Marine and coastal ecosystem impacts
The decline of marine water quality associated with terrestrial runoff from the adjacent catchments is a major cause of the current poor state of many of the key marine ecosystems of the Great Barrier Reef.
Great Barrier Reef marine ecosystems and their associated catchments are part of a dynamic, interconnected system. Activities within the catchments affect the condition of coral reefs and seagrass meadows, which have both declined severely in the period since 2008. Marine water quality continues to be negatively affected by the discharge of excess nutrients, fine sediments and pesticides from the adjacent catchments, and poor marine water quality is a major cause for the poor state of many of the key marine ecosystems (coral reefs, seagrass meadows, coastal wetlands and estuaries) of the Great Barrier Reef.
Summary of evidence
- Great Barrier Reef-wide coral cover has declined by approximately 50 per cent since 1985, while coral cover on inshore reefs has declined by 34 per cent since 2005. Coral cover in the northern Great Barrier Reef has remained stable. Causes of coral loss vary from reef to reef, depending on exposure to tropical cyclones, outbreaks of crown-of-thorns starfish or coral disease, elevated temperatures causing coral bleaching and exposure to flood plumes.
- Evidence of the link between poor water quality, specifically nutrients, and crown-of-thorns starfish outbreaks has been greatly strengthened.
- Inshore seagrass meadows along the developed Great Barrier Reef coast (i.e. south of Cooktown) have declined over the past three to five years and are in poor condition.
- Suspended sediment discharges, especially after extreme weather events, negatively affect turbidity in inshore waters, reduce the light required by corals and seagrass meadows and increase the sedimentation of fine particles and organic rich flocs (muddy marine snow) that can smother marine organisms.
- Poor water quality, especially elevated concentrations of and different ratios of nutrients and high turbidity, has been shown to increase the likelihood of bleaching in corals.
- There is evidence of increases in seagrass leaf tissue nitrogen concentrations since 2005. Epiphyte loads that reduce light availability and impair seagrass growth have increased, possibly as a consequence of increased nutrient supply.
- Pesticides pose a low to moderate risk to inshore coral reefs at current levels, but the consequences of long term exposure at concentrations below those known to affect coral is not understood.
- Many coastal and inshore seagrass meadows of the Great Barrier Reef are exposed to herbicide concentrations that adversely affect seagrass productivity. The contribution of herbicides to recent widespread seagrass losses is unknown.
- The interactions of poor water quality with other pressures such as climate change are largely unknown, but could increase the risk to Great Barrier Reef ecosystems.
- Significant new mangrove stands and landward range expansions in some areas of the Great Barrier Reef are correlated with increased sedimentation due to human activity. However, excessive sedimentation can reduce tree growth, bury seedlings and cause mortality. Increased productivity and growth in response to high nitrogen availability is offset by the increased probability of canopy loss and mortality during periods of drought or storm activity along gradients of increasing salinity. Remaining coastal wetlands are subject to sediment, nutrients and pesticides inputs from rainfall runoff and irrigation tailwater. These inputs and physical modifications to the wetlands contribute to loss of biodiversity and affect wetland structure and function, for example by facilitating weed growth, loss of connectivity between habitats, reduced oxygen levels and flow rate.