Signy Island South Orkney Islands

Location Signy Island (60°43' S, 45°36' W) is a 20 km2 island in the South Orkney archipelago. It lies at the confluence of the ice-bound Weddell Sea and the warmer Scotia Sea, and its climate is influenced by the cold and warm air masses from these two areas. The lakes on Signy Island share characteristics of Antarctic and subAntarctic environments, a fact reflected in the diverse flora and fauna. This region of the continent has been ice free for the past 6000 years and is referred to as the maritime Antarctic zone, with an annual mean air temperature near —4 °C. The island is small (7 x 5 km2; surface area 19.9 km2) and has relatively little relief (maximum elevation 279 m). Most of the 17 lakes on Signy Island lie in the valleys and plains of the narrow, coastal lowland, which is usually snow free during the summer. The lakes on Signy Island that have received extensive study include Heywood, Sombre, Amos, and Moss. Radiocarbon dates on basal sediment from the lakes on Signy Island show that these lakes did not exist more than ~12 000 years ago, making them similar in age to many of the continental lakes.

Formation and diversity These lakes share a common geology but cover a wide range of physical, chemical, and biological properties. Sombre lake is an ultraoligotrophic system receiving no significant nutrient input. The inflows consist primarily of snow melt streams which are frozen for 8-9 months in a year. As a result, the water column is relatively clear and most of the primary production comes from ben-thic cyanobacterial mats. Many of the lakes are suffering animal-induced (fur seal) eutrophication with Amos and Heywood lakes being the most severely affected. Unlike Sombre lake, the water column of Amos lake develops a dense phytoplankton bloom during spring and summer in response to elevated nutrient enrichment.

A summary of data averaged over a 6-year period (Table 2) reveals that the pH in these lakes is circum-neutral, whereas the conductivity varies from 40 to 233 mS cm—1, reflecting waters of extremely low to moderate ionic strength. The chlorophyll a levels generally increase with conductivity and represent waters ranging from mesooligotrophic to mesoeutrophic. Levels exceeding 10 mg l—1 chlorophyll a in Heywood lake result from wildlife-induced nutrient loading. Dissolved inorganic nitrogen (DIN; NO—+ NH4) and soluble reactive phosphorus (SRP) concentrations also vary considerably across the lakes on Signy Island, reflecting various degrees of nutrient loading and biological consumption. The ratio of DIN:SRP ranges from 3.6 to 445.3 revealing a wide range of potential

Table 2 Selected chemical and biological properties from 14 lakes on Signy Island

Lake

Depth

pH

Conductivity

Chla

Cl~

NO3-N

NH+-N

SRP

DIN:SRP

(m)

(mS cm1)

(mgr1)

(mgl1)

(mgr1)

(mgr1)

(mgr1)

(g:g)

Amos

4.3

8.12

120

4.11

33.9

520.6

214.4

20.3

3.6

Bothy

2

6.82

233

1.24

44.2

570.7

94.8

4.1

12.2

Changing

5.4

6.82

94

2.43

25.3

146.4

8.5

4.2

44.3

Emerald

15

6.62

67

1.33

23.4

97.3

10.3

2.1

33.6

Heywood

6.4

6.92

134

10.06

42.3

327

56.1

5.9

11.9

Knob

3.5

7.32

62

8.70

18.7

123.3

16.3

3.1

155.1

Light

4.4

6.82

121

9.21

44.1

33.4

11.2

4.7

31.9

Moss

10.4

6.82

40

1.85

23.2

111

5.6

1.4

50.7

Pumphouse

4

6.92

86

3.02

18.4

63.6

7.1

2.5

20.2

Sombre

11.2

6.82

78

3.99

25.8

181.9

12.9

4.6

74.9

Spirogyra

1.5

7.42

60

1.51

19.4

116.2

17.4

4.7

445.3

Tioga

4

7.42

134

4.29

20.9

105.7

53.3

6.1

5.7

Tranquil

8

6.92

52

1.53

16.7

80.7

1.8

2.7

275.0

Twisted

4

6.82

92

2.43

28.7

65.6

2.5

3.4

26.2

Adapted from Jones VJ, Juggins S, and Ellis-Evans C (1993) The relationship between water chemistry and surface sediment diatom assemblages In maritime Antarctic lakes. Antarctic Science 5(4): 339-348. Data averaged from collections during early winter, spring, summer open water between 1985 and 1991.

Adapted from Jones VJ, Juggins S, and Ellis-Evans C (1993) The relationship between water chemistry and surface sediment diatom assemblages In maritime Antarctic lakes. Antarctic Science 5(4): 339-348. Data averaged from collections during early winter, spring, summer open water between 1985 and 1991.

nutrient limitation (a ratio of —7 represents balanced growth of phytoplankton). Marked seasonal variations in the abundance, composition, and productivity of phytoplankton, bacterioplankton and protozoo-plankton within Heywood lake have been shown to be correlated with the seasonality of both physical factors and nutrient levels. The biota within the lakes on Signy Island can be expected to continue changing in response to animal-induced nutrient loading.

In addition to the well documented increases in nutrient loading in many of the lakes on Signy Island, the air temperatures in this region have been rising by —0.25 °C per decade, which has led to increasing lake water temperatures (0.6 °C per decade) and more icefree days each year. The higher air temperatures have caused elevated ice melt in the lake catchments, leading to elevated phosphorus loading from soil leaching. Measurements of dissolved phosphorus and chlorophyll a increased fourfold between 1980 and 1995 in response to climate warming. These data clearly point to the sensitivity of polar lakes to climate change, where small changes in air temperature result in extreme ecological change.

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