Seagrasses

Seagrass are flowering plants (angiosperms) which grow on soft bottoms below neap low tide and to depths of about 20 m. Extensive seagrass meadows are widely distributed around the Australian coast, but they are best developed in estuaries and bays or within the shelter of reefs. They are significant for their biological productivity and they form the basis for a number of food chains. They are also important in stabilising sediments and in providing habitat for a variety of fauna, including Dugongs (Dugong dugon), Green Turtles (Chelonia mydas) and a number of commercially important fish species. Seagrasses are often found immediately seaward of saltmarshes and mangroves, benefiting from the capacity of those areas to filter sediments, and together with mangroves and saltmarshes they provide nursery habitat for numbers of fish and crustaceans.

Within Australian waters there are 30 species of seagrasses. In temperate Australia Amphibolis, Halophila, Heterozostera, Posidonia and Zostera are the most common genera, often growing in dense and extensive beds. In tropical Australia Cymodocea, Halophila, and Thalassodendron are most prevalent, growing in low densities but widely distributed along the shore. A recent review by the CSIRO (Butler & Jernakoff 2000) noted that most research has focused on temperate Australia, and that much of the seagrass of northern Western Australia and the Northern Territory is unstudied.

Many and diverse organisms are associated with seagrasses, including bacteria living on the seagrass and in the detritus of the seabed, epiphytes on the seagrass leaves, infauna among the sediments and seagrass rhizomes, epifauna on the seafloor and the plants, and larger mobile animals moving in and out of seagrass beds. Fish feed on the plankton and other small creatures found in seagrass. Among the many fish that spend part of their life cycle within seagrass meadows are various whiting, leatherjackets and pipefish. Some studies support the notion that particular seagrass beds attract more fish than others, possibly because of their position relative to currents: if this is generally the case, it could be significant in estuarine management, and underlines our need for detailed knowledge of these environments.

The 1995 State of the Marine Environment Report highlighted widespread dieback amongst seagrasses in all parts of the coast, but particularly near the cities (see figure 2.38).

Figure 2.38 Major areas of seagrass die back around the continent

Source: Zann 1995

While there is a lack of detailed study on the subject, there is a clear association of seagrass dieback with increased growth rates of the epiphytic algae normally found on seagrass leaves, and also an association with an increase in turbidity (and the consequent reduced light levels). Epiphytes normally flourish in the elevated nutrient levels provided by stormwater and treated sewage discharge, and by farm and factory run-off. Increased turbidity is also associated with these conditions; but may in addition be caused by a variety of catchment management practices, coastal erosion, and particular storm events. Seagrasses may also be locally affected by a variety of activities, and sound coastal management seeks to minimise these effects. These activities include hard engineering structures, aquaculture, and trawl netting. For example, breakwater and groyne construction may lead to sediment entrainment and raised turbidity levels, oyster racks and pontoons may shade out the seagrass, aquaculture wastes may raise nutrient levels, and some trawling practices may scrape the seabed, physically disturbing the seagrass. In shallow water, small boat propellers may raise turbidity, to the detriment of seagrasses, and boat anchors may physically affect the meadows.

Seagrass meadows do not readily regrow, often because their loss is followed by erosion of the substratum and raised local energy conditions, and attempts to regenerate beds have had limited success. Lord, Paling and Gordon (in Butler & Jemakoff 2000) reviewed Australian efforts to rehabilitate and restore seagrass up to 1999. Detailed observation (e.g. Clarke 1987) suggests seagrass loss may often be patchy, and where it has occurred sediment may be eroded, leading to the development of small (< 2 m high) scarps, which recede under wave and current action, leading to further seagrass loss.

However, about 1000 m2 of Posidonia sinuosa, P. coracea and Amphibolis griffithii are reported to have been successfully transplanted on Success Bank, off Fremantle in Western Australia (Paling 1999) as part of a $6 million restoration scheme, following the seabed mining of shellgrit by Cockbum Cement Ltd. The seagrass is mechanically removed in 0.25 m2 sods ahead of the mining dredge and replanted after storage.