Feature

Scientific evidence for the impact of climate change in Antarctica

Prof William Froneman

It is now generally accepted that the increase in atmospheric concentrations of greenhouse gases resulting from human activities, including the burning of fossil fuels, deforestation and agricultural practices, have contributed to the observed rise in global atmospheric and seawater temperatures.

Because the oceans are the largest reservoir of carbon, the marine environment plays an important role in the regulation of the global climate. The role of the marine biosphere in the global climate system has thus over the past three decades, received considerable attention and formed the focus of a number of international studies including the Geosphere-Biosphere Programme (IGBP) and the World Climate Research Programme (WCRP). The main objectives of these programmes are to analyses and quantify the various physical (ocean circulation) and biochemical (biological) processes within the oceans that regulate global climate.

The Southern Ocean broadly comprises the south Indian, Atlantic and Pacific Oceans and accounts for ± 20% of the world's ocean surface area. The large expanse of the Southern Ocean suggests that it plays a key role in the global climate system. The ocean is dominated by one of the largest current systems on the planet, the easterly flowing Antarctic Circumpolar Current (ACC). The ACC is not a uniform water body but comprises a number of distinct bands that are separated from one another by transition zones, termed frontal systems. The frontal systems of the Southern Ocean are generally regions of elevated biological activity (primary and secondary production) and thus represent important foraging grounds for top predators including whales, flying seabirds and seals.

The Polar Frontal Zone (PFZ) is delimited by two of the main frontal systems of the Southern Ocean, the Sub-Antarctic Front (SAF) to the north and the Antarctic Polar Front (APF) to the south. The position of the frontal systems that delimit the PFZ are not fixed but vary according to wind patterns and bottom topography which may cause deviations in the flow direction of the fronts. The zone is characterised by several small oceanic islands that seasonally are home to millions of top predators including flying seabirds, penguins and seals. The high numbers of predators found on the various oceanic islands within the region contribute to their high ecological status. Importantly all predators that are found on the oceanic islands are reliant on the surrounding seas for their food which includes zooplankton, fish and squid.

The Prince Edward Islands are situated some 1600km southeast of Cape Town and lie within the Indian sector of the PFZ. The Prince Edward Islands, comprising Marion and Prince Edward islands were annexed by South Africa in 1948. The two islands are ± 19km apart and are separated from one another by a shallow shelf of < 500m depth. Marion Island is approximately 250km² in extent while Prince Edward Island is 45km² in extent. Like many of the oceanic islands within the PFZ, the Prince Edward Islands are seasonally home to up to 5 million breeding pairs of top predators. The presence of the large numbers of top predators on the islands resulted in the islands being proclaimed a special nature reserve in 1995. As part of the South African National Antarctic Programme (SANAP), members of the Southern Ocean Group, Rhodes University have over the past three decades attempted to explain how the large numbers of predators that are seasonally found on the islands are sustained in a region than is generally considered as unproductive. Two mechanisms have been identified that deliver the large quantities of food necessary to sustain the top predators on the islands. These mechanisms collectively are termed; "The life support systems of the Prince Edward Islands".

The first mechanism involves the interaction of the terrestrial island ecosystem with the surrounding oceans. The large numbers of predators on the islands produce enormous quantities of faecal/biological waste (e.g. plumage) material, which is washed off the islands into the surrounding waters following heavy rainfall on the islands. This in essence fertilises the waters in the immediate vicinity of the islands which promotes the growth of phytoplankton, a phenomenon known as the, " island mass effect". The phytoplankton in turn represents an important food source for the benthic organisms that inhabit the shallow island shelf region between the islands. One of the key components of the benthic community is the predatory swimming prawn, Nauticaris marionis, which represents a key prey item in the diets of several penguins and birds. The top predators that prey on the shrimp are termed the inshore feeders.

The second mechanism involves the advection (transport) of large quantities of prey within the easterly flowing ACC towards the island ecosystem. The top predators on the islands consume the prey when they are washed over the shallow island shelf region that separates the two islands. The alternation of the two mechanisms is largely responsible for delivering the food necessary to sustain the large number of predators that are found on the two islands.

Recent studies conducted in the vicinity of the islands by members of the Southern Ocean Group, Rhodes University have provided the first scientific evidence of the impact of global climate change on the islands ecosystem. Analysis of 25 year data set suggest that the zooplankton community in the vicinity of the islands has shifted from a cold water species dominated system to one where warmer species are most common. The observed shift in the zooplankton community structure is thought to reflect the warming of the waters in the region of the islands as a result of global climate change. The shift in the species composition is also likely to be associated with a decrease in prey availability for the tope predators as the warmer waters are generally considered to be less productive than the colder waters. More recently, feeding studies on the swimming prawn N. marionis, suggest that over the corresponding period, the diet of the shrimp has shifted from mainly benthic prey to pelagic prey. It is thought that the shift in the diet of the shrimp reflects the decline in the availability of benthic prey due to the decreased frequency of occurrence of the island mass effect

While much of the evidence presented is preliminary, it is becoming increasing evident that the marine ecosystem of the Prince Edward Islands is beginning to change in response to global climate change. It is worth noting that there is some evidence to suggest that the populations of a number of the so called inshore feeders that feed on the swimming prawn N. marionis have declined over the past three decades. It is tempting to speculate that the observed decline in the populations can be ascribed to the reduced availability of the shrimp due to the decrease in the frequency of occurrence of the so called island mass effect. There is clearly a need for further studies in the region of the islands to predict the overall outcome of the observed global climate change on the island ecosystem. Clearly global climate change is likely to be associated with a loss of biodiversity and change in ecosystem structure of the islands.

Much research lays ahead if we are to find ways to protect and preserve the biodiversity jewel of Antarctica and the southern Oceans.

More information:

 Images copyright to the Southern Oceans Group

Professor William Froneman is with the southern Oceans Research Group at Rhodes University. More information at: http://www.ru.ac.za/affiliates/sog/ 

South African Agency for Science and Technology Advancement for more on research and careers in Antarctica

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