break-up Pusceddu et al. For this purpose, we measured variations in the food web structure of the shallow-water Antarctic benthic community under two different sets of sea-ice coverage conditions. Since the number of species in shallow-water Antarctic benthic communities is subject to natural seasonal variation Gutt and Piepenburg, ; Thrush and Cummings, ; Clark et al.

Lastly, based on food web structure analysis, we sought to quantify food web vulnerability to biodiversity loss, identifying those taxa that occupied the most critical positions in the food web and quantifying their sensitivity to changes in resource availability associated with sea-ice dynamics. In accordance with optimal foraging theory Pyke et al.

Figure 1. The background image of Tethys Bay was obtained from Google Earth. Tethys Bay is characterized by marked seasonality in sea-ice dynamics, with coverage generally persisting until mid-December, leaving the area ice-free during the austral summer. This produces marked spatio-temporal variations in the availability of food for benthic communities Pusceddu et al. The chlorophyll-a concentrations in this area generally range between 0. Further information on the study area can be found in Calizza et al.

Samplings were performed before and after sea-ice break-up, which started around 18th December , leading to ice-free open water around 10th January The first round of sampling i. During this period, the whole study area was characterized by dark and non-turbid waters.

The second round of sampling i. During this period, icebergs flowed into and around the bay personal observation , as usually observed in the area Melchiori and Cincotti, The number of days elapsing between sea-ice break-up and sampling time were quantified by means of direct observations on the ground coupled with reference to satellite images Landsat 8, target paths: 61, target row: Three sampling stations in shallow waters were chosen Figure 1 , characterized by mixed rocky and soft bottoms and varying slopes in order to take full account of the environmental heterogeneity of this area Cattaneo-Vietti et al.

The collection and analysis of samples was carried out by the same field and laboratory research team in both sampling periods using similar sampling methods. Benthic invertebrates, detritus organic matter in coarse, fine and ultra-fine sediments and macroalgae Iridaea cordata and Phyllophora antarctica with associated epiphytes were collected by SCUBA divers at a depth of 10—30 m.

Small invertebrates, such as polychaetes and amphipods, were collected by carefully separating them from sediments or other sampled benthic specimens, mainly sponges. Sympagic algae diatoms and filamentous algal aggregates present in the ice core and at the ice-water interface were collected by coring the ice-pack three times at each sampling site.

Three replicates of sympagic algae per ice-core i. The sympagic diatoms and filamentous algal aggregates in Tethys Bay are mainly composed of species belonging to the genera Amphiprora and Nitzschia , respectively Lazzara et al.

Each zooplankton specimen was carefully separated from the bulk sample under a stereoscope. In our study area, the phytoplankton assemblages are dominated by Phaeocystis antarctica Fabiano et al. Since coprophagy is a common strategy among Antarctic benthic invertebrates McClintock, ; Norkko et al. The seal colony consisted of around 80— specimens usually stationed in the sampling area. Basal resources including the upper 5 cm layer of sediments and seal feces were collected within the first week of sampling activities in both sampling periods.

All collected samples were sorted, counted, identified to the finest possible taxonomic level and transported to Italy to be properly processed for the stable isotope analysis.

Specific soft tissues were taken from the large invertebrates Norkko et al. For small invertebrates such as amphipods and polychaetes , the whole body was used. These samples were individually analyzed. In addition, samples of very small organisms e.

The dry sample was homogenized to a fine powder using an agate mortar and pestle or a ball mill Mini-Mill Fritsch Pulverisette Fritsch Instruments, Idar-Oberstein, Germany. Where necessary, samples were pre-acidified using 1M HCl drop-by-drop Hobson et al. Aliquots of 0. Samples were then analyzed in two replicates using a continuous flow mass spectrometer IsoPrime, Isoprime Ltd. Measurement errors associated with the linearity and stability of the mass spectrometer were typically smaller than 0.

The isotopic niche metrics of populations at both sampling times were calculated in accordance with Layman et al. These metrics, originally proposed for application at community level, can also be used at population level to obtain information on trophic diversity within a single population Layman et al. CR provides information about the variety of food sources exploited by the population and hence its degree of trophic generalism.

NR quantifies the degree of omnivory of the population, accounting for inter-specimen variation in trophic position. The isotopic niche width was measured as the total area TA of the convex hull encompassing all the isotopic values of individuals within each population. The overlap between the isotopic niches of populations was calculated and expressed both as a percentage of TA and as the number of overlaps for each taxon R software ver.

Although low, this sample size allowed us to include naturally scarce taxa in the comparison of isotopic niche metrics between study periods. The number was also less limiting for species diet reconstruction based on Bayesian Mixing Models. The overall competitive effect was expressed as the diffuse overlap sensu MacArthur, and Pianka, , measured as the average overlapping TA multiplied by the average number of overlapping taxa.

The inputs of the model were: i the isotopic signatures of the target consumer, ii the mean isotopic signatures and relative standard deviation of potential food sources, and iii the Trophic Enrichment Factor TEF of the consumer.

Since a high number of resources can reduce the discriminatory ability of mixing models and thus the effectiveness of Stable Isotope Analysis in food web reconstruction, for each taxon, a multiple-step procedure, in accordance with Costantini et al. Based on the output of the Bayesian isotopic Mixing Models, the pool of real i. Finally, following the assignment of all the trophic links, the structures and the respective metrics of the food webs were obtained using the software Cytoscape 3.

In the food web, the nodes are the taxa S and the links L are the feeding links from resources to consumers. The proportional similarity of diet among taxa was measured by the Bray—Curtis similarity index.

The index is based on the contribution of each resource to the diet of each consumer and it ranges from 0, when no common resources are found for the compared groups, to 1, when the compared groups have the same resources in the same proportions Calizza et al. Topological food web metrics were also measured. BC is a measure of the centrality of a taxon in the food web. This parameter indicates the importance of a taxon in terms of the role it plays in connecting other nodes in the network Freeman, Neighborhood Connectivity NC indicates the average connectivity of each taxon in the food web and provides a measure of interconnection direct and indirect among all taxa in the food web.

Food web robustness to loss of taxa was estimated by simulating primary extinctions sensu Dunne et al. Taxa were sequentially removed from the most to the least connected, in accordance with Calizza et al. Secondary extinction occurs when a taxon loses all of its food items.

The Shapiro—Wilk normality test was applied to each dataset to verify that it followed a Gaussian distribution. A Mantel test was performed in order to assess the correlation between the matrices of isotopic Euclidean distances and the Bray-Curtis diet similarity Signa et al.

In order to avoid the potential confounding effects of differences in taxonomic composition between the BEFORE and AFTER communities, further comparisons of isotopic and food web metrics included only consumers common to both sampling periods and were performed using paired t -tests and Wilcoxon paired tests for inter-period comparison of variables with normal and non-normal distribution, respectively. Sediments, sympagic algae and animal-derived food live prey or carrion were the resource items that most contributed to the diet of benthic organisms in both sampling periods Figure 3.

Table 1. Figure 2. Each point represents a taxon. The legend shows the respective colors of consumers and basal resources. Figure 3. Yet differences between groups were also observed. The mean isotopic niche width Total Area of taxa common to both periods decreased from 7. While the brittle star Ophionotus victoriae , the sea-star Diplasterias brucei and the anemone Urticinopsis antarctica mirrored the general trends, with decreasing Total Area and CR after the sea-ice break-up, the scallop Adamussium colbecki , the brittle star Ophioplinthus gelida and the sea-urchin Sterechinus neumayeri increased their Total Area along with NR.

Table 2. The closer the taxa in the isotopic niche space, the more similar their diet according to the Bayesian mixing models Mantel test, Correlation R: 0. Within trophic guilds, the isotopic signatures of taxa were rather heterogeneous e. Consistent with the changes in the isotopic distribution of animal taxa, the contribution of sympagic algae to their diet increased after the sea-ice break-up t -test, t : 2.

As a result, the diet composition of the various taxa became more similar Bray-Curtis: The mean potential competition strength based on isotopic niches increased but the number of competitor taxa decreased t -test, t at least: 2.

The lower number of competitors, together with the generally narrower isotopic niche in AFTER Table 2 , was reflected in a lower diffuse niche overlap between taxa from The two periods were characterized by different food web topology and metrics Table 3.

Figure 4. Each node represents one taxon in the community. Arrows point from a resource to its consumer and their thickness is proportional to the linkage strength. Basal resources are shown in green. The food web graphs were developed using Cytoscape software. Bold IDs indicate taxa sampled in both study periods. For the list of taxa please refer to Table 1. Figure 5. The trophic position of consumers varied between 2.

Urticinopsis antarctica , Odontaster meridionalis , Ammothea sp. Another 14 taxa had a similarly, high trophic position BEFORE but, of those species present in both periods, the sea-spiders Colossendeis s p.

As observed for the community as a whole Table 3 , the taxa common to both sampling periods were less connected both directly and indirectly with others in the AFTER food web NC: Basal resources and S. Except for A. Antarctic benthic organisms are markedly adapted to their extreme environment, which is characterized by long-term low availability of food resources, which are closely linked to seasonal sea-ice dynamics Pusceddu et al.

Thus, changes in sea-ice spatio-temporal dynamics, which are expected in all climate change scenarios Schofield et al. In this study, community isotopic characterization, coupled with diet and food web reconstruction, provided crucial support for understanding and describing the response of Antarctic coastal benthic communities to the availability of basal resources under two different sea-ice coverage conditions Calizza et al. The results highlighted significant changes in the trophic niches of Antarctic benthic populations dwelling in shallow waters, as well as in food web architecture, after the sea-ice break-up and its associated input of sympagic algae.

In addition, they allowed us to identify the main carbon pathways that support the benthic populations, including taxa that play a key role in the organization of these complex communities.

The isotopic signatures of basal resources were clearly distinct and highly conserved over time. In addition, values were similar to what has been observed in other study areas Kaehler et al. The observed isotopic similarity of phytoplankton over time suggests that samples collected in dark waters before the sea-ice break-up are the result of horizontal transport to the coast from open waters, as already reported for the study area Pusceddu et al.

Other studies have found isotopic similarity between sediments and sympagic algae, suggesting that these algae are available to benthic organisms both as fresh material when released into the water column and as a small but constant supply of organic matter in the sediment Lizotte, ; Wing et al.

The latter constitutes an important food bank in the unproductive season Mincks et al. Biochemical studies show that the organic matter on the bottom in the neritic zone of the area was largely undegraded Fabiano and Pusceddu, ; Pusceddu et al. The isotopic similarity suggests negligible biogeochemical transformations of organic carbon and nitrogen following production, and thus minimal change in food quality, which might impact the proportions of carbon and nitrogen sources and their apparent incorporation in benthic communities over time.

By comparing the short- and long-term diets of the sea urchin Sterechinus neumayeri , Calizza et al. Similarly, Wing et al. However, this may not represent a general pattern in polar areas Gibson et al. The loss of coral reefs could also worsen coastal erosion due to their role in protecting shorelines from storms and cyclones. But if we reduce additional stressors such as overfishing and pollution, we can give species a better chance to adapt to climate change.

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