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In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom‐bound 15N/14N (δ15Ndb) and reduced export production during the ice ages indicates more complete nitrate (NO3−) consumption. This evidence points to an ice age decline in gross NO3− supply from the deep ocean to the surface wind‐mixed layer, which may help to explain the reduced CO2 levels of the ice age atmosphere. We use a seasonally resolved, two‐layer model of the N isotopes in the Antarctic Zone upper ocean to quantify the ice age decline in gross NO3− supply implied by the data. When model parameters are varied to reflect reduced gross NO3− supply, the concentration of wintertime upper ocean NO3− is lowered, but with a much weaker increase in NO3− δ15N than predicted by analytical models such as the Rayleigh and steady state models. Physical mixing is the dominant cause, with a modest contribution from foodweb dynamics. As a result, the observed δ15Ndb rise of ~3‰–4‰ must be explained mostly by a greater summertime increase in NO3− δ15N during the ice ages. The high degree of NO3− consumption required to generate this summertime δ15N rise indicates a >80% reduction in gross NO3− supply. Half or more of the modern gross NO3− supply is from wind‐forced Antarctic upwelling that drives the upper cell of Southern Ocean overturning. Thus, the decrease in NO3− supply cannot be achieved solely by a decline in vertical mixing or wintertime convection; rather, it requires an ice age weakening of the upper cell. Key Points: A two‐layer model of Antarctic Zone seasonality that includes upper ocean N cycling captures key N isotopic patternsThe model indicates that an ice age rise of ~3‰‐4‰ in diatom‐bound δ15N requires a >80% reduction in gross NO3− supply from deep watersThis large change implies a weakening of the upper cell of global ocean overturning that passes through the Antarctic surface [ABSTRACT FROM AUTHOR]

The Antarctic Peninsula (AP) region is one of the areas under faster warming rates worldwide, and where food web changes have been observed in the last decades. Among these changes are the development of cryptophytes under warmer conditions in detriment of diatoms, and the reduced krill availability in the environment. An isotopic approach was used to investigate whether the temporal and spatial patterns of energy transfer from phytoplankton (using particulate organic matter – POM – as a proxy of primary producers) to baleen whales (humpback – Megaptera novaeanglieae, fin – Balaenoptera physalus, Antarctic minke – Balaenoptera bonaerensis) , and killer whales – Orcinus orca – is similar in areas under different effects of warming around the northern Antarctic Peninsula (NAP). Samples of POM (n = 65), krill (n = 29) and skin of baleen (n = 106) and, opportunistically, killer whales (n = 5) were collected in Gerlache and Bransfield Straits (western AP) and the Powell Basin (northeastern AP) during the austral summers of 2013–2016. Mean isotope values for δ 13 C and δ 15 N values were, respectively, ‐26.3‰ (± 2.9) and 0.9‰ (± 1.7) for POM, ‐25.6‰ (± 0.9) and 5.3‰ (± 1.1) for krill, ‐24.1‰ (± 2) and 8.9‰ (± 1.5) for humpback, ‐24.6‰ (± 1.2) and 8.2‰ (± 0.7) for fin, ‐24.4‰ (± 1.6) and 8.7‰ (± 1) for Antarctic minke whales, and −23.6‰ (± 1.2) and 8.9‰ (± 1.7) for killer whales. Interannual significant differences were found for δ 13 C values of POM and fin whales’ samples, while spatial differences were found for δ 13 C values of POM samples and humpback whales and for δ 15 N values of POM, humpback and Antarctic minke whales. Lower δ 13 C and δ 15 N values for the base of the food web tended to be observed towards open sea regions (Powell Basin and an area under the influence of the Bellingshausen Sea waters). The isoscapes generated for the baseline of the NAP ecosystem provided unprecedented information, to the best of our knowledge, of how the δ 13 C and δ 15 N values of POM varied spatially and temporally in the region. HPLC-CHEMTAX pigment analysis indicated that two of the main phytoplankton groups in the study region were diatoms and cryptophytes. The contribution of these groups to the total phytoplankton biomass was positively and negatively correlated with the POM δ 13 C values, respectively. Despite the spatial and temporal limited interpretation of our results due to our reduced sampling effort to the east of the AP and to the relatively short temporal range investigated, the differences observed in the isotopic composition are considered representative of contrasting environmental conditions. The present study provides new insights on stable isotope values in the Antarctic ecosystem and may help to foresee the consequences of physico-chemical changes in water properties to the biota due to global warming. [ABSTRACT FROM AUTHOR]

Abstract: A new fishing gear was used to sample the macrozooplankton and micronekton community in the surface layer (0–2m) under ice and in open water, the Surface and Under Ice Trawl (SUIT). In total, 57 quantitative hauls were conducted in the Lazarev Sea (Southern Ocean) during 3 different seasons (autumn 2004, winter 2006, summer 2007/2008). At least 46 species from eight phyla were caught in all 3 seasons combined. Biomass density was dominated by Antarctic krill Euphausia superba. The average biomass density was highest under the winter sea ice and lowest under the young ice in autumn. In summer, macrozooplankton biomass was dominated by ctenophores in open water and by Antarctic krill under ice. The community composition varied significantly among seasons, and according to the presence of sea ice. The response of the community composition to the presence of sea ice was influenced by species that were significantly more abundant in open water than under ice (Cyllopus lucasii, Hyperiella dilatata), only seasonally abundant under ice (Clione antarctica), or significantly associated with sea ice (Eusirus laticarpus). A number of abundant species showed distinct diel patterns in the surface occurrence both under ice and in open water, indicating that the surface layer serves as a foraging ground predominantly at night. Our results emphasize the potential of a number of non-euphausiid macrozooplankton and micronekton species to act as energy transmitters between the production of sea ice biota and the pelagic food web. By providing a regional-scale quantitative record of macrofauna under Antarctic sea ice covering 3 seasons, this study adds new and direct evidence that the ice-water interface layer is a major functional node in the ecosystem of the Antarctic seasonal sea ice zone. [Copyright &y& Elsevier]