Spatial distributions of Southern Ocean mesozooplankton communities have been resilient to long-term surface warming

Vertical distribution of the zooplankton in the Antarctic Peninsula during the austral summer of 2017

Zooplankton plays a crucial role as the primary consumers in the Southern Ocean and its ecological processes, particularly in the Antarctic Peninsula, influenced by regional glaciological and oceanographic changes. To assess the overall composition of these communities, vertical samples were collected at various depths using a Multinet at oceanographic stations in the Antarctic Peninsula during the XXXV OPERANTAR expedition in summer 2017. Abiotic data (temperature, salinity and chlorophyll-a) were collected using a CTD and a fluoremeter. Organisms were identified to a higher level, with Chaetognatha and Euphausiacea identified to species whenever possible. Copepoda were the most abundant (73.4%), with Calanoida present in all samples and more abundant at 300m. Salpidae ranked as the second most abundant taxon (16.6%) up to 100m. Three species of Chaetognatha were identified, with Eukrohnia hamata being the most abundant, particularly at 300m. Two species of Euphausiacea were found, Euphausia superba and Thysanoesa macrura, with low abundances. Abiotic parameters showed significant relationships with the taxa. The region exhibits complex oceanography associated with zooplankton communities. The recorded data align with the zooplankton characterization of this region, uncovering a prevalence of Copepoda and surface abundant Salpidae, along with Chaetognatha (particulary E. hamata) and Euphausiacea.

Monitoring and modelling marine zooplankton in a changing climate

Zooplankton are major consumers of phytoplankton primary production in marine ecosystems. As such, they represent a critical link for energy and matter transfer between phytoplankton and bacterioplankton to higher trophic levels and play an important role in global biogeochemical cycles. In this Review, we discuss key responses of zooplankton to ocean warming, including shifts in phenology, range, and body size, and assess the implications to the biological carbon pump and interactions with higher trophic levels. Our synthesis highlights key knowledge gaps and geographic gaps in monitoring coverage that need to be urgently addressed. We also discuss an integrated sampling approach that combines traditional and novel techniques to improve zooplankton observation for the benefit of monitoring zooplankton populations and modelling future scenarios under global changes.

A database of zooplankton abundance in the Atlantic sectors of the Southern and sub-Arctic Oceans

Scientific sampling of zooplankton in the Atlantic sector of the Southern Ocean has been undertaken since the 1920s, but few analyzed datasets are available to the research community. We provide a database of standardized data derived from samples collected by Bongo nets in this sector between 1996 and 2013, amounting to almost 94,000 individual records. The study region contains some of the highest levels of pelagic biomass in the Southern Ocean and is also undergoing rapid ocean warming and changing seasonality in sea-ice distribution. Data from a single expedition to the sub-Arctic where the same sampling methodology was used are also included. Atlantic water is an increasing influence in that region, as is the prevalence of boreal plankton taxa within Arctic plankton communities. These data will be of value in supporting studies assessing the impacts of climate change on the structure and function of polar pelagic systems.

WOS, Steinberg DK, Swadling KM, Tarling GA, Thorpe SE, Veytia D, Ward P, Weldrick CK, Yang G. Status, Change, and Futures of Zooplankton in the Southern Ocean

In the Southern Ocean, several zooplankton taxonomic groups, euphausiids, copepods, salps and pteropods, are notable because of their biomass and abundance and their roles in maintaining food webs and ecosystem structure and function, including the provision of globally important ecosystem services. These groups are consumers of microbes, primary and secondary producers, and are prey for fishes, cephalopods, seabirds, and marine mammals. In providing the link between microbes, primary production, and higher trophic levels these taxa influence energy flows, biological production and biomass, biogeochemical cycles, carbon flux and food web interactions thereby modulating the structure and functioning of ecosystems. Additionally, Antarctic krill (Euphausia superba) and various fish species are harvested by international fisheries. Global and local drivers of change are expected to affect the dynamics of key zooplankton species, which may have potentially profound and wide-ranging implications for Southern Ocean ecosystems and the services they provide. Here we assess the current understanding of the dominant metazoan zooplankton within the Southern Ocean, including Antarctic krill and other key euphausiid, copepod, salp and pteropod species. We provide a systematic overview of observed and potential future responses of these taxa to a changing Southern Ocean and the functional relationships by which drivers may impact them. To support future ecosystem assessments and conservation and management strategies, we also identify priorities for Southern Ocean zooplankton research.

Stepping stones towards Antarctica: Switch to southern spawning grounds explains an abrupt range shift in krill

Poleward range shifts are a global-scale response to warming, but these vary greatly among taxa and are hard to predict for individual species, localized regions or over shorter (years to decadal) timescales. Moving poleward might be easier in the Arctic than in the Southern Ocean, where evidence for range shifts is sparse and contradictory. Here, we compiled a database of larval Antarctic krill, Euphausia superba and, together with an adult database, it showed how their range shift is out of step with the pace of warming. During a 70-year period of rapid warming (1920s-1990s), distribution centres of both larvae and adults in the SW Atlantic sector remained fixed, despite warming by 0.5-1.0°C and losing sea ice. This was followed by a hiatus in surface warming and ice loss, yet during this period the distributions of krill life stages shifted greatly, by ~1000 km, to the south-west. Understanding the mechanism of such step changes is essential, since they herald system reorganizations that are hard to predict with current modelling approaches. We propose that the abrupt shift was driven by climatic controls acting on localized recruitment hotspots, superimposed on thermal niche conservatism. During the warming hiatus, the Southern Annular Mode index continued to become increasingly positive and, likely through reduced feeding success for larvae, this led to a precipitous decline in recruitment from the main reproduction hotspot along the southern Scotia Arc. This cut replenishment to the northern portion of the krill stock, as evidenced by declining density and swarm frequency. Concomitantly, a new, southern reproduction area developed after the 1990s, reinforcing the range shift despite the lack of surface warming. New spawning hotspots may provide the stepping stones needed for range shifts into polar regions, so planning of climate-ready marine protected areas should include these key areas of future habitat.

An assessment of climate change vulnerability for Important Bird Areas in the Bering Sea and Aleutian Arc

Recently available downscaled ocean climate models for the Bering Sea and Aleutian Arc offer the opportunity to assess climate vulnerability for upper trophic level consumers such as marine birds. We analyzed seasonal and annual spatial projections from three climate models for two physical climate variables (seawater temperature and sea ice) and three forage variables (large copepods, euphausiids, and benthic infauna), comparing projected conditions from a recent time period (2003–2012) to a future time period (2030–2039). We focused the analyses on core areas within globally significant Important Bird Areas, and developed indices of the magnitude of projected change and vulnerability agreement among models. All three climate models indicated a high degree of change for seawater temperature warming (highest in the central and eastern Aleutian Islands) and ice loss (most significant in the eastern Bering Sea) across scales, and we found those changes to be significant for every species and virtually every core area assessed. There was low model agreement for the forage variables; while the majority of core areas were identified as climate vulnerable by one or more models (72% for large copepods, 73% for euphausiids, and 94% for benthic infauna), very few were agreed upon by all three models (only 6% of euphausiid-forager core areas). Based on the magnitude-agreement score, euphausiid biomass decline affected core areas for fulmars, gulls, and auklets, especially along the outer shelf and Aleutian Islands. Benthic biomass decline affected eiders along the inner shelf, and large copepod decline was significant for storm-petrels and auklets in the western Aleutians. Overall, 12% of core areas indicated climate vulnerability for all variables assessed. Modeling and interpreting biological parameters to project future dynamics remains complex; the strong signal for projected physical changes raised concerns about lagged responses such as distribution shifts, breeding failures, mortality events, and population declines.