36 CATCHMENT TO REEF
Catchment to Reef
• Coral reefs do not exist as sealed, isolated ecosystems unaffected by outside change, but are part of much larger regional and global systems, and rely on healthy ecosystems long distances away. For example, deforestation on the Himalayas can cause sedimentation on coral reefs in the Indian ocean, over 2000km away!
• “Catchment zones” are the areas within which all rain is “caught” and directed to a particular coastal area, and are where most impacts to coral reefs occur (although impacts such as climate change have global impacts).
• Rainfall returning to the seas through rivers carries with it nutrients and sediments and other materials that support aquatic life. These natural flows are however, being increased. Over a million people now live in the catchment of the Great Barrier Reef, with over a million vehicles, 5 million cattle and 11,000 km2 of crops. Half of the Great Barrier Reef Catchment area has already been cleared or severely modified, reducing water quality and the health of downstream ecosystems.
o Increase in sediments (from overgrazing, land clearing, cultivation and urban development) smothers seafloor organisms and reduces light penetration, slowing the growth of primary producers such as corals and sea grasses.
o Intensive fertiliser use and urban and industrial waste has doubled the quantities of nitrate and phosphate in rivers around the Great Barrier Reef. These promote excess algal growth, causing major changes to water and the makeup of coral reef ecosystems.
o Increase in chemicals from use in daily life (e.g. washing, killings weeds or pests, manufacturing goods) disrupt biological processes and can kill or damage many marine organisms.
Streams and rivers: Living links
• Streams and rivers transport water from land to the sea and support rich biodiversity of their own, whilst also acting as important links for coral reefs, delivering vital nutrients and other materials.
• Flood plumes spread the rivers load of sediment, nutrients and freshwater into the sea. Plumes are normal and have shaped the distribution of reef types and the species that live on them.
• Human activities in catchment zones alter the movement and quality of water, disrupting the normal functioning of ecosystems. For example clearing of vegetation
means that rainfall runs more quickly into the ocean, increasing the level of freshwater reaching the reef until it becomes damaging, leading to decreased salinity, increased sediment run off and further reaching sediment plumes.
• Wetlands capture and store floodwaters, allowing recharge of aquifers in the coastal plain and maintaining the flow of rivers in the dry season. At the same time they trap sediments and nutrients that support diverse and productive biological communities.
• Declining water levels cause wetlands to clog up with sediment. Combined with excessive use of fertilizers, pest plant and algae species can take over rapidly, which can be toxic and quickly use up all of the oxygen, starving other organisms.
Estuaries and Seagrass
• Estuaries are the meeting point for rivers and the sea. Mixing of fresh and salt water leads to murky water with varying salinity, which can nevertheless support extremely productive ecosystems.
• Estuaries can be impacted by numerous factors. Many estuarine mangroves are drained to allow coastal development. Excessive sediment and chemicals can reduce light penetration as they are resuspended by currents and waves.
• Many coral reef species spend part of their lifecycle within an estuary (for example the Red Emperor Snapper, whose juveniles inhabit mangroves), and may struggle to complete their lifecycle without healthy estuaries. Estuaries support specialised plants and animals eg. Mud crabs and mangrove trees. They are very productive due to the mixing of nutrients from the land in marine waters.
Completing the connection
• Man’s activity from mountain top to the deep sea can impact coral reef ecosystems in many ways.
• Directly, many organisms require several different ecosystems to complete their life cycles, and may struggle to survive if even one is degraded.
• Changes in landscape up and down the catchment area can change the extent, and content of flood plumes, and therefore can damage marine ecosystems which have adapted to live with the pre-existing plumes.
• In managing the catchment and the reef, it is vital to understand the natural connections between them, and human influences on them. Australia manages natural resources in “Natural Resource Management” areas, many of them are based on specific catchment zones.