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Tuatara: Volume 15, Issue 3, December 1967

Phytoplankton in Antarctic Lakes — a Problem of Survival

page 149

Phytoplankton in Antarctic Lakes — a Problem of Survival

THE CLIMATE of southern Victoria Land, Antarctica, appears to be inimical to plant life: the mean annual temperature is low (—20°C), winds are severe, and conditions of light and moisture are unfavourable. The extremely low atmospheric humidity of the area enables a narrow mountainous strip on the west side of McMurdo Sound to remain more or less ice-free during the summer. Precipitation is very low in this “Dry Valley” area, and the small amount of snow which does fall in the summer ablates rapidly, leaving little moisture available to plants. Abnormally long periods of summer sunlight may be as unfavourable to plants as the six months of polar night.

Despite these harsh environmental conditions, numerous small lakes in the Dry Valley area support a rich vegetation of algae. The Dry Valley lakes generally have a permanent ice cover, although some partly melt during the summer; others remain completely frozen throughout the year.

Early researchers, investigating Antarctic algal collections made by the expeditions of Scott and Shackleton, were astonished at the prevalence of blue-green algae in the samples. Extensive sheets of these algae grow in and around the ponds and lakes scattered throughout the McMurdo Sound region. They consist of a substratum of filamentous blue-green algae, with a large epiphytic flora of other blue-green and green algae, and diatoms.

Apart from the species making up the attached algal sheets, a number of planktonic diatoms and blue-green algae also exist in the lakes. Very little is known about the ecology and physiology of this Antarctic freshwater phytoplankton; however, evidence from recent investigations of lakes in the McMurdo area suggests that the algae may have rather interesting distribution patterns within the lakes. This paper is a preliminary comment on the distribution of algae in two Antarctic lakes, and the associated problem of survival under poor light conditions.

Limnological teams from Victoria University began studying some of the larger Dry Valley lakes in 1961. Their subsequent page 150 results have shown that the lake waters are inversely thermally stratified. The lakes are considered to be natural examples of the trapping and storing of solar energy by a salt water density gradient.

While investigating the chemical and physical characteristics of Lake Vanda (Wright Valley), Wilson and Wellman (1962) accidentally discovered free-floating blue-green algae in a strongly convecting layer some 50 ft below the surface. In 1964, I was able to confirm this discovery by finding several species of blue-green algae and diatoms between 20-100 ft in the same lake. In a study of Lake Miers (Miers Valley), I found a somewhat similar situation where planktonic algae were apparently concentrated in a narrow convecting layer at a depth of 50 ft. Live blue-green algae were also collected from an anaerobic environment on the lake floor (Baker, in press).

In lakes with thermal stratification, the restriction of phytoplankton to certain layers is not unusual. In temperate lakes the plankton is normally concentrated in the epilimnion (above the thermocline), where currents keep the plants within the photic zone. Below the thermocline, algae drop out of suspension into a layer with slow or no circulation, and sink to the lake floor. In ice-covered Lake Miers however, the phytoplankton seems to be separated from the surface by a thermocline — there is no true epilimnion. The physiological problems involved in living in a thermal and chemical gradient would naturally restrict plankton to the convecting layers of uniform composition and temperature below the thermocline. The relationship between water turbulence, cell buoyancy, and photosynthesis may be of secondary importance in this case; the small amount of light reaching the deep convecting layer may not be sufficient for significant photosythesis.

The Antarctic lakes possess thick ice covers (<22 ft), which remain permanently frozen except for a narrow summer melt-zone around the lake edge. The passage of sunlight through the ice is impeded not only by its thickness, but also by snow, layers of sand and pebbles, and the often very uneven ice surface. Preliminary studies of several lakes in Antarctica indicate that only the surface layers receive sufficient light to support photosynthesizing algae. Experiments in temperate latitudes have shown that significant photosynthesis of algae does not occur at levels where the light intensity is less than 1% of the surface insolation.

Lake Vanda, with a relatively smooth and clear 12 ft cover of ice, receives about 6% of the incident surface light directly beneath the ice layer, and the intensity of penetrating light is thereafter halved every 50 ft. Convecting layers are now known to occur near the surface of Lake Vanda (Hoare, 1966); these would enable plants to remain in the photic zone. However, it is doubtful whether algae living deep in Lake Miers, where 18 ft of ice cover absorbs 98-99% page 151 of the light, would be able to carry out appreciable photosynthesis. Whether such plants are adapted to photosynthesize at very low light intensities, or are able to live heterotrophically in darkened conditions, requires further study.

Algae showing an ability for heterotrophic growth under unfavourable summer conditions would have the added advantage of being able to maintain populations in the lakes throughout the dark winter. Plants receiving sufficient light in the summer (as in Lake Vanda), might have a dual ability for photosynthetic and heterotrophic metabolism; in dark conditions they could assimilate dissolved organic substances resulting from the earlier photosynthetic acitivity. Those plants which live a permanently heterotrophic existence in deep water can also utilize external organic metabolites washed down from the littoral region and inflowing glacial streams during the long summer.

Several types of organic substances are known to be effective carbon and energy sources for heterotrophic metabolism. Many substrates, such as sugars, fatty acids, alcohols, and amino acids, are intermediates in the major pathways of metabolism. Heterotrophic plants living in darkness could oxidise part of the substrate and use the energy released to convert the rest into cellular material, thus overcoming the lack of radiant energy.

Some algae may carry on autotrophic metabolism in the dark by oxidizing molecular hydrogen or hydrogen sulphide, instead of water, as in normal photosynthesis. This is an adaptive process dependent on the presence of a latent hydrogenase enzyme, which is activated only under dark, anaerobic conditions, and continues only if the light intensity remains low. Blue-green algae living in a dark, anaerobic environment near the bottom of Lake Miers may metabolize in this manner.

These suggestions are based on incomplete data and must be regarded as preliminary. It is very clear that a detailed investigation of the ecology and physiology of Antarctic lake phytoplankton will be necessary before any sound attempt can be made to elucidate their means of survival under rigorous polar conditions.

Literature Cited

Baker, Alan N., (in press). Algae from Lake Miers, a Solar-Heated Antarctic Lake. New Zealand J. bot.

Hoare, R. A., 1966. Problems of Heat Transfer in Lake Vanda, a Density Stratified Antarctic Lake. Nature, 210 (5038): 787-789.

Wilson, A. T. and H. W. Wellman, 1962. Lake Vanda: an Antarctic Lake. Nature, 196 (4860): 1171-1173.

* Biologist, VUWAE 9