Factors influencing the Region’s values
Since the Outlook Report 2009, understanding of the effects of water quality changes on Great Barrier Reef species and habitats has continued to improve.73 Research has strengthened evidence for causal relationships between water quality change and the ecosystem health of corals, seagrasses and mangroves, and for the effects of increased nutrients and sedimentation on the health and resilience of coral reefs.73 Nutrients from land-based run-off are considered one of the greatest threats to the Reef ecosystem.216 Once dissolved inorganic nutrients enter the marine system, they are taken up by phytoplankton, bacteria and seafloor plants such as macroalgae and seagrasses. The addition of excess nutrients, to a certain level, can increase productivity across large areas and if the conditions are right can increase the survival rates of certain species.213,263 This includes a variety of plants and animals in the marine system such as phytoplankton264, macroalgae that compete with corals220,265 and epiphytes that compete with seagrass266. An excess of nutrients can even lead to a change in the trophic status of an area of the marine environment and it is believed that this eutrophication is happening more regularly in the inshore waters of the Great Barrier Reef.213,267,268 Examples of the consequences of imbalances in the nutrient cycle as a result of elevated nutrients include: • extensive, observable phytoplankton blooms in flood discharges and likely shifts in the species composition of phytoplankton213,269 • links to an increase in the frequency of crown-of-thorns starfish outbreaks (see Section 3.6.2) • may contribute to a shift in the balance between macroalgal and coral abundance • may make corals more sensitive to temperature stress270,271 • may facilitate disease outbreaks in coral by increasing the virulence of their pathogens or reducing their immune responses272,273; the nutrient organic carbon contributes to this effect274 • increased growth of phytoplankton, macroalgae and algal epiphytes that lower ambient light levels, thus increasing competition for light and reducing photosynthesis in seagrass275 and corals (particularly in deeper waters). Sediments in land-based run-off have far-reaching effects on the Great Barrier Reef ecosystem. For example: • Heavier erosion sediments infill freshwater stream beds and deep waterholes, with the reduced water depth affecting the distribution, abundance and recruitment of many freshwater species and some marine-related species such as sawfish276.
Elevated sediment loads and resuspension reduce light and smother plants and animals.
Nutrient cycle imbalances are affecting the ecosystem.
• Increases in suspended sediment are significantly altering light regimes — lower light levels reduce primary production in both the water column and on the seafloor.277,278 • Increased amounts of sediments are settling on seafloor organisms such as seagrass and corals, making it harder or impossible for them to grow, survive and reproduce.279,280 This has significant flowon effects to organisms and animals dependent on these habitats. • In some areas increased fine sediments from land-based run-off have resulted in mangrove forests replacing beaches.33 • The suspension and resuspension of sediments is increasing the turbidity of open waters and releasing additional nutrients previously bound up or buried in sediments.269,278,281 Inshore areas, particularly in the southern two-thirds of the Region, frequently exceed the water quality guidelines threshold for suspended sediment concentrations of two milligrams per litre219 (Figure 6.15). This threshold correlates strongly with declines in ecosystem condition such as increased growth of macroalgae.282 Concentrations above 6.6 milligrams per litre have been linked with coral stress283, declines in seagrass cover284, fish habitat changes285, and home range movement286.
Ecosystem effects of long-term exposure to pesticides are not well understood.