Professor William Froneman of the Department of Zoology and Entomology presented his inaugural lecture at Eden Grove recently entitled Biological Oceanography at the Prince Edward Islands: a Review of Achievement. Professor Froneman took the opportunity to give an overview of his work with the Southern Ocean Group. The group is largely concerned with examining the spatial and temporal variability in the marine food web of the Southern Ocean and its response to global climate change.
The Prince Edward Islands are located some 2500km southeast of southern Africa. Like many of the oceanic islands within the Southern Ocean, the islands are seasonally home to enormous populations of air breathing top predators including flying seabirds, penguins and seals.
The accumulation of these top predators on these oceanic islands, estimated to be in the vicinity of 450 million individuals, reflects the virtual absence of any significant land masses within the region. The Prince Edward Islands are seasonally home to some 44% of the world’s population of wandering albatross and are therefore, of high conservation status.
Over the past three decades, research conducted as part of the South African National Antarctic Programme (SANAP) has attempted to ascertain how the islands can seasonally sustain the large populations of top predators on the islands in a region which is largely considered to be unproductive.
Results of several research cruises indicate that the food necessary to sustain the large numbers of predators on the islands is derived from both the offshore (away from the islands) and inshore region of the islands.
The inshore component results from the complex interaction between the oceanographic environment and the islands’ ecosystem. The top predators produce enormous quantities of organic waste which when washed off the islands serve to fertilise the waters surrounding the islands. This contributes to rapid growth of phytoplankton in the immediate vicinity of the islands, a phenomenon known as the, “Island mass effect”.
Much of the phytoplankton generated by the island mass effect sinks and provides an important food source for the benthic organisms living on the island shelf which represent an important food source for the swimming shrimp, Nauticaris marionis. The shrimp represents a significant food source for a number of the top predators found on the islands.
The offshore food delivery mechanism involves the transport of food towards the islands by the prevailing easterly flowing current, the Antarctic Circumpolar Current. The food advected towards the islands is concentrated in the shallow shelf waters surrounding the islands and is therefore accessible to the top predators on during the daytime.
The relative importance of the inshore and offshore component of the food delivery mechanism is determined by the prevailing oceanographic conditions in the vicinity of the Prince Edward Islands. Collectively, these two food delivery mechanisms are termed, “The Life support system of the Prince Edward Islands”.
More recently, studies conducted by postgraduate students within the Southern Ocean Group have highlighted the importance of the extensive kelp beds in the vicinity of the islands as a food source for the benthic organisms in the shallow shelf waters of the islands.
Global climate change and Southern Ocean ecosystems form a great part of the research of the Southern Ocean Group. A 2002 study demonstrated that the subsurface waters of the Southern Ocean have warmed in response to global warming.
The warming of the waters has been associated with a range expansion of warm water species southwards towards the Antarctic continent with a concurrent change in the zooplankton species composition within the region. In the region of the Prince Edward Islands, the warming of the waters has contributed to a decline in the frequency of occurrence of the so called, ‘Island Mass effect”.
This has been associated with a change in the inshore ecosystem, with the benthic organism feeding mainly on food transported to the islands by the prevailing current rather than the island mass effect. This change has resulted in the decline in the populations of those top predators that feed extensively on the swimming shrimp.
By Jeannie McKeown
Photo by Adrian Frost