General physical characteristics
The inland aquatic ecosystems of Antarctica include surface, subglacial and groundwater systems1,2,3,4, and are centers for biodiversity, biogeochemistry and biological production. Sources of liquid water vary across the continent, with increasing dependence on melting of glacial ice over recent snowfall (and rain) from the maritime to higher latitudes. The summertime duration of free-flowing water follows a similar gradient, from several months in the Maritime Antarctic to only a few days per year at 80oS. On the Antarctic Peninsula, relatively high precipitation results in high catchment water-yields, and lakes and ponds are mostly well flushed and diluted. At higher latitudes, increasing aridity results in a greater proportion of endorheic lakes and ponds (catchments that have no outflow), within which salinisation (accumulation of salts) becomes prevalent. Salinisation is a feature of the few groundwater systems analysed to date. Lakes and ponds in the colder inland parts of the continent can be perennially covered with many metres of ice, within which loss by ablation and basal melting is balanced by gains during winter freezing. Lakes and ponds at lower latitudes may receive sufficient summer heat to often become seasonally ice free.
Given the dramatic differences in fundamental aspects of inland waters in different parts of Antarctica, we have identified five inland-water catchment systems that largely match the“Environmental Domains Analysis for the Antarctic Continent”5 : 1. Maritime; 2. Continental Oases; 3. Polar Deserts; 4. Supra-glacial ecosystems and, 5. Subglacial ecosystems.
Examples: King George Island, Byers Peninsula, Alexander Island, Signy Island
This is the wettest part of Antarctica, where summer temperatures often exceed zero and precipitation as rain is significant. This, plus melting of winter snow, results in streams which flow over relatively long periods. Catchments may be partially vegetated with microbial mats, mosses and liverworts (and grasses in some northern sites) that provide some organic and nutrient inputs to water bodies6. Inland waters are generally highly dilute and unproductive (ultra-oligotrophic), though in coastal areas, marine animals may cause nutrient enrichment (eutrophication). Salt is generally low and mostly from sea spray7. Lakes and ponds are usually ice-free in summer7. Plentiful meltwater means that most have outflows and have relatively stable levels, but some are ice-dammed. Maritime lakes are generally well mixed in summer in their ice-free state and stratified under ice in winter. These are termed holomictic lakes.
2. Continental Oases:
Examples: Bunger, Vestfold and Larsemann Hills, Schirmacher and Syowa Oases
Continental Oases are found on the continental margins in relatively recent landscapes often formed from the Holocene retreat of the ice sheet and accompanying uplift of the land surface (isostatic uplift). Some lakes existed during at least the last glacial-interglacial cycle8. The streams, that flow in summer only, are usually short, some with large flows (e.g. up to 6 m3s-1 in Algae River, Bunger Hills), more often from the inland ice-sheets than glaciers. Ponds are often rich in cyanobacteria and sometimes have aquatic mosses. Lakes of the Oases demonstrate a mix of summer ice-free and ice-capped conditions. Most are freshwater, but brackish to hypersaline systems may result from evaporation and evolution of trapped seawater. Lakes here are often chemically stratified (meromictic) where meltwater has overridden relic seawater. An example is Deep Lake (Vestfold Hills) a saline end member with evaporation-concentrated seawater that remains ice free at -15oC in winter9. Tidal epishelf lakes2 are found here and in Maritime Antarctica. Epishelf lakes occur where freshwater accumulates at the junction between oases and floating ice shelves. The freshwater overlays the seawater at the lake base.
3. Polar Deserts:
Examples: McMurdo Dry Valleys, James Ross Island.
Polar deserts are characterized by very low precipitation, and barren landscapes (sometimes >106 years old). Water sources are primarily glaciers and ice-fields. Melting is sensitive to temperature and solar radiation so that melt-stream discharges are characterized by variability at diurnal, intra-summer and inter-annual timescales10,11. Some streams are large (e.g. up to 10 m3 s-1 in Onyx River, Wright Valley that is 30 km long). There are many endorheic catchments and saline and brackish ponds are common. Ponds typically remain ice-capped in summer with only a few examples gaining sufficient heat to become ice-free12,13. All polar desert lakes have thick, perennial ice-caps10 (e.g. Lakes Fryxell, Bonney) and some are frozen to the base with perhaps a basal brine layer14 (e.g. Lakes Vida, House). Historic dry-downs and re-fillings in response to long-term climate variability have left most Polar Desert lakes chemically stratified with increasing salt concentrations toward the base (Meromictic). Lake Vanda (Area: 7.5 km2, Depth: 78 m) has multiple brine layers and year-round bottom temperatures of >20°C10. Recent studies have indicated extensive saline groundwater beneath much of Taylor Valley4 and parts of the Wright Valley15.
4. Supra-glacial ecosystems:Examples throughout coastal Antarctica
Free water in summer forms supra-glacial streams and ponds that develop on the ablation zones of many coastal and inland glaciers, ice sheets and ice shelves. Variability in supra-glacial streamflow is high and is characterized by diurnal freeze-thaw cycles3,16,17. Ponds and lakes accumulate in ice depressions, usually under a thin ice-cap and waters are dilute and unproductive (ultra-oligotrophic). It has been suggested that meltwater accumulations may affect ice-shelf stability16.
Cryoconite holes are also a common feature across glacier ablation zones, forming where surface sediment accumulations (cryoconite) melt small cylindrical “ponds” (cm2 to m2 diameter, cm deep), frequently colonized by planktonic and benthic microbial life18.
5. Subglacial ecosystems: Widespread under both East and West Antarctic ice sheets.
Over 400 subglacial lakes have been identified19. Lake Vostok is the largest at 14,000 km2 in area with an average water depth of 410 m (6th largest lake on the planet in terms of volume). Subglacial lakes are overlain by 1-4 kilometres of ice and many are hydrologically connected 1,19,20. Their filling and draining has been associated with ice-sheet movements. Some may have been isolated from the planetary surface for millions of years with implications for their life forms and need for protection1.
Conservation and Management
Most Antarctic lakes, ponds and streams are in fragile landscapes sensitive to disturbance and contamination21,22. However, research and tourism in several ice-free areas has intensified dramatically over the past few decades potentially placing these features at higher risk. Some inland waters are already impacted by field station usage and long-term field investigations23 which may leave legacies of disturbance21,22,23.
Climate change will significantly influence Antarctic inland water ecosystems because their existence and properties are critically dependent on melting. Information on human activities near inland waters is needed for: prioritisation of sites for conservation; restoration and special management; identification of habitats at risk of biological invasions; identification of locations suitable for sustainable tourism activities within environmental limits; and vulnerability to climate change. Management of direct human activities on freshwater systems is through (just a few) ASPAs, three ASMAs and SCAR’s Environmental Codes of Conduct for ‘Terrestrial Scientific Field Research in Antarctica’, and ‘Exploration and Research of Subglacial Aquatic Environments’, often with national programme overlays.