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= Ace Lake – Antarctica =

''Ace Lake is located in the Vestfold Hills. For more information, see Vestfold Hills.''

Ace Lake is located in the Vestfold Hills in East Antarctica. It is a shallow, saline meromictic lake that is permanently stratified, and possesses strong chemical and physical gradients. The post-glacial lake is mostly concealed with ice for around 11 months of the year, with a maximum depth of 25 metres. Ace Lake has been studied since the 1970’s, due to its short distance from a permanently manned research facility, Davis Station. This has allowed year-long investigations, as well as investigations that are summer only. The combination of the findings from these studies has established a significant database on Ace Lake’s limnology. Similar to other continental meromictic lakes, Ace Lake exhibits a condensed food web, subjugated by eukaryote and prokaryote micro-organisms.

History
Ace Lake (68°28’S, 78°11’E) is positioned in Princess Elizabeth Land in the Vestfold Hills on the Eastern side of Antarctica. This area is an ice-free, low-lying coastal region roughly 400km2, containing around 300 lakes consisting of freshwater, hyposaline, saline, through to hypersaline, with each being formed in basins created by glaciers. There have been limnological investigations conducted at Ace Lake ever since the 1970’s.

The lakes paleolimnology has been well documented. Studies on animal fossils, diatom, radio carbon dating as well as sulphur chemistry have developed an accurate picture of its evolution. Originally, Ace Lake was a marine inlet that was affected by the mixture of meltwater and ocean inputs. During this time, the lake inhabited marine plankton, marine organisms and sea-ice diatoms as well as protist remains deposited in sediment. As the sheet of ice receded, Ace Lake became isolated as a result of isostatic uplift, flushing it with meltwater from the melting ice sheet. The lake became meromictic over the course of 800 years, supporting a freshwater diatom assembly. Approximately 6700 years ago, diatom assemblages coinciding with the maximum Antarctic sea level indicate that seawater flooded into Ace Lake over the sill, disturbing the meromixis of freshwater. During this period, the sediments were laminated and contained elemental sulphur suggesting that the extent and energy of the marine input was limited. This marine input stopped about 5500 years ago, and the lake again became a meromictic basin that stabilized over 1700 years to become the lake it is today and has been with little change for about 4000 years.

Since the level of meltwater inputs has reduced, water level has also reduced due to evaporation, resulting in the physiochemical status currently observed. Now, the Lake is saline and meromictic, situated less than 200m from the closest marine inlet, Long Fjord, divided by a ledge only 2 metres above current lake level.

Paleolimnology studies the sediment of Ace Lake and fossils have revealed a diverse collection of organisms in the early stages of the Lake’s evolution. This implies that these organisms were a part of a widespread Antarctic flora and fauna population that existed prior to the last glacier in the area, surviving in glacial refugia, or that intercontinental dispersal had occurred.

Chemical and Physical Environment
Ace Lake consists of an area between 0.13-0.18km2 with a maximum depth between 23 and 25 metres. Snow melt fills the lake, which differs contingent upon annual rainfall amounts. The Lakes level of salinity increases with depth, with haloclines appearing in the water column. Similar to most lakes that are meromictic, salinity is the main factor maintaining the lakes stratification.

The mixolimnion, the upper water column, possesses a level of salinity roughly ranging from 11.2–20.9 psu (Practical Salinity Unit). Over summer, meltwater inflows and the melting process of the ice sheet that covers the lake dilute the surface waters of the mixolimnion generating a layer of water with low salinity. A major halocline and accompanying thermocline exist at approximately 8 metres, with the upper monimolimnion waters becoming more saline below this at further depths. Methane exists at depths greater than 11 metres in the monimolimnion, with the temperature of the bottom water constant between 1°C and 2°C.

The slow rate of methanogenesis suggests that Ace Lake's methanogens may operate well below their optimum temperature, although definitive statements about the presence of psychrophilic methanogens in the Antarctic lake must await attempts at isolation or longer field studies using alternative methodologies.

Community Structure
There are four main biological groups supported by Ace Lake. A microbial community exists in the anoxic monimolimnion. A planktonic community in the oxygenated euphotic zone of the mixolimnion. Within the littoral zone, a benthic algal mat community exists, and below the chemocline there’s a benthic community in the sediment. The chemocline’s upper zone separating the monimolimnion and mixolimnion is an area of higher plankton activity and a deep chlorophyll maximum. In this zone, oxygen diminishes and nutrient concentrations are reasonably high resulting from upwards diffusion from the monimolimnion.

Functional Dynamics of Ace Lake
Although the environment surrounding Ace Lake is harsh in nature, studies have revealed a sustainable community that subsists throughout the winter. Mixotrophic PNAN and dinoflagellates, as well as HNAN, are active year-round. However, winter reduces the biomass of most of these groups. Bacterial production in Ace Lake is sustained throughout winter and primary production continues despite low light levels in autumn and spring. A M.rubrum population, a phototrophic ciliate, remains active through winter, although a portion of the population encysts, prevailing in spring to augment the continual growing population. Copepod P. Antarctica, the lake’s single planktonic zooplankter, also exists in winter and doesn’t seem to have a resting phase or use eggs to survive winter. Because summer is short, its essential for actively growing populations to implement a survival strategy to be prominent as summer arrives. There’s inadequate time to grow numbers and biomass from cysts or resting stages. Ice is a major controlling influence on Ace lakes function and dynamics, and what happens in winter influences what occurs in the following spring and summer.