Contribution of sea ice microbial production to Antarctic benthic communities is driven by sea ice dynamics and composition of functional guilds

Individual diet variability shapes the architecture of Antarctic benthic food webs

Antarctic biodiversity is affected by seasonal sea-ice dynamics driving basal resource availability. To (1) determine the role of intraspecific dietary variability in structuring benthic food webs sustaining Antarctic biodiversity, and (2) understand how food webs and the position of topologically central species vary with sea-ice cover, single benthic individuals' diets were studied by isotopic analysis before sea-ice breakup and afterwards. Isotopic trophospecies (or Isotopic Trophic Units) were investigated and food webs reconstructed using Bayesian Mixing Models. As nodes, these webs used either ITUs regardless of their taxonomic membership (ITU-webs) or ITUs assigned to species (population-webs). Both were compared to taxonomic-webs based on taxa and their mean isotopic values. Higher resource availability after sea-ice breakup led to simpler community structure, with lower connectance and linkage density. Intra-population diet variability and compartmentalisation were crucial in determining community structure, showing population-webs to be more complex, stable and robust to biodiversity loss than taxonomic-webs. The core web, representing the minimal community 'skeleton' that expands opportunistically while maintaining web stability with changing resource availability, was also identified. Central nodes included the sea-urchin Sterechinus neumayeri and the bivalve Adamussium colbecki, whose diet is described in unprecedented detail. The core web, compartmentalisation and topologically central nodes represent crucial factors underlying Antarctica's rich benthic food web persistence.

Temperature and nutrients alter the relative importance of stochastic and deterministic processes in the coastal macroinvertebrates biodiversity assembly on long-time scales

Macroinvertebrates play a vital role in coastal ecosystems and are an important indicator of ecosystem quality. Both anthropogenic activity and environmental changes may lead to significant changes in the marine macroinvertebrate community. However, the assembly process of benthic biodiversity and its mechanism driven by environmental factors at large scales remains unclear. Here, using the benthic field survey data of 15 years at large spatial and temporal scales from the Yellow Sea Large Marine Ecosystem, we investigated the relative importance of environmental selection, dispersal processes, random-deterministic processes of macroinvertebrates community diversity assembly, and the responses of this relative importance driven by temperature and nutrients. Results showed that the macroinvertebrates community diversity is mainly affected by dispersal. Nitrogen and phosphorus are the most important negative factors among environmental variables, while geographical distance is the main limiting factor of β diversity. Within the range of 0.35-0.70 mg/L of nutrients, increasing nutrient concentration can significantly facilitate the contribution of the decay effect to β diversity. Within the temperature range studied (15.0-18.0°C), both warming and cooling can lead to a greater tendency for species diversity assembly processes to be dominated by deterministic processes. The analysis contributes to a better understanding of the assembly process of the diversity of coastal marine macroinvertebrates communities and how they adapt to global biogeochemical processes.

Mercury concentrations in Antarctic zooplankton with a focus on the krill species, Euphausia superba

The Antarctic is the most isolated region in the world; nevertheless, it has not avoided the negative impact of human activity, including the inflow of toxic mercury (Hg). Hg deposited in the Antarctic marine environment can be bioavailable and accumulate in the food web, reaching elevated concentrations in high-trophic-level biota, especially if methylated. Zooplankton, together with phytoplankton, are critical for the transport of pollutants, including Hg to higher trophic levels. For the Southern Ocean ecosystem, one of the key zooplankton components is the Antarctic krill Euphausia superba, the smaller euphausiid Thysanoessa macrura, and the amphipod Themisto gaudichaudii - a crucial food source for most predatory fish, birds, and mammals. The main goal of this study was to determine the Hg burden, as well as the distribution of different Hg forms, in these dominant Antarctic planktonic crustaceans. The results showed that the highest concentrations of Hg were found in T. gaudichaudii, a typically predatory taxon. Most of the Hg in the tested crustaceans was labile and potentially bioavailable for planktivorous organisms, with the most dangerous methylmercury (MeHg) accounting for an average of 16 % of the total mercury. Elevated Hg concentrations were observed close to the land, which is influenced by the proximity to penguin and pinniped colonies. In areas near the shore, volcanic activity might be a possible cause of the increase in mercury sulfide (HgS) content. The total Hg concentration increased with the trophic position and ontogenetic stage of predation, specific to adult organisms. In contrast, the proportion of MeHg decreased with age, indicating more efficient demethylation or elimination. The Hg magnification kinetics in the study area were relatively high, which may be related to climate-change induced alterations of the Antarctic ecosystem: additional food sources and reshaped trophic structure.

Biological responses to change in Antarctic sea ice habitats

Sea ice is a key habitat in the high latitude Southern Ocean and is predicted to change in its extent, thickness and duration in coming decades. The sea-ice cover is instrumental in mediating ocean–atmosphere exchanges and provides an important substrate for organisms from microbes and algae to predators. Antarctic krill, Euphausia superba, is reliant on sea ice during key phases of its life cycle, particularly during the larval stages, for food and refuge from their predators, while other small grazers, including copepods and amphipods, either live in the brine channel system or find food and shelter at the ice-water interface and in gaps between rafted ice blocks. Fish, such as the Antarctic silverfish Pleuragramma antarcticum, use platelet ice (loosely-formed frazil crystals) as an essential hatching and nursery ground. In this paper, we apply the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO) to review current knowledge about relationships between sea ice and associated primary production and secondary consumers, their status and the drivers of sea-ice change in this ocean. We then use qualitative network modelling to explore possible responses of lower trophic level sea-ice biota to different perturbations, including warming air and ocean temperatures, increased storminess and reduced annual sea-ice duration. This modelling shows that pelagic algae, copepods, krill and fish are likely to decrease in response to warming temperatures and reduced sea-ice duration, while salp populations will likely increase under conditions of reduced sea-ice duration and increased number of days of >0°C. Differences in responses to these pressures between the five MEASO sectors were also explored. Greater impacts of environmental pressures on ice-related biota occurring presently were found for the West and East Pacific sectors (notably the Ross Sea and western Antarctic Peninsula), with likely flow-on effects to the wider ecosystem. All sectors are expected to be impacted over coming decades. Finally, we highlight priorities for future sea ice biological research to address knowledge gaps in this field.

Experimentally derived biochemical modelling parameters to improve understanding of aquaculture's effect on marine food webs

To construct robust biogeochemical models for application to marine-based aquaculture settings, careful selection of appropriate model parameters is necessary. This study used an experimental approach to establish biomarkers of farm and marine-derived organic matter, and to derive isotopic turnover rates, and trophic discrimination factors specific to aquaculture associated food webs. A shift towards a farm-derived resource base resulted in consumer tissues more depleted in the carbon-13 isotope (indicated by more negative δ¹³C values) and a higher proportion of oleic acid, linoleic acid, and alpha-linoleic acid in the fatty acid profile of consumers over time. Measured trophic discrimination factors between dietary sources and consumer tissues demonstrated high variability among species and tissue types, ranging from -0.25‰ to 0.82‰ for Δ¹³C and from -0.77‰ to 6.8‰ for Δ¹⁵N. Stable isotope half-lives were also diverse among species and tissue types, ranging from <7 days to 462 days. Results demonstrated that construction of robust models for tracing assimilation of farm-derived organic matter through marine food webs requires the use of taxa and tissue specific parameters. Turnover rates have applications for understanding assimilative capacity of communities and for managing populations within the ecological footprint of farms.

Climate cascades affect coastal Antarctic seafloor ecosystem functioning

Polar seafloor ecosystems are changing rapidly and dramatically, challenging previously held paradigms of extreme dynamical stability. Warming-related declines in polar sea ice are expected to alter fluxes of phytoplankton and under-ice algae to the seafloor. Yet, how changes in food flux cascade through to seafloor communities and functions remains unclear. We leveraged natural spatial and temporal gradients in summertime sea ice extent to better understand the trajectories and implications of climate-related change in McMurdo Sound, Antarctica. McMurdo Sound was expected to be one of the last coastal marine environments on Earth to be affected by planetary warming, but the situation may be changing. Comparing satellite observations of selected coastal sites in McMurdo Sound between 2010-2017 and 2002-2009 revealed more ice-free days per year, and shorter distances to open water during the warmest months each year, in the more recent period. Interdecadal Pacific Oscillation (IPO), Oceanic Niño Index (ONI) and Antarctic Oscillation (AAO) climate indices peaked concurrently between 2014 and 2017 when sea ice breakouts in McMurdo Sound were most spatially and temporally extensive. Increases in sediment chlorophyll a and phaeophytin content (indicating increased deposition of detrital algal food material) were recorded during 2014-2017 at three coastal study sites in McMurdo Sound following the major sea ice breakouts. Soft-sediment seafloor ecosystem metabolism (measured in benthic incubation chambers as dissolved oxygen and inorganic nutrient fluxes) was correlated with sediment algal pigment concentration. Epifaunal invertebrate density, particularly opportunistic sessile suspension feeders, and infaunal community composition also shifted with increased food supply. The ecological characteristics and functions measured at the food-poor sites shifted towards those observed at richer sites at a surprisingly fast pace. These results indicate the sensitivity of the benthos and shed light on Antarctic marine trophic cascades and trajectories of response of iconic high-latitude seafloor habitats to a warming climate.

Sea Ice Dynamics Drive Benthic Microbial Communities in McMurdo Sound, Antarctica

Climate change is driving dramatic variability in sea ice dynamics, a key driver in polar marine ecosystems. Projected changes in Antarctica suggest that regional warming will force dramatic shifts in sea ice thickness and persistence, altering sea ice-associated primary production and deposition to the seafloor. To improve our understanding of the impacts of sea ice change on benthic ecosystems, we directly compared the benthic microbial communities underlying first-year sea ice (FYI) and multi-year sea ice (MYI). Using two tractable coastal habitats in McMurdo Sound, Antarctica, where FYI (Cape Evans) and MYI (New Harbour) prevail, we show that the structure and composition of the benthic microbial communities reflect the legacy of sea ice dynamics. At Cape Evans, an enrichment of known heterotrophic algal polysaccharide degrading taxa (e.g., Flavobacteriaceae, unclassified Gammaproteobacteria, and Rubritaleaceae) and sulfate-reducing bacteria (e.g., Desulfocapsaceae) correlated with comparatively higher chlorophyll a (14.2±0.8μgg^-1) and total organic carbon content (0.33%±0.04), reflecting increased productivity and seafloor deposition beneath FYI. Conversely, at New Harbour, an enrichment of known archaeal (e.g., Nitrosopumilaceae) and bacterial (e.g., Woeseiaceae and Nitrospiraceae) chemoautotrophs was common in sediments with considerably lower chlorophyll a (1.0±0.24μgg^-1) and total organic carbon content (0.17%±0.01), reflecting restricted productivity beneath MYI. We also report evidence of a submarine discharge of sub-permafrost brine from Taylor Valley into New Harbour. By comparing our two study sites, we show that under current climate-warming scenarios, changes to sea ice productivity and seafloor deposition are likely to initiate major shifts in benthic microbial communities, with heterotrophic organic matter degradation processes becoming increasingly important. This study provides the first assessment of how legacy sea ice conditions influence benthic microbial communities in Antarctica, contributing insight into sea ice-benthic coupling and ecosystem functioning in a polar environment.

A quantitative analysis of organic matter inputs to soft sediment communities surrounding salmon farms in the Marlborough Sounds region of New Zealand

Local differences in trophic structure and composition of organic matter subsidies can influence the capacity of soft sediment communities to assimilate recycled organic matter from processes such as salmon farm enrichment. The present study combines biochemical analysis with biomass density information on soft sediment taxa collected within the depositional footprint of salmon farms and at reference sites in the Marlborough Sounds, New Zealand. Distinct biochemical signatures confirmed that the flux of organic matter from salmon farms was an important subsidy for soft sediment communities. Isotopic modelling demonstrated that the proportion of biomass supported by farm-derived organic matter did not change in a consistent pattern along the 300 m gradient from each farm site, whereas the average trophic level of communities decreased with increasing proximity to farms. High variability in both the total biomass and the distribution of biomass across trophic levels occurred among sites downstream of farms and among individual farms. Consequently, estimates of basal organic matter assimilation per unit area by communities differed by several orders of magnitude among sites. Total organic matter assimilation tended to decrease with increasing proximity to farms due to a shift towards a more detrital based community. Differences in basal organic matter assimilation among farms did not appear to be directly related to local flow regime, but instead was closely linked to differences in the soft sediment community composition likely influenced by an array of anthropogenic and environmental factors. The results presented here highlight the importance of considering local variability in basal organic matter source pools, and the potential for synergistic and cumulative effects to drive changes in food web trophodynamics when assessing the impacts of aquaculture on soft sediment communities.

Antarctic ecosystems in transition - life between stresses and opportunities

Important findings from the second decade of the 21st century on the impact of environmental change on biological processes in the Antarctic were synthesised by 26 international experts. Ten key messages emerged that have stakeholder-relevance and/or a high impact for the scientific community. They address (i) altered biogeochemical cycles, (ii) ocean acidification, (iii) climate change hotspots, (iv) unexpected dynamism in seabed-dwelling populations, (v) spatial range shifts, (vi) adaptation and thermal resilience, (vii) sea ice related biological fluctuations, (viii) pollution, (ix) endangered terrestrial endemism and (x) the discovery of unknown habitats. Most Antarctic biotas are exposed to multiple stresses and considered vulnerable to environmental change due to narrow tolerance ranges, rapid change, projected circumpolar impacts, low potential for timely genetic adaptation, and migration barriers. Important ecosystem functions, such as primary production and energy transfer between trophic levels, have already changed, and biodiversity patterns have shifted. A confidence assessment of the degree of 'scientific understanding' revealed an intermediate level for most of the more detailed sub-messages, indicating that process-oriented research has been successful in the past decade. Additional efforts are necessary, however, to achieve the level of robustness in scientific knowledge that is required to inform protection measures of the unique Antarctic terrestrial and marine ecosystems, and their contributions to global biodiversity and ecosystem services.

Culture-Dependent and -Independent Analyses Reveal the Diversity, Structure, and Assembly Mechanism of Benthic Bacterial Community in the Ross Sea, Antarctica

The benthic bacterial community in Antarctic continental shelf ecosystems are not well-documented. We collected 13 surface sediments from the Ross Sea, a biological hotspot in high-latitude maritime Antarctica undergoing rapid climate change and possible microflora shift, and aimed to study the diversity, structure and assembly mechanism of benthic bacterial community using both culture-dependent and -independent approaches. High-throughput sequencing of 16S rRNA gene amplicons revealed 370 OTUs distributed in 21 phyla and 284 genera. The bacterial community was dominated by Bacteroidetes, Gamma- and Alphaproteobacteria, and constituted by a compact, conserved and positively-correlated group of anaerobes and other competitive aerobic chemoheterotrophs. Null-model test based on βNTI and RCBray indicated that stochastic processes, including dispersal limitation and undominated fractions, were the main forces driving community assembly. On the other hand, environmental factors, mainly temperature, organic matter and chlorophyll, were significantly correlated with bacterial richness, diversity and community structure. Moreover, metabolic and physiological features of the prokaryotic taxa were mapped to evaluate the adaptive mechanisms and functional composition of the benthic bacterial community. Our study is helpful to understand the structural and functional aspects, as well as the ecological and biogeochemical role of the benthic bacterial community in the Ross Sea.

Antarctic food web architecture under varying dynamics of sea ice cover

In the Ross Sea, biodiversity organisation is strongly influenced by sea-ice cover, which is characterised by marked spatio-temporal variations. Expected changes in seasonal sea-ice dynamics will be reflected in food web architecture, providing a unique opportunity to study effects of climate change. Based on individual stable isotope analyses and the high taxonomic resolution of sampled specimens, we described benthic food webs in contrasting conditions of seasonal sea-ice persistence (early vs. late sea-ice break up) in medium-depth waters in Terra Nova Bay (Ross Sea). The architecture of biodiversity was reshaped by the pulsed input of sympagic food sources following sea-ice break up, with food web simplification, decreased intraguild predation, potential disturbance propagation and increased vulnerability to biodiversity loss. Following our approach, it was possible to describe in unprecedented detail the complex structure of biodiverse communities, emphasising the role of sympagic inputs, regulated by sea-ice dynamics, in structuring Antarctic medium-depth benthic food webs.

In situ response of Antarctic under-ice primary producers to experimentally altered pH

Elevated atmospheric CO₂ concentrations are contributing to ocean acidification (reduced seawater pH and carbonate concentrations), with potentially major ramifications for marine ecosystems and their functioning. Using a novel in situ experiment we examined impacts of reduced seawater pH on Antarctic sea ice-associated microalgal communities, key primary producers and contributors to food webs. pH levels projected for the following decades-to-end of century (7.86, 7.75, 7.61), and ambient levels (7.99), were maintained for 15 d in under-ice incubation chambers. Light, temperature and dissolved oxygen within the chambers were logged to track diurnal variation, with pH, O₂, salinity and nutrients assessed daily. Uptake of CO₂ occurred in all treatments, with pH levels significantly elevated in the two extreme treatments. At the lowest pH, despite the utilisation of CO₂ by the productive microalgae, pH did not return to ambient levels and carbonate saturation states remained low; a potential concern for organisms utilising this under-ice habitat. However, microalgal community biomass and composition were not significantly affected and only modest productivity increases were noted, suggesting subtle or slightly positive effects on under-ice algae. This in situ information enables assessment of the influence of future ocean acidification on under-ice community characteristics in a key coastal Antarctic habitat.

Horizontal and vertical food web structure drives trace element trophic transfer in Terra Nova Bay, Antarctica

Despite a vast amount of literature has focused on trace element (TE) contamination in Antarctica during the last decades, the assessment of the main pathways driving TE transfer to the biota is still an overlooked issue. This limits the ability to predict how variations in sea-ice dynamics and productivity due to climate change will affect TE allocation in the food web. Here, food web structure of Tethys Bay (Terra Nova Bay, Ross Sea, Antarctica) was first characterised by analysing carbon and nitrogen stable isotopes (δ¹³C, δ¹⁵N) in organic matter sources (sediment and planktonic, benthic and sympagic primary producers) and consumers (zooplankton, benthic invertebrates, fish and birds). Diet and trophic position were also characterised using Bayesian mixing models. Then, relationships between stable isotopes, diet and TEs (Cd, Cr, Co, Cu, Hg, Ni, Pb and V) were assessed in order to evaluate if and how horizontal (organic matter pathways) and vertical (trophic position) food web features influence TE transfer to the biota. Regressions between log[TE] and δ¹³C revealed that the sympagic pathway drives accumulation of V in primary consumers and Cd and Hg in secondary consumers, and that a coupled benthic/pelagic pathway drives Pb transfer to all consumers. Regressions between log[TE] and δ¹⁵N showed that only Hg biomagnifies across trophic levels, while all the others TEs showed a biodilution pattern, consistent with patterns observed in temperate food webs. Although the Cd behavior needs further investigations, the present findings provide new insights about the role of basal sources in the transfer of TEs in polar systems. This is especially important nowadays in light of the forecasted trophic changes potentially resulting from climate change-induced modification of sea-ice dynamics.

Benthic responses to an Antarctic regime shift: food particle size and recruitment biology

Polar ecosystems are bellwether indicators of climate change and offer insights into ecological resilience. In this study, we describe contrasting responses to an apparent regime shift of two very different benthic communities in McMurdo Sound, Antarctica. We compared species-specific patterns of benthic invertebrate abundance and size between the west (low productivity) and east (higher productivity) sides of McMurdo Sound across multiple decades (1960s-2010) to depths of 60 m. We present possible factors associated with the observed changes. A massive and unprecedented shift in sponge recruitment and growth on artificial substrata observed between the 1980s and 2010 contrasts with lack of dramatic sponge settlement and growth on natural substrata, emphasizing poorly understood sponge recruitment biology. We present observations of changes in populations of sponges, bryozoans, bivalves, and deposit-feeding invertebrates in the natural communities on both sides of the sound. Scientific data for Antarctic benthic ecosystems are scant, but we gather multiple lines of evidence to examine possible processes in regional-scale oceanography during the eight years in which the sea ice did not clear out of the southern portion of McMurdo Sound. We suggest that large icebergs blocked currents and advected plankton, allowed thicker multi-year ice, and reduced light to the benthos. This, in addition to a possible increase in iron released from rapidly melting glaciers, fundamentally shifted the quantity and quality of primary production in McMurdo Sound. A hypothesized shift from large to small food particles is consistent with increased recruitment and growth of sponges on artificial substrata, filter-feeding polychaetes, and some bryozoans, as well as reduced populations of bivalves and crinoids that favor large particles, and echinoderms Sterechinus neumayeri and Odontaster validus that predominantly feed on benthic diatoms and large phytoplankton mats that drape the seafloor after spring blooms. This response of different guilds of filter feeders to a hypothesized shift from large to small phytoplankton points to the enormous need for and potential value of holistic monitoring programs, particularly in pristine ecosystems, that could yield both fundamental ecological insights and knowledge that can be applied to critical conservation concerns as climate change continues.