Edited by: Monica M. C. Muelbert, Federal University of São Paulo, Brazil
Reviewed by: Fabien Roquet, University of Gothenburg, Sweden; Leticia Cotrim Da Cunha, Rio de Janeiro State University, Brazil
This article was submitted to Global Change and the Future Ocean, a section of the journal Frontiers in Marine Science
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The manuscript assesses the current and expected future global drivers of Southern Ocean (SO) ecosystems. Atmospheric ozone depletion over the Antarctic since the 1970s, has been a key driver, resulting in springtime cooling of the stratosphere and intensification of the polar vortex, increasing the frequency of positive phases of the Southern Annular Mode (SAM). This increases warm air-flow over the East Pacific sector (Western Antarctic Peninsula) and cold air flow over the West Pacific sector. SAM as well as El Niño Southern Oscillation events also affect the Amundsen Sea Low leading to either positive or negative sea ice anomalies in the west and east Pacific sectors, respectively. The strengthening of westerly winds is also linked to shoaling of deep warmer water onto the continental shelves, particularly in the East Pacific and Atlantic sectors. Air and ocean warming has led to changes in the cryosphere, with glacial and ice sheet melting in both sectors, opening up new ice free areas to biological productivity, but increasing seafloor disturbance by icebergs. The increased melting is correlated with a salinity decrease particularly in the surface 100 m. Such processes could increase the availability of iron, which is currently limiting primary production over much of the SO. Increasing CO2 is one of the most important SO anthropogenic drivers and is likely to affect marine ecosystems in the coming decades. While levels of many pollutants are lower than elsewhere, persistent organic pollutants (POPs) and plastics have been detected in the SO, with concentrations likely enhanced by migratory species. With increased marine traffic and weakening of ocean barriers the risk of the establishment of non-indigenous species is increased. The continued recovery of the ozone hole creates uncertainty over the reversal in sea ice trends, especially in the light of the abrupt transition from record high to record low Antarctic sea ice extent since spring 2016. The current rate of change in physical and anthropogenic drivers is certain to impact the Marine Ecosystem Assessment of the Southern Ocean (MEASO) region in the near future and will have a wide range of impacts across the marine ecosystem.
The Southern Ocean (SO) physical environment is shaped by permanent cold and predictable seasonal cycles. However, despite its relative isolation within the Antarctic Circumpolar Current (ACC) inter-annual variation of the SO is strongly influenced through atmospheric and oceanic teleconnections. In an era of rapid climate change it is vital to identify the global drivers of the SO marine ecosystems. It is important to understand the regions within the SO where their effects are greatest, the expected impacts of any changes in these drivers, and crucially their interactions. For the working group, Marine Ecosystem Assessment of the SO (MEASO), global drivers are classified as those that influence the whole of the SO, even though their affects may manifest more strongly in some regions than others. Local drivers are defined as those that influence a particular location or series of locations within the SO (Grant et al., to be published in this research topic). While there is a global demand for protein to feed the ever increasing human population, southern ocean fisheries have regional impacts on stocks and ecosystems. A key difference is that local drivers can be managed with local interventions, e.g., regional regulation of fisheries. Yet, global drivers can only be managed externally to the region, e.g., climate change, ozone and plastic. Global tourism is driven by global drivers such as increasing wealth and demand for wilderness experiences. However, the regional impacts of tourism can also be managed through local regulation. Tourism therefore appears as both a global and local driver.
The manuscript outlines the global phenomena that impact the SO now and are expected to continue to impact it in the future. Atmospheric drivers considered here include changes in ozone, trends in Southern Annular Mode (SAM), variability in El Niño Southern Oscillation (ENSO), elevated air and ocean temperatures and ongoing increases in atmospheric CO2. Global oceanic connections considered here include the global thermohaline (overturning) circulation as well as changes in the SO, including eddies currents and the exchange of water masses (
Description of the global drivers affecting the Southern Ocean. Northern drivers are global drivers whose influence reaches from North of the Southern Ocean.
These drivers, and their interactions, affect a range of attributes of the SO environment. Those considered here are: the cryosphere including marine ice, ice shelves and glaciers as well as salinity. For the purposes of MEASO the SO is separated into five sectors and three zones (
Areas for assessing effects of global drivers in the Marine Ecosystem Assessment of the Southern Ocean (black lines) with the Antarctic Peninsula highlighted (red dashed box). The five sectors (Atlantic, Central Indian, East Indian, West Pacific and East Pacific) and zones (Antarctic, Subantarctic and Northern) are labeled. Zonal boundaries are the Southern ACC Front between Antarctic and Subantarctic zones, and the Subantarctic Front between Subantarctic and Northern zones, with the Subtropical Front to the north. Icons indicate currently identified critical locations for increase and decrease in temperature and sea ice, potential for invasive species, and where circumpolar deep water is increasingly shoaling onto the continental shelf. They also illustrate general effects of ozone depletion, pollution and ocean acidification.
The majority of the biological consequences within pelagic and benthic marine ecosystem are discussed in the specific MEASO biota papers (as described in
There have been several recent attempts to assess the dynamism of stressors impacting the Southern Ocean and its inhabitants (
Stratospheric ozone concentration in the middle and high latitudes of the southern hemisphere has been decreasing in spring, summer and winter since the 1970s (e.g.,
The tropospheric effect is an indication that stratospheric ozone depletion directly influences the leading mode of atmospheric circulation variability in the southern extratropical regions, the Southern Annular Mode (SAM). A measure of the SAM is an index calculated as the pressure gradient between mid-latitudes and Antarctica (
The SAM interacts with ENSO during the austral summer, particularly in the Pacific sector (e.g.,
Intensifying warm and moist westerly winds during more frequent positive phases of the summer SAM cause pronounced atmospheric warming of the Northern and central sectors of the West Antarctic Peninsula (WAP). Strong interannual variations of atmospheric warming/cooling patterns are strongly related to climatic modes of SAM and ENSO (
The SO has been a stable cold environment for millions of years, shaping unique ecosystems of cold adapted Antarctic biota. Within this stable regime there have been 100’s to 1000’s year time scale variations of 2–4°C on the WAP, which are associated with reconstructions of the SO westerly winds (
At the northwestern tip of Bransfield Strait and the South Shetland Islands (East Pacific), ocean temperatures are cooled by water entering from the Weddell Sea, while mean air temperatures increased by 0.4°C per decade between 1991 and 2012 (
Other regions of the SO either remained stable or cooled in the last two-three decades. These differences originate from varying patterns in the ocean heat transport around the Antarctic (
Many SO marine ectotherms are adapted to a narrow annual thermal range of only 2–4°C, and are therefore vulnerable to the effects of warming (
The SO circulation forms a major horizontal connection between ocean basins as well as a vertical link between shallow and deep water masses. The ACC consists of a strong, multi-jet, and turbulent eastward flow around Antarctica, transporting approximately 173.3 ± 10.7 × 106 m3.s–1 (
As indexed by the SAM, a clear trend in strengthening of westerly winds has been observed in recent decades (
The use of sea surface temperature (SST) and sea surface height (SSH) contours to track front locations has suggested there has been a latitudinal change in the mean location of the ACC. This could have major impacts on marine ecosystems by modifying the environmental conditions experienced by numerous marine organisms (
The assessments of trends in the overturning circulation is challenging due to the high inter-decadal variability related to wind stress (
Future projections for the SO under different warming scenarios from CMIP5 and CMIP6 models suggest that the trends observed over the last few decades will continue in the coming century at a rate that depends on future emission scenarios (
The observed changes in ocean properties and circulation in Antarctica reveals the central role that regional variability plays in understanding the evolution of this system (
The rapid warming of the WAP (
The combination of strong regional and temporal variability in observed ocean properties, combined with what are still relatively limited datasets, result in significant challenges when trying to attribute the origin of the changes. Elsewhere in Antarctica, teasing out anthropogenic change from natural variability has proven challenging for many variables, suggesting that the observed change is consistent with the pattern expected from natural variability (
Every year Antarctic seasonal sea ice undergoes a sixfold change in ice-covered area; it is one of the largest seasonal signals on Earth, with consequent effects on air-sea exchanges of heat, momentum and gases (most notably CO2), water mass properties and finally, on marine ecosystems. Antarctic sea ice is, however, highly variable and frequently exposed to strong storms, high winds and waves. These factors affect Antarctic sea ice growth and melt processes, thickness evolution and drift, and impart high regional and seasonal variability.
Over the past four decades there has been a modest increase in spatially averaged Antarctic sea ice extent. However, this Antarctic-wide average hides important regional and seasonal details, as Antarctic sea ice is not increasing everywhere and shows high seasonal and regional variability (e.g.,
These contrasting regional sea ice distributions can in part be explained by the SAM and ENSO changes detailed previously. The westerly winds have strengthened mostly in austral summer and autumn, i.e., when SAM shows the strongest positive trends (
This last decade, 2010–2020, has been most notable for showing the strongest changes in Antarctic-wide sea ice. During 2012–2014 numerous records were broken for Antarctic-wide high sea ice extent (based on satellite observations since 1979) (
Previous work has highlighted the effects of changes in sea ice and how they can transform shallow water benthic ecosystems associated with changes in light regimes (
The floating ice shelves shape unique conditions in the SO for marine life living below them. Ice shelves are floating masses of ice slowly advancing from the glaciers of the interior toward the ocean, at least regionally and temporarily since the late Oligocene (
Besides impacts on ocean physics the disintegration of large ice shelves causes one of the most effective changes in marine ecosystems (
A third, smaller but important area of marine ice loss is glacier retreat. Like sea ice and ice shelves, glaciers are also showing decreases, and in fairly complex geographic patterns. Nearly 90% of glaciers along the WAP are now retreating, and their retreat rates are also increasing (
Seawater salinity in the whole SO has decreased during the last 60–70 years (e.g.,
Freshening has other effects besides salinity. Stratification has a key controlling influence on phytoplankton blooms as do sources of iron from glacial meltwater and icebergs (
One of the most important functions of the SO in the Earth System is its uptake of atmospheric CO2 by physical and biological processes and its release of oceanic CO2 to the atmosphere (
At the global scale, oceans have absorbed ∼30% of the anthropogenic atmospheric CO2 and this has already caused shifts in seawater carbonate chemistry by reducing seawater pH, carbonate ion concentrations and therefore saturation states of the carbonate minerals aragonite and calcite (Ω, where values < 1 indicate undersaturated conditions that result in the dissolution of carbonate structures;
The SO has taken up around 40% of the total oceanic uptake of anthropogenic CO2 (
The coastal regions around Antarctica are characterized by highly variable topography, bathymetry, hydrodynamics and carbonate chemistry, which leads to highly variable seawater pH,
The Southern Ocean is a large high nutrient low chlorophyll (HNLC) zone where primary production is not limited by the availability of macronutrients, as in other oceans, but of the essential micronutrient, iron (e.g.,
The ongoing and projected increase in wind speeds and storminess over large parts of the Southern Ocean is likely to increase upwelling and upper ocean mixing, thus micro- and macronutrient flux from the CDW source waters into the mixed layer (
The large-scale climatic and environmental drivers of changes in the abundance and distribution of marine organisms will also have important consequences for Southern Ocean biogeochemistry through export fluxes, carbon storage in pelagic and benthic food webs, and for benthic-pelagic coupling (
Antarctic Tourism started over 100 years ago (
The nature of Antarctic ecotourism is also changing with the increased number of itineraries expanding the market to attract more active tourists, with options to kayak, camp on the ice, dive and snorkel with wildlife as well as skiing, including heli-skiing
Today the large majority of all tour operators operating in Antarctica are members of the International Association of Antarctic Tour Operators (IAATO), a self-governing body, including all commercial SOLAS passenger ship operators (see footnote). The association’s membership comprises 105 companies and organizations from all over the world. The future protection of Antarctica from the impacts of human activity requires collaboration on a global scale. To promote effective visitor management, IAATO annually shares detailed information on its activities with Antarctic Treaty Parties. Without further regulation the geographic spread of visits is virtually certain to increase.
All of these developments are highly likely to increase the footprint of Antarctic tourism, potentially increasing the impact on marine ecosystems, particularly through the risk of the introduction of non-indigenous species. There are also concerns about the risk of tourists transferring anthropengic diseases to Antarctic seabirds. However, the greatest health concern for the cruise ship industry are over human disease transmission within the close quarters of cruise ships (
Many anthropogenic pollutants are resistant to environmental degradation and can be transported from human population sources to remote regions of the world, such as the Southern Ocean. Heavy metals have been detected in ice core samples, with, for example, lead from Australian mining operations appearing at concentrations of up to 6 pg.g–1 after the start of the industrial revolution (
POPs consist of a number of different chemicals, including polychlorinated biphenyls (PCBs), organochloride pesticides (O) and polybrominated diphenyl ethers (PBDEs). A recent study of these compounds in the East Pacific region found comparable PCB and OCP concentrations to those detected 10 years earlier, but detected PBDEs for the first time (
Plastics are a global problem with levels in remote regions of the Atlantic. There has been a 10 fold increase in the quantity of plastic debris on beaches in the remote Falkland, Tristan da Cunha, St Helena and Ascension Islands in the last decade (
The shallow seas of the SO and its outer lying archipelagos have high levels of endemism in the region; up to 70% in some taxa (
However, there are “barriers” to species movement. The Polar Front is both semi-porous and migratory over time (
Preventative/biosecurity measures have been considerably ramped up and enforced within the last decade, including with the production of a Non-native Species Manual by the Committee for Environmental Protection (
The majority of NIS records have been in the Atlantic and East Pacific regions where the majority of boat transits into the Southern Ocean occur (
Locations of records of non-indigenous species within the MEASO region. References cited in text and
New approaches to threats to native species (
Poleward movement of species to bottlenecks such as Patagonia, South Africa and Australasia is likely to intensify over the coming decades, increasing propagule pressure of potential NIS (
Migratory species in Southern Ocean ecosystems include pinnipeds, cetaceans and seabirds. Many of these species travel long distances to Antarctic and sub-Antarctic waters during the austral summer to forage on low-trophic level prey. Migratory species are uniquely vulnerable to changing environmental conditions due to their long-distance travel and reliance on a number of unique integrated factors (
Migratory species moving through Southern Ocean ecosystems face numbers of direct and indirect pressures from changing physical environments and human activities across their range. Within Antarctic waters, changing climate drivers will alter productivity regimes, which will very likely affect low trophic prey such as krill (
The long distance movements of Southern Ocean migratory species make them susceptible to other external anthropogenic impacts, such pollution, ship strikes fishing gear interactions diminishing food supplies, and reduced breeding habitats, all of which can reduce population numbers or negatively affect the fitness of an animal (
While pollutant exposure is generally low within the SO, pollutants pose a serious risk to SO migratory species. Migratory seabirds such as south polar skuas (
Due to this exposure when outside of the SO, migratory species likely act as vectors for transporting pollutants into the SO (
Since the early 90s, efforts conducted by the Committee for the Conservation of Antarctic Living Marine Resources (CCAMLR), via a series of conservation measures, have been very successful in addressing the problem of incidental mortality, especially seabirds, in the Convention area. However, during migrations outside of the SO bycatch and entanglement in fisheries gear are recognized as the most significant threat to the survival of cetacean and seabird species and populations globally (
Other external drivers affecting SO migratory species include vessel traffic and mining activities. Ship strike/collision can result in injury, displacement, disturbance, and behavioral modification (
Some migratory animal populations may also be susceptible to diminishing food supplies in regions where fisheries are poorly managed, or where climate change impacts may affect lower trophic prey populations. Overfishing can have profound implications for higher-order predators. Population-level impacts on seabirds such as albatrosses and petrels may occur through direct competition with fisheries for prey (
The global physical and human drivers, along with their interactions, were summarized from the proceeding text into a qualitative network diagram (
Qualitative network diagram describing the linkages between the global drivers and the benthic and pelagic Southern Ocean assemblages. Northern drivers are global drivers whose influence comes from North of the Antarctic cirucm-polar current (ACC). Contrasting effects are highlighted by separate west Pacific (western Antarctic Peninsula) and east Pacific regional networks. Range shifts from north of the ACC (Non-indigenous species) and within the Southern Ocean are indicated separately. An arrow head indicates a positive influence, an open circle indicates a negative influence.
The future projections of the global drivers highlighted in the proceeding literature review are summarized through the assessment in
Assessment of the current direction of each of the identified global drivers within the MEASO sectors, in recent times (R) and in the future (F).
Region | Zone | Ozone |
SAM |
ENSO |
Temperature |
Tourism |
Pollution |
Sea Ice |
Ice Shelves |
Salinity |
pH |
NIS |
|||||||||||
R | F | R | F | R | F | R | F | R | F | R | F | R | F | R | F | R | F | R | F | R | F | ||
Atlantic Ocean | A | + | + | + | + | + | + | + | ? | + | + | − | ? | − | − | − | − | ? | − | No | + | ||
S | + | + | + | + | + | + | + | + | + | + | + | − | ? | N/A | N/A | − | ? | − | + | + | |||
N | + | + | + | + | + | + | + | + | + | + | + | N/A | N/A | N/A | N/A | − | + | − | + | + | |||
Central Indian | A | + | + | + | + | + | + | N/A | ? | + | + | − | ? | No | No | − | − | ? | − | No | + | ||
S | + | + | + | + | + | + | + | + | + | N/A | N/A | N/A | N/A | − | ? | − | + | + | |||||
N | + | + | + | + | + | + | + | + | + | N/A | N/A | N/A | N/A | − | + | − | + | + | |||||
East Indian | A | + | + | + | + | + | − | + | + | + | + | ? | No | No | ? | − | ? | − | No | + | |||
S | + | + | + | + | + | − | + | + | + | + | ? | N/A | N/A | − | ? | −− | No | + | |||||
N | + | + | + | + | + | − | + | + | + | N/A | N/A | N/A | N/A | − | + | − | ? | + | |||||
West Pacific | A | + | + | + | + | + | − | + | + | + | + | ? | No | No | − | − | ? | − | No | + | |||
S | + | + | + | + | + | − | + | + | + | + | ? | N/A | N/A | − | ? | − | No | + | |||||
N | + | + | + | + | + | − | + | + | + | N/A | ? | N/A | N/A | − | + | − | ? | + | |||||
East Pacific | A | + | + | + | + | + | + | + | + | ? | + | + | − | ? | − | − | − | ? | ? | − | No | + | |
S | + | + | + | + | + | + | + | + | ? | + | + | − | ? | N/A | N/A | − | + | − | No | + | |||
N | + | + | + | + | + | + | + | + | ? | + | + | N/A | ? | N/A | N/A | + | + | − | No | + |
Modeling the trajectories of climate change is one of the biggest challenges to understanding the future impacts of drivers on the Southern Ocean and a key limitation to future projections. Whereas arctic environmental responses to climate over the last decade have been continuous and homogeneous, Antarctic and SO changes are regional and variable (
There is also a degree of uncertainty over when tipping points will be reached (e.g.,
The very limited availability of long-term time series hinders our ability to discern between interannual/interdecadal changes and long-term trends in ecosystem shift. It is difficult to inform policy and decision makers without a clear distinction between natural variability and global change impacts. Funding agencies must allocate money to maintain long-term sampling programs. Continued and improved data collection through international effort to obtain standardized datasets, developments in modeling and integrated work programs, such as the Southern Ocean Observing System (
As with all remote regions, data coverage is better near locations with the greatest human footfall. This footfall is also higher in summer than winter. Remote sensing from satellites and remote monitoring stations are key tools to increase the spatio-temporal coverage and improve the reliability of projections that can significantly reduce the cost of long term sampling programs. The increase in continuous observing systems will be key to improving data coverage. Innovation, technological advancements on autonomous vehicles and other platforms (e.g., oceanographic moorings), including the ability to measure under the seasonal ice cover and floating ice sheets should contribute towards improving our understanding of processes occurring during the winter, and how they change in longer time-scales.
Understanding if human behavior will change, and how international treaties will limit the pace of climate change, will be key. Industrial process are developing novel chemicals, some of these are designed to replace substances that are currently controlled, such as CFCs. However, some of these replacements, such as SF6, are known to be potent climate gases (
Many of the changes in global drivers that have affected SO marine ecosystems over recent decades were strongly linked to the loss of atmospheric ozone since the 1970s (
While the reversal of the winds may slow the shoaling of warm water onto SO shelves (
Some pollutants such as CFC’s, are subject to global agreements, their levels are controlled and their impacts are reversing (
Other anthropogenic drivers also have degrees of uncertainty. Until very recently global tourism was expected to increase in parallel with economic prosperity, however, growing health concerns over mass tourism (
In particular the necessity of determining between trends that are the result of natural variability and those that are anthropogenic or the result of climate change requires an increased number of extended time series. Process studies, to understand the underlying mechanisms of this variability, must be a priority. Efforts to improve the observational records around Antarctica are critical to understand the nature of change around the continent and to predict the impacts on marine ecosystems.
All authors listed have made a substantial, direct and intellectual contribution to the work, and approve it for publication.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This work is a core contribution to the first Marine Ecosystem Assessment for the Southern Ocean (MEASO). We thank the MEASO Support Group and Steering Committee for assisting with figures, coordination and editing of the text.