Putting the western Indian Ocean on the map.
Suppose you had to navigate with a map that read, "This mountain may lie three kilometres farther east than it is drawn." The world's best nautical charts of the more remote parts of many oceans are still littered with equivalent statements. The sea monsters are gone, but many marine charts still lack some vital information.
The western Indian Ocean is one such mare incognitum. Although traders from Arabia and East Africa sailed the coastal waters for millennia, few ventured into the open ocean.
The first efforts to explore these waters took place almost 100 years ago. The Percy Sladen Trust Expedition, led by J Stanley Gardiner, on a British hydrographic vessel, HMS Sealark. Over a period of seven months it covered a vast area, producing maps and marine charts (many of which are still the best available) and describing the flora and fauna.
In the years that followed, there was little else, except some small-scale scientific work, until the 1990s, when the Royal Geographical Society (with IBG) proposed the Shoals of Capricorn Programme, a comprehensive multidisciplinary research project that aimed to provide information about the region's environment and a plan for its management. In January 2004, the Royal Society hosted a meeting at which some of the more than 200 scientists who took part in the six-year programme presented their findings.
The area of interest stands out quite clearly on marine charts. It is some 90,000 square kilometres of shallow waters, known as the Mascarene Ridge or Plateau, bound by Madagascar to the east and an arc of islands from the Seychelles in the north to Mauritius in the south. Ten thousand years ago, at the end of the ice ages and the dawn of human civilisations, it would have been dry land, partly enclosing the Mascarene Basin as a sea. Today, unconnected to any continental shores, with only a few tiny islands, it is unique among the world's oceans.
One of the first tasks in exploration is the development, or improvement, of maps. Bathymetric charts of the oceans have been created and refined since the ages of European discovery. Back then, the relative simplicity of the available navigational and depth-sounding tools meant that gathering even single data points in the deep ocean was very laborious. Nevertheless, over the centuries, a broad picture of the sea floor gradually developed.
Modern technology has revolutionised this process. Satellite positioning, along with broad-beam and side-scan sonar, produces detailed, textured maps of the sea floor. Satellite altimetry has enabled researchers to map comprehensively the sea floor by measuring differences in the height of the sea's surface. The gravitational pull exerted by variations in the topography of the sea floor affects the height of the water, causing it to be higher, perhaps by just a few tens of centimetres, over an undersea mountain than over a deep trench. When the influence of waves and tides is averaged out, scientists can use these differences to calculate the sea floor's topography.
But there is still a long way to go. Satellite altimetry is low resolution and can't be used to develop fine-scale navigational charts, because it only picks up features of ten kilometres width or more. Developing more accurate charts using sonar systems still requires boats, time and resources. Information has poured in from the shipping lanes, but elsewhere the maps are bare.
The Mascarene Ridge is one such area. But the Shoals programme has precipitated great advances. In 1999 two naval survey vessels, HMS Scott and HMS Beagle, gathered detailed information on the deeper slopes to either side of the ridge. Workers at Southampton University combined their findings with new data from ship tracks and a range of other sources to produce the best ever chart, which was published in 2001.
Unfortunately, the shallowest points remain poorly covered. Over these areas the charts are still peppered with observations that point out potential shallow patches, some perhaps just a few metres across. Once marked on a chart, such areas are given a wide berth by all vessels, for obvious reasons. Although some may simply represent errors or false readings, others may describe fabulous coral-heads, teeming with life, but threatening disaster to any vessel that strikes them.
The Mascarene Ridge, particularly between Ritchie Bank and Cargados Carajos, presents a formidable barrier to the passage of the South Equatorial Current, one of the major water flows in the Indian Ocean. The origins of the current have been traced in the Pacific Ocean and a passage of water known as the Indonesian Throughflow, but little is known about its passage over, and around, the shallow banks of the Mascarene Ridge.
In May 2001, the research yacht SRV Zuza joined the Shoals programme to undertake a research cruise from the Seychelles to Mauritius and back. By examining plankton in the surface waters, Dr Chris Gallienne of Plymouth Marine Laboratory provided the first evidence of the ridge's influence on ocean productivity.
Gallienne found that the densities of zooplankton increased tenfold along the western edges of the Mascarene Ridge and that this pattern continued far out into the Mascarene Basin, particularly around the Nazareth Channel. This indicated that the passage of water may be producing upwellings, drawing deeper oceanic water up to the sea surface. Such upwellings bring with them a flux of nutrients that, in many parts of the world, can be critically important in sustaining marine life. From a human perspective, such areas convert, quite directly, into places of high productivity and fish catches.
The term 'biological pump' describes the removal of carbon dioxide from the ocean/atmosphere system by organisms that die and sink into the ocean sediments. A number of studies have begun to look at how areas of high biological productivity, such as the western Indian Ocean, influence global climates in this way. These need to be included in global climate models, as they may be playing an important role in removing at least some atmospheric C[O.sub.2] and slowing the rate of climate change.
One Shoals study, led by Denise Smith-Wright of the Southampton Oceanography Centre, looked at how plankton are helping to reduce global warming. Smith-Wright found that certain microplankton release a rare compound, dimethyl sulphide, when they die. As this chemical enters the atmosphere, it attracts water, acting as what is known as a cloud-concentration nucleus. The resultant clouds reflect sunlight and thus reduce heating of the Earth's surface.
Another type of plankton may be having the opposite effect. Research in the western Indian Ocean suggests that the smallest living organisms in the plankton, the so-called picoplankton, produce relatively high levels of halogens in this region. Halogens are powerful greenhouse gases, and they also attack and remove ozone from the upper atmosphere in much the same way as chlorofluorocarbons.
Despite the broad coverage of the studies, our knowledge of the shallowest waters of the Mascarene Ridge remains scant. These vast areas above the 100-metre contour have barely been seen by scientists. As part of the Shoals programme, the research vessel Searcher surveyed the Providence Cerf Bank in 1998, and several expeditions made brief visits to the Saya de Malha Bank.
All of these studies found that wide, dense fields of seagrass were covering large parts of the banks. Seagrasses are the only higher plants to have moved completely from land back into the ocean (mangroves are their coastal companions, but keep their leaves firmly in the air). They are highly productive perhaps 1,000 times as productive as phytoplankton communities per unit area--and all around them are rich, diverse fish communities. Green turtles, which feed on seagrasses, appear to be widespread, and whales and dolphins have been seen.
Corals are also found in places on these banks. None of these have been mapped, but it seems possible that there could be some important coral reefs in a few places. Unknown and unmapped, such reefs could nevertheless be an important refuge for corals and fish, particularly as various phenomena--ranging from pollution and overfishing to climate change--already have a negative impact upon many coral reefs right across the Indian Ocean.
The surveys to date have provided tantalising glimpses of the western Indian Ocean. We're beginning to get a picture of an important, productive and fascinating region. It plays a critical role in fisheries and food production for several countries, and may influence productivity on an oceanic scale and even global climate systems.
Written in stone: a geological history
The Seychelles Bank, and its high granite islands, is a tiny deposit of continental rock left behind when India began its great passage northwards, leaving Africa and Madagascar behind. Officially called a microcontinent, this shallow bank has been isolated from other continents for 50-60 million years. The great granite rocks on the white sand beaches of the Seychelles stand testament to the islands' continental origin, while in the cool shadows of the Vallee du Mai World Heritage site, this geological antiquity Is reflected in the island flora. The forest here is composed entirely of palm trees, unique relicts from an earlier era that were left shipwrecked by the shifting continents.
At the southern tip of the Mascarene Ridge volcanic activity has shaped the islands and seascape. Reunion--the home of one of the world's most active volcanoes--has grown over a volcanic hotspot beneath the Earth's crust, but as this crust has moved, so it has left other islands in its wake. Mauritius is the most recent, and beyond this, the remote island of Rodrigues. The differing age of each of these islands is reflected in the coral reefs that fringe their margins. While Reunion has a rich marine fauna, its corals are only just beginning to build small reefs. On Mauritius, the corals have been growing for millennia and encircle much of the island with fringing reefs and even a small barrier reef. Rodrigues is slowly sinking and on its way to becoming a coral atoll. A prolific growth of corals has produced a huge lagoon stretching far offshore around the coast.
Anatomy of an expedition
As well as establishing long-term field stations in the Seychelles and Mauritius, the Shoals programme supported a series of boat-based expeditions that explored the remote parts of the Mascarene Ridge and Basin. One of the first of these, in 1998, travelled to the southern Seychelles.
Led by the author and Tom Spencer of Cambridge University, the team was made up of four principal researchers, three research assistants, a photographer, the ship's captain and three crew. Our equipment included inflatable boats, diving equipment, fuel and food to last more than 40 days, all crammed into our 18-metre research vessel.
The expedition covered some 3,000 kilometres, from the Seychelles Bank to Aldabra and back. It documented corals, fish and island morphologies in places such as Alphonse Atoll, the Providence-Cerf Bank and St Pierre. We were among the few witnesses to the heavy losses of coral that year, caused by unprecedented warming in the Indian Ocean. Our observations of this mass bleaching event have contributed significantly to the understanding of this phenomenon and of the threat posed by climate change, not only to coral reefs, but to the communities and nations that depend on them.
In 1999, some of the team returned to observe the rate of change in the same locations. Using baseline data from the first expedition, they saw that some of the corals were beginning to recover, but also noted the extent of the destruction and its influence on the fish communities of the region.
A legacy of hope
Scientific research is only the beginning of facing environmental challenges. Creating practical solutions needs political and public interest, education and enthusiasm. On the small island of Rodrigues, an NGO, Shoals Rodrigues (SR), has been tackling environmental issues in a variety of ways.
SR provides fishermen with training and education in order to reduce the negative impacts of overfishing and damage to coral caused by reef walks and anchors. And with a rise in tourism, it is training hotel staff to act as snorkelling guides.
Just as important, SR has also engaged the wider public, particularly children. It has provided all of the island's schools with educational material that addresses the marine environment. And on Saturdays, an oversubscribed Club Met gives children a hands-on understanding of marine life and lessons in swimming and snorkelling. These children are among the island's most passionate advocates of marine conservation and share their enthusiasm with their families and friends.
The populations of small islands such as Rodrigues depend on the sea for food and the reefs for protection from storms. And as tourism takes off, healthy reefs will be a valuable attraction, all the more so, as marine resources in other countries become increasingly degraded. Thanks to SR, the positive and exciting messages of science and conservation are already paying dividends. Today, the Rodriguans are working together to establish fishing reserves, with the prospect of improved catches in the years ahead as fish stocks recover.
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|Date:||Jul 1, 2004|
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