Environmental Threats To The Great Barrier Reef - Water Quality

Water Quality

Water quality was first identified as a threat to the Great Barrier Reef in 1989. Thirty "major rivers" and hundreds of small streams comprise the Great Barrier Reef catchment area, which covers 423,000 square kilometres (163,000 sq mi) of land. Queensland has several major urban centres on the coast including Cairns, Townsville, Mackay, Rockhampton and the industrial city of Gladstone. Dredging in the Port of Gladstone is raising concern after dead and diseased fish where found in the harbour. Cairns and Townsville are the largest of the coastal cities, with populations of approximately 150,000 each.

There are many major water quality variables affecting coral reef health including water temperature, salinity, nutrients, suspended sediment concentrations, and pesticides. The species in the Great Barrier Reef area are adapted to tolerable variations in water quality however when critical thresholds are exceeded they may be adversely impacted. River discharges are the single biggest source of nutrients, providing significant pollution of the Reef during tropical flood events with over 90% of this pollution being sourced from farms.

Due to the range of human uses made of the water catchment area adjacent to the Great Barrier Reef, some 700 of the 3000 reefs are within a risk zone where water quality has declined owing to the naturally acidic sediment and chemical runoff from farming, and to coastal development and the loss of coastal wetlands which are a natural filter. Industries in the water catchment area are cotton growing, comprising approximately 262 km²; 340 dairy farms with an average area of 2 km² each, 158 km² cattle grazing, 288 km² horticulture including banana growing, sugarcane farming, and cropping of approximately 8,000 km² wheat, 1,200 km² barley, and 6,000 to 7000 km² sorghum and maize. Fertiliser use in the cotton, dairy, beef, horticulture and sugar industries is essential to ensure productivity and profitability. However, fertiliser and byproducts from sugar cane harvesting methods form a component of surface runoff into the Great Barrier Reef lagoon. Principal agricultural activity is sugar cane farming in the wet tropics and cattle grazing in the dry tropics regions. Both are considered significant factors affecting water quality. Copper, a common industrial pollutant in the waters of the Great Barrier Reef, has been shown to interfere with the development of coral polyps. Flood plumes are flooding events associated with higher levels of nitrogen and phosphorus. In February 2007, due to a monsoonal climate system, plumes of sediment runoff have been observed reaching to the outmost regions of the reef.

Runoff is especially concerning in the region south of Cairns, as it receives over 3000 mm of rain per year and the reefs are less than 30 kilometres (19 mi) away from the coastline. Farm run off is polluted as a result of overgrazing and excessive fertiliser and pesticide use. Mud pollution has increased by 800% and inorganic nitrogen pollution by 3,000% since the introduction of European farming practices on the Australian landscape. This pollution has been linked to a range of very significant risks to the reef system, including intensified outbreaks of the coral-eating Crown of Thorns Starfish which contributed to a loss of 66% of live coral cover on sampled reefs in 2000.

It is thought that the mechanism behind excess nutrients affecting the reefs is due to increased light and oxygen competition from algae, but unless herbivory is unusually low, this will not create a phase shift from the Great Barrier Reef being primarily made up of coral to being primarily made up of algae.

It has been suggested that poor water quality due to excess nutrients encourages the spread of infectious diseases among corals. In general, the Great Barrier Reef is considered to have low incidences of coral diseases. Skeletal Eroding Band, a disease of bony corals caused by the protozoan Halofolliculina corallasia, affects 31 species of corals from six families on the reef. The long-term monitoring program has found an increase in incidences of coral disease in the period 1999-2002, although they dispute the claim that on the Great Barrier Reef, coral diseases are caused by anthropogenic pollution.

Elevated nutrient concentrations result in a range of impacts on coral communities and under extreme conditions can result in a collapse. It also affects coral by promoting phytoplankton growth which increases the number of filter feeding organisms that compete for space. Excessive inputs of sediment from land to coral can lead to reef destruction through burial, disruption of recruitment success or deleterious community shifts. Sediments affect coral by smothering them when particles settle out, reducing light availability and potentially reducing photosynthesis and growth. Coral reefs exist in seawater salinities from 25 to 42%. Salinity impacts to corals are increased by other flood-related stresses.

The Australian and Queensland Governments have committed to act to protect the reef, and water quality monitoring programmes are in place. However, the World Wildlife Fund has criticised that progress against these commitments has been slow, saying that as many as 700 reefs are at risk from sediment runoff.