Seagrass beds play a very significant role in coastal ecosystems

eagrass beds are of considerable ecological importance in coastal and marine ecosystems (Poiner et al. 1992) where they play a significant role in the processes and resources of nearshore coastal ecosystems. They are among the most productive and dynamic elements of an aquatic ecosystem.

The growth and survival of seagrass communities is of major importance to coastal waters as seagrasses are:

1. primary producers that contribute large quantities of fixed carbon (the basis of all food chains) to coastal ecosystems (Larkum et al. 1989);
2. important in stabilising bottom sediments (Fonesca and Kenworthy 1987) because they slow water movement which promotes sedimentation of particulate matter (Phillips and Menez 1988);
3. part of the nutrient cycle in the aquatic system (Moriarty et al. 1984);
4. important in supplying shelter and refuge for adult and juvenile animals and contributing large amounts of substrate for encrusting animals and plants; and
5. essential food for dugongs (Dugong dugon) and green turtles (Chelonia mydas).

Seagrasses grow in shallow-water ecosystems, notably the inshore lagoon of the Great Barrier Reef. The Great Barrier Reef lagoon is a largely sheltered area and offers special protection to seagrass beds within the reef itself and on the lee or landward side of reefs or embayments (Larkum et al. 1989).

Surveys conducted of seagrasses between Cape York and Hervey Bay show that they are most often found in areas that receive shelter from the prevailing winds, such as in bays, behind northerly facing peninsulas, behind islands, reefs and shoals, and on some reef platforms and fringing reefs (Lee Long et al. 1993). The regional contribution of these seagrass beds to primary production and as habitat for marine fauna is likely to be extremely important.

The large majority of seagrasses found in the Great Barrier Reef Region grow in the inshore lagoon in waters no deeper than 10 metres and no greater than 10 kilometres from the coast (Lee Long et al. 1993; Larkum et al. 1989). Large areas of deepwater seagrass (in waters of between 10 and 30 metres depth) have also recently been found in the Great Barrier Reef. Seagrasses in close proximity to land are more likely to be affected by material flowing from land and vulnerable to changes in coastal processes. Recent studies of the factors contributing to seagrass decline have shown that increased anthropogenic inputs to the coastal zone are often linked to seagrass loss (Walker and McComb 1992; Dennison et al. 1996; Short et al. 1996).

The species of seagrass found between Cape York and Hervey Bay are common throughout northern Australia, including the Gulf of Carpentaria and Torres Strait. Fourteen species have been recorded from the seagrass habitats of north-eastern Australia (Larkum et al. 1989). Tropical Australia supports well-developed seagrass communities and a large proportion of all known seagrass species (> 22%). Tropical Australia has a greater diversity of seagrass species than elsewhere in the tropical Indo-West Pacific (Larkum et al. 1989).

Key environmental factors

The distribution and growth of seagrasses is regulated by a variety of water quality factors such as temperature, salinity, nutrient availability, substratum characteristics, turbidity and submarine irradiance (Dennison and Kirkman 1996; Abal and Dennison 1996). For example, it is well known from overseas and temperate Australian studies that the availability of nutrient resources affects the growth, distribution, morphology and seasonal cycling of seagrass communities (Short et al.1995). In addition, seagrasses depend on an adequate degree of water clarity to sustain productivity in their submerged environment (Short and Wyllie-Echeverria 1996). Increased turbidity and sedimentation reduce water clarity, which can affect the health and productivity of seagrass communities (Abal and Dennison 1996).

Although natural events have been responsible for both large-scale and local losses of seagrass habitat, recent evidence suggests that human population expansion is now the most serious cause of seagrass habitat loss. Increasing anthropogenic inputs to the coastal oceans are primarily responsible for the enhanced nutrient input from the land and the worldwide decline in seagrasses (Abal and Dennison 1996). Human activities that most affect seagrasses are those that alter water quality or clarity. These activities can include nutrient and sediment loading from agricultural run-off and sewage disposal, dredging and filling, urban stormwater, upland development, and certain fishing practices.

How do increased nutrients affect seagrass survival?

Nutrient loading is the primary factor responsible for both reduction of water quality and stimulation of algal growth in coastal marine waters (Short et al. 1996; Short and Wyllie-Echeverria 1996). Several studies (Neverauskas 1987; Johansson and Lewis 1992; Phillips and Menez 1988; Short et al. 1996) have related the decline of seagrass distribution to the degree of nutrient loading within various catchments. Causes of seagrass degradation include various forms of nutrient loading, including sewage enrichment (Neverauskas 1987; Johansson and Lewis 1992), enrichment of groundwater supplies (Short et al. 1996) and run-off from agricultural lands (Phillips and Menez 1988). Loss of seagrasses in Cockburn Sound in Western Australia is strongly correlated with the increase of discharge rich in plant nutrients over a period of increasing industrial development (Cambridge and McComb 1984).

Once impacted, seagrass colonisation and regrowth can be very slow, or nonexistent, because of possible ongoing impacts and poor dispersal capabilities of most seagrass species (Preen et al. 1995; Dennison and Kirkman 1996). Loss of seagrasses can bring about a change in the marine food chain with an accompanied shift in main primary producers from benthic to planktonic and a reduction in leaf detritus production. Continued seagrass loss can result in an ecosystem shift to a lagoonal system dominated by high turbidity and algal growth or bare sandy/silty substrate which may remain after the decline of the seagrass beds (Cambridge and McComb 1984). This change results in a considerable loss of diversity.

Seagrasses respond to changes at both a global and local scale but, for the scope of this paper, only local or regional changes in the environmental nutrient regime will be considered. At a regional scale, increases in nutrient loading associated with eutrophication and changes in light quality can adversely affect seagrass beds, resulting in either their reduction or disappearance (Short et al. 1995). Effects on seagrasses can be evident in four different stages, these being structural impacts, diseases, reduced photosynthesis (directly linked with reduced light) and ecosystem shifts.

Structural impacts
Under conditions of high nutrient loading, seagrasses take up additional nutrients from the water. This can cause stress in the plant as there is little intercellular tissue space available for nitrate accumulation. As a consequence, high quantities of nitrate will be converted into ammonia, either immediately, or following vascular storage (Brown 1993). Ammonia production, in turn, requires the plant to divert substantial carbon resources for immediate conversion into amino acids. After an extended period of elevated nutrient uptake, the plant, even with abundant carbon available, will not have the capacity to fix enough carbon to meet its total carbon demand. Lack of carbon in the cellular tissue ultimately severely affects the structural integrity of the seagrass, and results in the death of the plant.

Diseases
Physiological stresses imposed by nutrient supply imbalances may also affect weakened plants by enhancing susceptibility to opportunistic pathogens (i.e. wasting disease) (Brown 1993; Den Hartog 1996). This may be due to a decrease in the production of anti-microbial compounds under conditions of enriched nitrate (Buchsbaum et al. 1990).

Reduced photosynthesis
A reduction in light reaching the seagrass can be brought about by increased turbidity arising from living or non-living particulates in the water, or increased shading by the deposition of silt on photosynthetic tissue (Larkum et al. 1989). Elevated algal growth on the leaf surfaces or stems, resulting from the uptake of additional nutrients by the epiphytic algae, can also limit the amount of light reaching the underlying seagrasses. A reduction in light reaching the seagrass chloroplast precludes effective seagrass photosynthesis. Loss of structural integrity and increased incidence of disease may be exacerbated by reduced photosynthesis.

Many documented cases of seagrass loss have followed eutrophication of coastal embayments where enhanced nutrients have resulted in a reduction in light penetration of the water column, or a reduction in light reaching seagrass levels due to its interception by epiphytic algae (Walker and McComb 1992; Preen et al. 1995). In enhanced nutrient regimes of coastal areas, there is a strong potential for interactions between water-column nitrate and suspended sediment loading (or other sources of light reduction, such as macroalgal overgrowth).

Figure 1. Summary of effects of nutrients on shallow seagrass beds

Ecosystem shifts
Nutrient enrichment can enhance the growth of macroscopic and microscopic algae on seagrass leaf surfaces (Neverauskas 1987). Nutrients are required for seagrass growth but the concentrations in tissues are lower than in macroalgae. Due to differences in the carbon:nitrogen:phosphorus ratio, macroalgae can dominate seagrasses under conditions of marked eutrophication, both as epiphytes and as free-floating species which may originate as attached epiphytes (Batyan 1986). Increased epiphytic growth results in shading of seagrass leaves by up to 65%, which reduces the photosynthetic rate and leaf densities (Walker and McComb 1992).

Nutrient concentrations and seagrasses in Great Barrier Reef waters

Eutrophication effects on seagrass beds are most severe in sheltered habitats with reduced tidal flushing, where nutrient loadings are both concentrated and frequent, and where temperatures fluctuate more widely than in areas with greater water exchange. Shallow seagrass beds found in the inshore Great Barrier Reef lagoon are exposed to potentially higher nutrient inputs, infrequent flushing and temperature variability, making them vulnerable to any changes in the nutrient and light regime.

In protected waters similar to those facing northward along the Queensland coast, epiphytes and macroalgae respond so quickly to water-column enrichment that they can seasonally outgrow grazing pressure, leading to severe light reduction and decline of the underlying seagrass (Burkholder et al. 1992).

Recent studies (Burkholder et al. 1992; Short et al. 1996) have shown that under conditions simulating poorly flushed coastal habitats, even low levels of nitrate enrichment can promote the decline of seagrasses. Growth and survival of seagrass species significantly decreased at all enrichment levels, with the most rapid decline occurring at the highest nitrate loading. Plant death was preceded by loss of structural integrity in above-ground tissues.

Laboratory studies have found that the seagrass species Zostera marina declined under low to moderate (3.5-7.0 mM) water-column nitrate enrichment (Short et al. 1995; Burkholder et al. 1992). Long-term nitrate additions cause severe seagrass decline, likely to be enhanced by increasing temperatures and light reduction. Enriched levels of ammonia (1.85-5.41 mM) and phosphate (0.22-0.50 mM) lead to a reduction in shoot density and biomass of the seagrass population (Short et al. 1995). Conversely, laboratory studies on Great Barrier Reef algae have demonstrated increased algal growth associated with nutrient enrichment (Schaeffelke and Klumpp 1997). Growth of epiphytic algae is also likely to be promoted by excess water column nutrients. Small increases in water column nutrient concentration can also result in increased growth of seagrasses. This has occurred around Green Island following prolonged discharge of untreated sewage (Steven et al. 1990; van Woesik et al. 1990).

On nearshore reefs, the water column nutrients are highly variable, ranging from non-detectable to levels indicative of a eutrophication state (Schaeffelke and Klumpp 1997; Bell 1992). Approximate ranges for (non-flood) inshore water quality concentrations have been measured between non-detectable and 2 mM for dissolved inorganic nitrogen (predominantly ammonia) and non-detectable and 0.2 mM for phosphate (Furnas et al. 1995; Furnas and Brodie 1997; Devlin et al. 1997; Schaeffelke and Klumpp 1997).

Nutrients and suspended particulate concentrations associated with cyclones and floods are the highest that most Great Barrier Reef communities are likely to be exposed to (Brodie and Furnas 1996). Inshore seagrass communities are episodically subjected to high dissolved nutrient and suspended loads more typical of a eutrophic system. Water samples taken in flood plumes have consistently recorded elevated ammonia (0.6-4.2 mM), nitrate-nitrite (0.24-14.36 mM) and phosphate (0.13-1.98 mM) (Steven et al. 1997). In large flood events, nutrient levels have remained high in the inshore lagoon for a number of days to weeks (Brodie and Furnas 1996).

Conclusion

Within the past few decades, catastrophic losses of thousands of hectares of seagrass habitat have occurred throughout the world (Gieson 1990). Seagrass losses in recent years in Australian coastal waters have been extensive with over 45 000 hectares lost (Walker and McComb 1992). This loss may result from natural events, e.g. 'wasting disease' (Den Hartog 1996) or flooding resulting from cyclones or high energy storms (Preen et al. 1995) but most seagrass losses have been correlated with increases in human activities (Neverauskas 1987; Walker and McComb 1992). Evidence suggests that human population expansion and the increasing input of anthropogenic materials into the coastal oceans are primarily responsible for the worldwide decline in seagrasses (Short and Wyllie-Echeverria 1996).

Out of all possible scenarios, the factors most frequently correlated with the disappearance of seagrass beds are nutrient enrichment from sewage and agricultural drainage, and reduction in available light caused by increased suspended solids and floating and epiphytic algae (Abal and Dennison 1996). This paper has presented a brief summary of the potential for seagrass decline with increases in nutrient loading. Seagrasses are an important ecological system in the Great Barrier Reef lagoon and while there is no evidence to date of any major decline in seagrass abundance or destruction on the Great Barrier Reef, ongoing monitoring and conservation are essential. Conservation of the seagrass beds existing in the Great Barrier Reef Region will be a major part of any sustainable management plan for the Region.

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