River Sediment Stays Close to Coast

People have become increasingly concerned in recent years that sediment washed down by rivers into the sea might be harming corals on the Great Barrier Reef. However, such fears may be unfounded, according to James Cook University sedimentology lecturer Dr Ken Woolfe.

Not only does the sediment remain close inshore - contrary to popular belief that it settles on and destroys coral reefs - but some species of corals are remarkably resilient to turbid waters, he says. There is no doubt that the sediment load in rivers has increased in recent decades, but it is mainly being deposited in sheltered coastal bays and inlets.

CRC Reef researcher Dr Woolfe, together with a team of James Cook University marine geologists, collected sediment samples - using surface grabs and cores - from hundreds of sites in the Great Barrier Reef lagoon. The scientists found almost no evidence of sediment from rivers accumulating on mid-shelf and outer reefs. Furthermore, by measuring tides, waves and turbidity at inshore sites where river sediment accumulates, they found that any sand or mud is quickly removed from corals by physical processes such as wave action.

After organic and carbonate matter from coral reef sediment samples in mid and outer-lagoon had been removed the scientists found that more than 90 per cent of the remainder was ancient volcanic air-carried ash, most probably from Indonesia and Papua New Guinea. "The amount of mainland sediment carried to these reefs by river plumes is small compared to the background flux of volcanic dust," Dr Woolfe said.

Sediments entering the Great Barrier Reef lagoon generally move north, keep close to the mainland, and accumulate in northern-facing bays. Some in-shore corals adapt to turbid waters and even benefit from organic sediments suspended in the water.

Samples taken from deeper mid-shelf areas, 20-40 metres from the surface, also showed little evidence of accumulated sediment. "In most places the sediment is less than half a metre thick and most of it is carbonate debris," Dr Woolfe explained. "This indicates that processes along the Queensland coast ensure river-borne sediment stays close to the shoreline instead of being carried out to sea."

As south-east trade winds that prevail for nine months of the year generate waves travelling in a north-westerly direction, wave and wind-driven currents run parallel to the coast. "The net result is that riverine sediment entering the Great Barrier Reef lagoon is by and large moved to the north, and keeps close to the mainland. Most sediment is found within a few kilometres of the coast."

Dr Woolfe and his team found that this wedge of riverine sediment extends to about 20 metres in water depth off Townsville, but only to 10 metres further north. Sediments can be kicked up in the water column above this wedge during windy periods, sometimes to more than 10 times the concentration than a river plume following a flood. The entire water column can be turbid as opposed to plumes that are mostly on the surface. However, prevailing wind and sea conditions keep pushing suspended sediment inshore.

The CRC team found that a wedge of sediment extends to about 20 metres in water depth off Townsville, but only to the 10 metre mark in the northern Great Barrier Reef region.

Sediment carried north in this boundary layer gradually settles in large north-facing bays, such as Cleveland Bay offshore Townsville, Trinity Bay offshore Cairns, and Princess Charlotte Bay in the far north. Wave energy in these sheltered bays is much less than along exposed reaches of the coast. Some is deposited on the ocean floor and the rest is carried into mangrove-lined estuaries by tides.

"Mangrove swamps act like giant vacuum cleaners, sucking up the sediment," Dr Woolfe said. "Because wind-driven coastal currents are already carrying as much sediment as the wave regime will permit, a rapid increase in sediment from river systems will not result in an increase in turbidity. The limiting factor in transporting sediment is wave and wind action."

An interesting management implication of research this research is that the bulk of sediment entering the Great Barrier Reef lagoon may come from well south of the Great Barrier Reef itself, and that rivers adjacent to the Great Barrier Reef region contribute a relatively small amount of sediment.

"Bays at the southern end of the Barrier Reef are already full of sediment which may have come from New South Wales rivers. Northern bays still have considerable capacity, and there is not much spilling out from them," Dr Woolfe said.

He estimates it will take thousands of years for northern bays to reach sediment capacity. "The amount of sediment reaching the Great Barrier Reef lagoon is about 30 million tonnes per year, compared with the 100 million tonnes discharged into the Gulf of Papua from the Fly River in Papua New Guinea."

The team has found that corals that do live in the turbid coastal current are remarkably tolerant to high turbidity. "We worked on reefs with turbidities regularly 10 times greater than what most biologists previously believed corals could tolerate," Dr Woolfe said. These observations are supported by another CRC funded project that measured the tolerance levels of certain species of filter-feeding corals found on inshore reefs. PhD marine biology student, Ken Anthony, has found that organic sediment suspended in water can actually contribute to energy needs of these corals for growth and respiration. Inshore populations of some species even appear to adapt their nutritional requirements to higher levels of sediment in surrounding waters.

"It is quite likely that nearly all reefs on the Great Barrier Reef started out life in turbid conditions," Dr Woolfe said. "So long as there is enough light, and the sediment is not accumulating, the corals don't seem to mind."

Although he is convinced that sediment itself poses no danger to the Great Barrier Reef, Dr Woolfe sounds a few cautions. "Just because river plumes are not posing a direct sediment threat to the coral reefs doesn't mean they are not posing a contaminant threat. Contaminants adhere to the smallest particles and remain suspended in plumes. "And a few reefs along the coast are under threat from sediment - eventually they will be physically overwhelmed as the coast moves out. Over the past 6,000 years, the coast has advanced up to 10 kilometres."

Although there is little doubt that sediment loads in rivers in Queensland have increased since human settlement, it remains close inshore and does not generally accumulate on corals, contrary to popular belief. "Although this increased sediment load has greatly modified the rivers themselves, it has had little identifiable effect on the coral reefs," Dr Woolfe said.

By Colleen Davis