Coastal Ecosystems

Estuaries support many of the key ecological processes that underpin the functioning of coastal and marine ecosystems throughout the tropical world. They are primary nurseries for many estuarine, coastal and offshore fishes and crustaceans, including many of great fisheries importance. Not only are their rich waters the sites of substantial primary productivity, estuarine waters and habitats are also vital in integrating primary productivity from the land into marine food webs.

Estuarine productivity powers estuary ecosystem processes (e.g. nursery ground functioning) and is exported to offshore waters through the direct flow of dissolved and particulate material, and via the movement of estuarine juveniles to offshore habitats. They are also the homes to iconic species such as barramundi, saltwater crocodiles, turtles and dugongs.

Tropical estuaries comprise a mosaic of interacting habitats ranging from mangrove and seagrass, to rock and coral reefs, to open sandy and muddy areas. This mix of habitats is vital to ecological functions because mobile organisms like fish and crustaceans need to access different habitats for certain life functions (e.g. feeding or refuge) and many species move through different habitats as they grow. Tropical estuaries exist in a range of environmental settings, from muddy mainland estuaries to crystal clear estuaries on oceanic islands. They also face a diversity of tidal influences, from areas with almost no tidal change to others with tidal ranges of ten meters or more.

Coastal wetlands occur adjacent to estuaries and coasts, and many are influenced by both tidal inundation and seasonal freshwater flooding. Depending on the amount of tidal influence they can range from fresh to very saline, and may change their extents and salinity in response to seasonal variations. Coastal wetlands contain a diversity of plant communities from freshwater swamps with melaleuca and water chestnut, to salt marsh and mangrove forests in more saline areas.

These are transitional zones between terrestrial and freshwater habitats and estuaries and coasts, and they provide primary nursery habitats for fish such as barramundi, mangrove jacks, bream and tarpon. They are also vitally important to water birds, providing resting, nesting and feeding habitats for migratory birds, and acting as refuges for inland water birds during periods of droughts.

Coastal wetlands are often heavily impacted by human development with many lost or altered through reclamation or cut off from other coastal habitats by the construction of dams, bund walls and roads.

Nearshore ecosystems are the areas of the ocean adjacent to the coast, extending out to depths of about 15 to 20 meters. They are closely connected to estuary environments and are the part of the ocean most influenced by freshwater from the land. On the landward margin are beaches, bays and headlands. In deeper water there are large areas of smooth bottom habitats that may be bare of vegetation or contain seagrass beds, as well as coastal reefs and coral isolates.

Many of the same animal species that are found in estuaries occur in the nearshore zone, as well as species more typical of offshore waters. Extensive seagrass beds in this zone can provide important habitats for dugongs and turtles. Because this zone is very accessible it faces a diversity of human impacts. Despite this, nearshore ecosystems have received relatively little research making them some of least understood of tropical marine ecosystems.

Estuaries, coastal ecosystems and wetlands are vital nursery habitats for many species, such as barramundi, mangrove jack, trevally, mud crabs and banana prawns. This nursery role means these ecosystems are vital to the life cycle development of these species, and therefore support fish stocks throughout continental shelf and reef waters. The nursery ground values of these areas are particularly high due to high productivity and because of the complex mosaic of interacting habitats they comprise; such as seagrass, mangroves, sandy channels and coastal swamps.

This diverse mixture of habitats is necessary to support the rapid changes in resource requirements of juveniles as they transition through their early life-history stages. Animals move among these habitats as they growth through successive juvenile stages, and often used different habitats for different purposes, with areas used as refuge from predators often different from their feeding habitats.

Coastal freshwater and tidal wetland habitats are being transformed as a result of increasing demand for commercial, residential and tourism activities. The result is a habitat seascape complex, comprising a mosaic of natural and engineered coastal features. The extent of man-made coastal structures in the Great Barrier Reef World Heritage Area (GBRWHA), a marine ecosystem of global biodiversity and cultural significance, is rapidly increasing.

Mapping has revealed that equivalent to 10% of the entire coastline in the Great Barrier Reef has been modified for urban and coastal development. Rock breakwalls, marinas, swimming enclosures, residential canal estates, pontoons, port facilities, boat ramps and bridges are now an obvious part of the coastline in the GBRWHA. The result is now a mixed seascape of natural (mangroves and seagrass) wetlands, with engineered, hard, structures.

Beyond urbanization, industrialization is impacting coasts throughout the tropics, with the proliferation of pop-up ports. Pop-up ports occur where individual ships directly access coastal locations in response to short term export needs such as logging. This presents a raft of challenges for poorly resourced countries with remote coasts.