Run-off of sediment and nutrients to the Great Barrier Reef has increased several-fold as a result of past and current land-use practices. There is significant concern that coastal ecosystems in the Great Barrier Reef World Heritage Area (GBRWHA) are being adversely affected as a consequence of this increase.
Supply of Pollutants
Most pollutants from the land are delivered to the GBRWHA during major flood events. Episodic high inputs of particulate and dissolved matter during flood events are an important and natural part of the ecology of the Great Barrier Reef and the associated continental shelf and estuarine environments.
Flood plumes are generally constrained to within 20km of the coast by prevailing south-east winds. Under unusually calm or northerly wind conditions, plumes can travel further offshore but do not persist. Because of the behaviour of flood plumes and the maximum depth of sediment re-suspension by non-cyclonic waves, significant
adverse effects of land-based inputs on the GBRWHA are likely to be restricted to nearshore areas – broadly within 20km of the coast and in waters less than 20m deep.
Run-off from the land delivers nutrients, sediment and various chemical pollutants to the GBRWHA. Because of documented impacts on reefs elsewhere in the world, most attention has focused on the potential impacts of enhanced sediment and nutrient delivery to the GBRWHA.
The most recent estimates of change in sediment run-off from the land since 1800 is a 3 to 4-fold increase. Many biologically active and toxic trace elements in agrochemical products bind to soil and sediments. Nearly half of the nitrogen (40 per cent) and most of the phosphorus (80 per cent) delivered to the GBRWHA is associated
with fine sediment carried by run-off. Most of this sediment is deposited within 10km of the coast in northward facing bays (e.g. Bowling Green Bay, Trinity Bay, Princess Charlotte Bay).
The significance of the increased sediment delivery from the rivers is subject to active debate. While delivery has increased, sedimentologists argue that the increased sediment supply will not increase sediment accumulation or turbidity at most coastal coral reefs, because these factors are not currently limited by sediment supply.
Turbidity in nearshore areas is primarily caused by wind-driven re-suspension of bottom sediment. Most of this sediment has accumulated over the past 5-6000 years, during which the sea has remained close to its current level.
Thriving coral reefs with high coral cover, and in some cases high diversity, occur in episodically turbid nearshore waters of the GBR. Deposition of sediments near river mouths may, however, threaten seagrasses and there are anecdotal, but unconfirmed accounts of coastal coral reefs in the Wet Tropics being buried by sediment. It’s
not clear whether this happened to active reefs or reefs where corals had already died as a result of other causes.
Significant increases in sedimentation on nearshore coral reefs would be likely to cause changes in community structure and create less favourable habitats for hard corals, zooxanthellate soft corals and calcareous coralline algae. Loss of reef structure to sediment infill may lead to a reduction in numbers of herbivorous fish and a
subsequent increase in macroalgae.
Most of the nutrients (principally N and P) required by the pelagic and reef communities of the GBR are derived from recycled biological material. Terrestrial run-off is the largest external source of nutrients to inshore regions of the GBRWHA. The most recent estimates of nutrient runoff places average annual exports of phosphorous
and nitrogen at 6 to 10-fold and a 2-fold above 1800 levels, respectively. The extent to which this nutrient run-off has increased concentrations of nutrients in the marine environment, and the nearshore zone in particular, is uncertain.
The growth of phytoplankton and seagrasses in GBR waters appears to generally be constrained by the availability of nitrogen, rather than by phosphorus or silicate. The nitrogen most immediately available to plants is dissolved inorganic nitrogen (DIN), primarily ammonia and nitrate. Land use increases the export of DIN to the GBR.
It was long thought that the increased nutrients in run-off stimulated the growth of large algae (or macroalgae) that would displace corals on nearshore reefs. We now know that many factors influence the interaction between corals and macroalgae. Algae quickly re-colonise disturbed surfaces, including corals that are killed by other
factors such as freshwater, high water temperatures or crown-of-thorns starfish. If algae are established, the regrowth of hard corals can be slowed or disrupted. Algal cover on reefs is often related to the abundance (or absence) of herbivores, especially fish. Factors which control the abundance of herbivores on nearshore reefs of
the GBR are not well understood, but fishing is not a direct problem as it is in other tropical regions.
Experimental studies exposing corals to artificially high levels of nutrients have demonstrated direct effects on corals including changes in coral growth and calcification, disruption of reproduction (embryo development, fertilization rates) and changes in settlement success of planulae. Most of these changes are small and the
overall effect is likely to be subtle.
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