Status and trends of seagrass habitats in the Great Barrier Reef World Heritage Area

[Extract] Seagrasses are specialised marine flowering plants that grow in the nearshore environment of most of the world's continents. Most are entirely marine although some species (such as Enhalus acoroides) cannot reproduce unless emergent at low tide. Surviving in a range of conditions; from upper esturine to marine, there are relatively few species globally (about 60) and these are grouped into just 13 Genera and 5 Families. Seagrasses support food webs by virtue of their physical structure and primary production and are breeding grounds and nurseries for important crustacean, finfish and shellfish populations. Providing food for green sea turtles, nearly 100 fish species, waterfowl and for the marine mammal the dugong; which is on the IUCN red list as vulnerable to extinction (IUCN 2000); seagrasses are also the basis of a detrital food chain. Theses plants filter nutrients and contaminants from the water, stabilise sediments and act as dampeners to wave action. Seagrasses rank with coral refs and mangroves as productive coastal habitats and strong linkages among these habitats make the loss of seagrasses a contributing factor in the degredation of the world's oceans. There is now a broad understanding of the range of species and seagrasses habitats around the world (Green and Short 2003) although shallow sub-tidal and intertidal species distributions are better recorded than seagrasses in water greater the 10 m below Mean Sea Level (MSL). Surveying deeper water seagrass is time consuming and expensive. Further areas of deepwater seagrass are likely to be located (Lee Long et al. 1996a). It is important to document seagrass species diversity and distribution and the contribution seagrass species and seagrass meadows make to broader ecosystem services. This information is essential in order to identify those areas requiring conservation measures. Destruction loss of seagrasses has been reported from most parts of the world (Short and Wyllie-Echeverria 1996; Larkum et al. 2006), often from natural causes (den Hartog 1987) or storms (Poiner et al. 1989). However, destruction commonly has resulted from human activities, e.g. as a consequence of eutrophication, or land reclamation and changes in land use (Cambridge and McComb 1984; Coles et al. 2003b). Increases in dredging, development of the shoreline, damage associated with over expoitation of coastal resources, and recreational boating activities along with nutrient and sediment loading has dramatically reduced seagrass distribution in some parts of the world (Short and Wyllie-Echeverria 1996). Anthropogenic impacts on seagrass meadows continue to destroy or degrade coastal ecosystems and decrease seagrass functions and values, including their contribution to fisheries (Walker et al. 1989; Green and Short 2003). Efforts are being made toward rehabilitation of seagrass habitat in some parts of the world through transplantation, improvement of water quality, restrictions on boating activity, fishing and aquaculture, and protection of existing habitats through law and environment policy. A number of general parameters determine whether seagrass will occur along any coastline. These include the biophysical parameters that regulate the physiological activity of seagrasses (such as temperature, salinity, waves, currents, depth, substrate, day length, light, nutrients, water currents, wave action, epiphytes and diseases), the availability of propagules and anthrpogenic inputs that inhibit available plant resources (such nutrient and sediment loading). Combinations of these parameters will permit, encourage or eliminate seagrass from a specific location. The depth range of seagrass is likely to be controlled at its deepest edge by the availability of light for photosynthesis. At the shallow edge exposure (with associated high temperatures and drying at low tide), wave action and associated turbidity and low salinity from fresh water inflow determine seagrass species survival. Seagrasses survive in the inter-tidal zone especially in sites sheltered from wave action or where there is entrapment of water at low tide, (e.g. reef platforms and tide pools), protecting the seagrasses from expossure. Regular reporting on the distribution, status and ecosystem role of seagrasses at a global scale is now possible with electronic communication and dedicated monitoring programs such as Seagrass-Watch and SeagrassNet. This information is available for use by coastal zone management to aid planning and development decisions.