Increased sedimentation and the flow of nutrients and pesticides into the ecosystem affect inshore areas, causing higher algal growth, build-up of pollutants in sediments and marine species, and reduced light and smothered corals.

The coastal zones, especially areas close to river mouths, are most exposed to increased sediments, nutrients and pesticides.

Floods

Floods in the Great Barrier Reef catchment increase the levels of sediments, nutrients and pesticides in river plumes reaching mid-shelf reefs. Concentrations of dissolved inorganic nitrogen, dissolved inorganic phosphorous and suspended sediment are all many times higher during floods than other times.

Nutrients

Under natural conditions, the rivers flowing into the Great Barrier Reef provide nutrients to the marine ecosystem. However, more nutrients now enter the Great Barrier Reef Region than before agricultural development.

Dissolved inorganic nitrogen and phosphorous continue to enter the Great Barrier Reef at greater levels, two to five times for nitrogen and four to ten times for phosphorous, compared to European settlement.

Sediments

The amount of sediments flowing into the waters of the Great Barrier Reef is estimated to have increased four to eight times since European settlement.

Most sediments come from catchments that have large grazing areas because of soil erosion caused by forest clearing and reduced ground cover.

Soil erosion in cane farming areas has reduced since green harvesting and trash blanketing replaced burnt cane harvesting.

Pesticides

Pesticides (including herbicides, insecticides and fungicides) are widely found in the waters and animals of the Great Barrier Reef and in waters in its catchments. This is worrying because they are slow to break down and can accumulate in marine plants and animals.

Mining

There have been many small-scale mining operations across much of the Great Barrier Reef catchment. Water from mining voids and underground passageways can have raised levels of heavy metals, arsenic, salts, acid-leachate and sediments.

Water barriers

Dams, weirs and drainage in most catchments have altered freshwater flows to the Great Barrier Reef. These structures can alter the timing and extent of major flows and restrict links between marine and freshwater habitats.

Water released from dams and weirs has a different temperature, chemistry and level of sediments from that of natural river flows.