page_title: Environmental threats to the Great Barrier Reef
section_title: Water quality
hierarchical_section_title: Environmental threats to the Great Barrier Reef / Water quality
caption_attribution_description: Trichodesmium bloom off Great Barrier Reef
context_page_description: The Great Barrier Reef is the world's largest reef systems, stretching along the East coast of Australia from the northern tip down to the town of Bundaberg, is composed of roughly 2,900 individual reefs and 940 islands and cays that stretch for 2,300 kilometres and cover an area of approximately 344,400 square kilometres. The reef is located in the Coral Sea, off the coast of Queensland in northeast Australia. A large part of the reef is protected by the Great Barrier Reef Marine Park.
  According to the 2014 report of the Australian Government's Great Barrier Reef Marine Park Authority, says that climate change is the most significant environmental threat to the Great Barrier Reef, while the other major environmental pressures are listed as decreased water quality from land-based runoff, impacts from coastal development and some persistent impacts from fishing activities. The reef is also threatened by storms, coral bleaching and ocean acidification. The 2014 report also shows that, while numerous marine life species have recovered after previous declines, the strength of the dugong population is continuing to decline.
context_section_description: 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 centers 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 harbor. 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. When the 2019 Townsville flood waters reached the Great Barrier Reef, the flood plumes covered a large area of corals, even reaching 60 km out to sea.
  As of 1995, water visibility had decreased to 10 metres.
  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 due to the naturally acidic sediment and chemical runoff from farming. Coastal development and the loss of coastal wetlands—the latter acts as natural filter—are also major factors From mid 2012 to mid 2016, 596,000 hectares of forest in the catchment zone was cleared.
  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.
  The principal agricultural activity in the wet tropics is sugar cane farming, while cattle grazing is the primary industry in the dry tropics regions. Both are considered significant threats to high 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 out most 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 fertilizer 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 th
caption: Trichodesmium bloom off the Great Barrier Reef
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