Coral reef history books

Diver swimming over a massive Porites coral.
Massive Porites corals, such as this specimen in the northern Great Barrier Reef, are an important archive of environmental information. (Photo: Eric Matson)
A positive print of an X-ray of a coral slice showing the annual density banding pattern.A slice of coral under ultraviolet light showing annual luminescent banding which records freshwater river flood events.

Massive corals, growing several metres in size, contain an archive of information on environmental change, providing valuable reference material for monitoring the progression of ocean acidification and its effects on marine life.

Tropical coral reefs are the most charismatic of marine ecosystems and uniquely defined by both their diverse biological components and the immense physical structures they create. At their heart is a special relationship between coral animals and single-celled algae (zooxanthellae). This mutual dependence allows the corals to build their calcium carbonate skeletons (calcify) at a rate that exceeds the natural forces of erosion, creating a coral reef. These calcium carbonate structures, cemented together by coralline algae, provide complex habitats that support a vast array of flora and fauna. The backbone of Australia's Great Barrier Reef, for example, consists of more than 360 species of hard (calcifying) corals. The reef is home to over 1500 species of fish, 4000 species of molluscs, 800 species of starfish and sea urchins, 400 species of sponges…the list goes on. It is also the breeding ground for humpback whales from Antarctica.

Alarm bells have been ringing about the health of the world's coral reefs for several decades. Many reefs in Southeast Asia and the Caribbean are now seriously degraded due to local over-exploitation and pollution. Coral reefs, including even the least disturbed and best-protected Great Barrier Reef, are now facing a troubled future as a result of rising ocean temperatures and progressive ocean acidification – both due to human activity.

Coral bleaching, due to the loss of the coral's zooxanthellae in unusually warmer water, can result in death of the coral and thus compromise the whole array of organisms that rely on a healthy reef. The frequency of bleaching events affecting large areas of reef (including the Great Barrier Reef in 1998, 2002 and 2006) has increased in recent decades and is clearly linked to global warming.

Ocean acidification is a more insidious threat that strikes at the structural heart of coral reefs. Calcifying marine organisms extract calcium and carbonate ions from seawater to make their skeletons and shells. As the oceans absorb our extra carbon dioxide, the pH decreases and there are fewer carbonate ions available. This increasingly compromises the ability of corals, and the important reef 'cements' (coralline algae) to produce calcium carbonate. This 'osteoporosis' will make reef structures weaker and more susceptible to the natural forces of erosion and more intense tropical cyclones (another projected consequence of global warming).

However, detecting changes requires observations. Early European descriptions of the Great Barrier Reef date to the late 18th century (Sir Joseph Banks, 1770), with more detailed scientific observations in the late 19th century (William Saville Kent The Great Barrier Reef, 1893) and early 20th century (The Great Barrier Reef Expedition, 1928-29). However, it was not until the invention of the aqua-lung in the 1940s that more systematic scientific observations of these tropical wonderlands could be made. New species are still being found today!

Fortunately, coral reefs contain their own history books. Some massive coral species grow to be several metres high (Fig. 2) and contain annual bands similar to tree rings. Growing at 1-2 cm per year, such corals can record several hundred years of growth and the changes that have occurred in their environment.

To 'read' these changes, cores are removed from the centre of the colony (the plugged hole is rapidly overgrown by the surrounding coral tissue). X-ray analysis of slices of the core reveals the annual banding in the density of the calcium carbonate skeleton and each year can be dated from the outer edge – the year the coral was collected. The coral skeleton also records various geochemical tracers of the water (such as delta18O – the stable oxygen isotope which records water temperature and salinity) and, from nearshore corals of the Great Barrier Reef, records of river flood events, when sea water was diluted by fresh.

The world's largest collection of long coral cores at the Australian Institute of Marine Science provides a longer historical context to assess current changes in the reef environment. They have already told us that water temperatures are at their warmest for at least the past several centuries; that European settlement and land clearing in Australia in the late 19th century increased the amount of sediment entering the reef lagoon; and that although the average amount of freshwater entering reef waters has not changed, the wet years are becoming wetter and the dry years becoming drier.

The signatures of coral bleaching events (such as in 1998) are extremely rare in past centuries. Measurements of the amount of calcium carbonate laid down each year by the coral (using gamma densitometry) show that up to the 1980s, calcification rates increased in line with warming water temperatures. However, disturbing evidence is emerging that coral calcification rates now seem to be declining, despite continued warming waters. Is ocean acidification the culprit? We cannot be one hundred per cent certain yet (partly due to lack of long-term observations of ocean chemistry) but something is happening and, combined with the other effects of a changing climate, the future for the Great Barrier Reef is at risk. This well-protected reef will not disappear, but the mix of corals and associated organisms that make up this unique and diverse marine ecosystem will change.

JANICE LOUGH

Australian Institute of Marine Science