The ocean off the northern Antarctic Peninsula is characterised by steep environmental gradients, if not discrete boundaries (Dorschel et al. 2015), as well as speciose benthic habitats (Griffiths 2010; De Broyer et al. 2014) and hotspots of biological productivity (Grange and Smith 2013). This region of the Southern Ocean (SO) attracts particular attention from scientists, decision-makers as well as the general public, because here the pace of atmospheric and oceanic climate change has been clearly above the global average over the past decades (Turner et al. 2014), and the local ecosystems are strongly affected by multiple stressors (Gutt et al. 2015a, b). Moreover, commercial bottom trawling disturbed the sea floor until it became prohibited in 1990, and whaling, sealing and exploitation of penguin and krill stocks modified the pelagic system in the twentieth century (Kock 1992; Knox 2007). In the austral summer (January–March) of 2013, this region was the target area of the cruise PS81 (ANT-XXIX/3) of the German research icebreaker Polarstern (for the PS81 cruise report, see Gutt 2013) (Figs. 1, 2 and 3).

Fig. 1
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Research icebreaker Polarstern in thick pack ice in the western Weddell Sea during expedition PS81 (ANT-XXIX/3), © J. Gutt

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Fig. 2
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Participants of Polarstern expedition PS81 (ANT-XXIX/3) in January–March 2013

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Fig. 3
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Participants of the post-expedition workshop in Dijon/France, in September 2014

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During the cruise, Antarctic krill was investigated in the framework of a regular survey programme of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), and benthic ecosystems were studied applying an ecosystem-oriented multidisciplinary approach that integrated the analyses of a number of working groups and focussed on the overarching research question: How does the variability of various environmental factors impact biological patterns and processes? The project was developed to provide a sound basis for adequately addressing one of the scientific key issues identified in the First SCAR Antarctic and Southern Ocean Horizon Scan (Kennicutt et al. 2014) as being of highest priority for the next 20 years and beyond: How will threshold transitions vary over different spatial and temporal scales, and how will they impact ecosystem functioning under future environmental conditions? The project also contributed to the biology research programme Antarctic Thresholds-Ecosystem Resilience and Adaptation (AnT-ERA) of the Scientific Committee on Antarctic Research (SCAR), which addresses key questions (Gutt et al. 2013): (1) How do species traits impact community stability and key ecosystem processes? and (2) What are the likely consequences of a changing environment for key ecosystem functions and services?

In the framework of the overarching aim to investigate complex biological structures and link these to environmental factors, a number of different subprojects were carried out during the cruise. Their scientific results were discussed in the wider context of the expedition’s objectives during a post-expedition workshop hosted by the University of Burgundy in Dijon, France, in September 2014. Three case studies have already been published, one with an interdisciplinary approach focussing on a flat-topped hill on the the northwestern Weddell Sea shelf (Dorschel et al. 2014), a second one on the ecology and diversity of free-living benthic nematodes (Hauquier et al. 2015) and a third one on physiological effects of ocean acidification (Collard et al. 2015).

Like during the former Polarstern cruise PS69 that investigated an adjacent study area in 2006/2007, the basic sampling strategy was to deploy the wide set of sampling equipment at a number of selected stations. Such an integrative sampling within a spatially well-defined area, called a core station, has provided results that are not only representative for one spot, but also for a range of habitats in a region several square kilometres in size. The strategy to comparatively investigate several of such core stations comprising a set of habitats predefined according to their bottom topography provided the basis for distinguishing between faunas associated with different habitat types independently of a specific site. Some of the observed benthic and sediment parameters could serve as a proxy for food supply to the sea floor and thus provide evidence for the strength of the pelagic-benthic coupling across environmental gradients.

For the ecological studies presented in this special issue, the environmental setting was of high relevance. Thus, comprehensive ship-based information on bathymetry, seabed morphology, sediment properties and water-mass characteristics as well as satellite data on chlorophyll-a and sea-ice patterns was published by Dorschel et al. (2015), van Caspel et al. (2015) and Huneke et al. (2016).

The fauna investigated during the PS81 cruise ranged from the meiofauna collected from multi-corer samples, with emphasis on benthic free-living nematodes and copepods, to large sponges caught by the Agassiz trawl, as well as from the pelagic key organism krill surveyed within the regular CCAMLR survey to whales observed and counted along helicopter transects. A special focus among benthic systematic groups was on sponges (Berne et al. 2015; Kersken et al. 2016), amphipods (Verheye et al. 2016) and echinoderms (David et al. 2015; Ambroso et al. 2016). For echinoids, a shift in food preference with changing environmental conditions was studied (Michel et al. 2015). Not only organism-based aspects were investigated, but also ecological and sediment processes, including interactions between biota and biogeochemical factors (Isla 2015). The study on the spatial ecology of ascidians, which are assumed to be especially sensitive to environmental change (Segelken-Voigt et al. 2016), was based on seabed images, which are publicly available in the data repository PANGAEA (www.pangaea.de). An analysis of macrobenthic communities and their diversity was based on subsamples taken from Agassiz trawl catches (Gutt et al. 2015a, b). Interestingly, the most obvious large-scale macrobenthic distribution patterns separating the Drake Passage from the Bransfield Strait and the Weddell Sea resemble very much the composition patterns of whales (Herr et al. 2016), which utilise a different part of the SO ecosystem but are obviously directly or indirectly affected by the same set of environmental drivers.

The faunistic and environmental primary data collected during the PS81 cruise were uploaded to data repositories, e.g., ANTABIF (www.biodiversity.aq) or PANGAEA, to contribute to a more complete image of global and regional marine biodiversity and to a better understanding of marine ecosystem functioning. The results published in this special issue will provide a sound baseline to assess and predict the impact of ongoing and future environmental change in the sensitive marine ecosystems off the northern Antarctic Peninsula, where the pace of change is faster than elsewhere in the world ocean.