Environmental Status: Seagrasses

Seagrass meadows cover approximately 13% of the GBRWHA, and play a vital role in the Reef ecosystem.

Condition

Diversity of Great Barrier Reef seagrasses

Seagrasses are flowering plants and occur in the coastal, estuarine, reef and deepwater habitats of the Great Barrier Reef (Carruthers et al, 2002). Fifteen of the sixty known species of seagrass worldwide occur in Queensland, most of which are widespread (Waycott et al, 2004). Seagrasses are commonly found in shallow waters less than 10m deep. However, recent surveys have mapped seagrasses down to a depth of 60m. At these depths, the Halophila species are predominant. Recent estimates suggest that seagrass beds cover approximately 13% of the Great Barrier Reef World Heritage Area (GBRWHA), indicating the presence of some 6,000 square kilometres of shallow seagrass habitat and a further 40,000 square kilometres of deepwater seagrass habitat in the Great Barrier Reef (Coles et al, 2003).

Patterns of seagrass density and distribution

Shallow water seagrasses

Halophila seagrasses (pictured) are amongst the most common in the Great Barrier Reef, and are the dominant species between Cape York to Cairns. Photo courtesy of QDPI&F.

The most common genera of seagrass in the GBRWHA are Halophila and Halodule, although the average cover varies with region. Since 1984, key sites within the Great Barrier Reef have periodically been surveyed by the Queensland Department of Primary Industries and Fisheries (QDPI&F) to determine the nature and extent of seagrass communities. While the cover and condition of seagrass is highly variable, some regional trends are apparent. These surveys have shown that the Northern Great Barrier Reef and Torres Strait regions have the highest diversity of seagrass species. From Cape York to Cairns, seagrasses of the genus Halophila dominate with equal areas of sparse  and dense cover (Coles et al, 2003). Approximately 1,000 hectares of seagrass can be found close to Cairns, comprised mainly of Zostera, Halodule and Halophila species (Coles et al, 2004). Between Cairns and Bowen, seagrasses are mostly a mixture of Halodule and Halophila species, and the meadows are less dense with about 70% of seagrass area having less than 10% cover. Between Bowen and Yeppoon, seagrasses are mainly Halodule and Zostera species, with about half of the seagrass area being between 10 and 50% cover. South of Yeppoon, seagrass communities are mainly comprised of Zostera, Halodule and Halophila species. These communities tend to be denser with 60% of the seagrass area having a cover of 50% or greater (Coles et al, 2003; Coles et al, 2004).  Refer to http://www.reef.crc.org.au/discover/plantsanimals/seagrass/seagrassidentify.html for further information and illustrations of some seagrass species.

Deepwater seagrasses

Surveys by the QDPI&F in the early 1990s found extensive deepwater seagrass meadows at depths of up to 60m, in the relatively clear waters of the Great Barrier Reef lagoon. These deepwater meadows were dominated by five Halophila species (Lee Long et al. 1996). The distribution of deepwater seagrasses appears to be influenced by a combination of factors including water clarity, propagule dispersal, nutrient supply and water currents (Coles et al, 2000). Deepwater seagrass meadows were most likely to be found between Princess Charlotte Bay and Cairns, however, the overall density of deepwater seagrass communities is thought to be low (<5% cover) with a correspondingly low biomass (Coles et al, 2003). However, relatively little is known about deepwater seagrasses.

Ecological roles

Seagrasses provide food for threatened species such as green turtles and dugongs.

Seagrasses perform a number of important ecological functions in the Great Barrier Reef ecosystem (Carruthers et al, 2002). Seagrass meadows support a wide diversity of plants and animals and have high rates of primary production. Seagrasses are consumed by many animals, including threatened species such as green turtles and dugongs, and are also a source of detritus for coastal food webs (Klump et al, 1989). The quantity and quality of seagrass may directly affect other species such as dugongs. For instance, data from the Queensland Parks and Wildlife Service (QPWS) Strandings Database suggest that increases in dugong mortality may be related to declines in seagrass pasture quality caused by abnormal wet seasons (Preen and Marsh, 1995; Haines and Limpus, 2000).

As a habitat, seagrasses provide nursery areas for juvenile prawns such as tiger and endeavour prawns, and fishes such as Spanish mackerel and king salmon (see Environmental status – fishes), all of which are important to commercial and recreational fisheries (Zeller, 1998). Larger predators such as birds, sharks and barramundi may also use seagrass beds as foraging grounds.

Seagrasses are thought to play an important role in the sediment and nutrient dynamics of coastal systems by stabilising sediments and recycling nutrients. However, recent research suggests that this may not always be the case for tropical seagrass meadows comprised of structurally small seagrasses (Mellors, 2003). Further investigation into the dynamics of structurally small seagrasses is required.

Variability and current condition of Great Barrier Reef seagrass communities

Factors influencing seagrass condition

Seagrass cover and species composition change over time, and both natural and human related factors affect these changes. In general, seagrasses are influenced by environmental factors such as the geography and hydrology of the area, the amount of available light, weather patterns, waves, temperature, salinity, erosion and sediment deposition (Coles et al, 2003; Carruthers et al, 2002). For example, recent reports suggest that declines in shallow water seagrasses near Cairns, the Gulf of Carpentaria and the Whitsundays (in 2001-02) may be related to elevated seawater temperatures that caused seagrass “burning” (Coles et al, 2004). Biological processes such as grazing by dugongs and turtles can also affect seagrass condition. The reduction in grazing pressure caused by declines in herbivores such as dugongs and green turtles (see Environmental status – marine mammals and Environmental status – Marine reptiles) has been suggested as a factor in the expansion of some seagrass beds on the Queensland East Coast (Coles et al, 2004). Other seagrass beds further away from the coast, such as those off Green Island, have also expanded and shown changes in species composition in recent times. Research suggests that these expansions are linked to increased levels of nutrient runoff from coastal areas, which has increased the growth of seagrasses normally limited by low nutrient levels (Udy et al., 1999).

Disturbance and recovery

Seagrass communities are very dynamic. Species such as Halophila ovalis (above) are often the first species to recolonise areas after a disturbance. Photo courtesy of QDPI&F.

Seagrasses are also affected by natural disturbances, including events such as storms and cyclones that generate pulses of sediment laden and nutrient rich water flowing from coastal rivers into the Great Barrier Reef. The impact of storms and cyclones may be worsened where erosion from the land causes excessive amounts of sediment to flow into coastal areas (see Pressure: declining water quality and coastal development ). For example, there are anecdotal reports of the loss of thousands of hectares of seagrass from the Torres Strait region, possibly due to flooding and sediment runoff from Papua New Guinea (Coles et al, 2004, 2003).

Given favourable conditions, these disturbance events are usually followed by a period of recovery where seagrasses re-colonise the area. The rate of recovery may depend on the magnitude of the disturbance, the species of seagrass affected, the physical and environmental conditions of the affected area and the existence of seed banks  that may aid recovery (Carruthers et al, 2002).

Current condition of Great Barrier Reef seagrasses

Overall, many seagrass communities are constantly changing, and the condition of a particular seagrass meadow will depend on the local environment, the seagrass species present and the environmental history of the area (Schaffelke et al, in press). This variability makes it difficult to identify long-term trends in seagrass condition. Further, little is known about long-term cycles in seagrass meadow size and biomass as monitoring programs such as Seagrass Watch have only recently been established. Nevertheless, the available data suggest that while some seagrass beds have declined, others are increasing with more seagrass beds remaining stable (Queensland Seagrass Watch News, February 2003). Across the Great Barrier Reef region, the changes observed in seagrass communities appear to be attributable to natural cycles of decline and recovery. At this time, there is little evidence of a widespread decline in the condition of the seagrasses of the Great Barrier Reef. However, seagrasses are subject to localised pressures and given their importance to the healthy functioning of the Great Barrier Reef ecosystem, these pressures are a cause of concern for Reef managers.

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