The redistribution of Coomera River estuary mangrove zones under rising sea levels: the capacity for landward migration

KATHLEEN BRANDER

Abstract

The geographical setting of the Coomera River estuary puts its mangrove communities at high risk from climate change. This study aimed to assess the impact of sea level rise on the overall coverage of mangroves within the Coomera River estuary, and to evaluate whether there will be sufficient space for landward migration to compensate for mangrove areas lost through inundation. Using geographical information systems, it was found that a minimum net reduction of 15 per cent of present-day mangrove coverage is likely by the end of the century. It will therefore be of vital importance to restrict any further developments within future mangrove zones, as this would result in even greater losses in the total area of mangrove habitat.

Key Words: climate change, sea level rise, mangrove communities, landward migration

Kathleen Brander wrote this report as a third year student in Geographical Sciences, in the School of Geography, Planning and Environmental Management, University of , .

1. Introduction

1.1 Values and Importance of Mangroves Mangrove forests provide a wealth of ecological, economic and societal resources. They serve as nurseries and feeding grounds for numerous species of fish, crustaceans and molluscs (Ley et al. 2002; Marshall 2004), and habitats for invertebrates and migratory birds ( Bridgewater and Cresswell 1999). In addition, mangroves improve water quality for coral reefs and seagrasses (Victor et al. 2004; McLeod and Salm 2006), and offer areas of public and scenic amenity (UNEP 2006). These values render mangroves invaluable to offshore and onshore commercial fisheries (Lovelock 1993), as well as coastal tourism industries. Moreover, mangroves act as natural buffers to coastal lands and protect shorelines from erosion (Ewel et al. 1998; Bridgewater and Cresswell 1999; Badola and Hussain 2005). Loss of mangroves could affect coastal societies by increasing the risk of flooding, saline intrusions, and storm surges (Field 1995; Simas et al. 2001). Australia’s mangroves are also of cultural significance to Indigenous peoples (Joyce 2006), and many Aboriginal and Torres Strait Islander communities continue to rely on these resources for food, medicine and timber (Duke 2006).

1.2 Ecology and Biogeography of Mangrove Forests The term ‘mangrove’ is used to refer to the woody trees, shrubs, palms and ferns that occur in the transitional zones between terrestrial and marine environments (Ellison 1994). The tidal habitats in which these plants occur are also described as mangroves (Tomlinson 1994). Mangrove plants are found in tropical and subtropical regions throughout the world, and form fringing forests along intertidal shorelines, estuaries, and riverbanks (Chapman 1976). They have developed a number of remarkable adaptations that enable them to survive in such harsh environments, with conditions of high salinity, regular inundation, wave action, and waterlogged soils with low oxygen and nutrient content (Pernetta 1993; Rogers 2004).

The complex interplay of many factors from global, regional, estuarine and intertidal scales ultimately determines the distribution of mangrove forests (Duke et al. 1998). From a global perspective, mangroves are generally constrained within the 20° C isotherms (Duke 2006) and are dependent upon the dispersal ability of their water- buoyant propagules (Rabinowitz 1978). On regional scales, temperature (Duke et al. 1998), rainfall (Smith and Duke 1987), catchment area (Bunt et al. 1982), and the dispersal capability of individual species (Clarke and Myerscough 1993) all contribute to mangrove biogeography. All mangroves are restricted to intertidal zones, and are usually found between local mean sea levels (MSL) and highest astronomical tides (HAT) (Pernetta 1993; Specht and Specht 1999).

Mangroves display intertidal zonation perpendicular to coastlines (Tomlinson 1994), which is thought to be a result of natural environmental gradients rather than succession (Woodroffe 1983). These zonations have been linked to a large number of variables, including salinity, competition, light effects and stand support structures (Duke et al. 1998), as well as soil type, and wave energy (Spenceley 1982; Woodroffe 1992; Lovelock 1993). However, species ecotones frequently form distinctive bands along tidal contours (Saenger et al. 1977; Duke 2006), which suggests that tidal inundation is a prominent driver of zonation (Vanderzee 1988). This notion is supported by numerous palaeoecological and present-day studies, which indicate a significant correlation between the extent of mangrove coverage and local sea level changes (Woodroffe 1992; Ellison and Stoddart 1991; Saintilan and Wilton 2001; Wilton 2002; Gilman et al. 2007).

1.3 Mangroves and Climate Change Climate change will directly impact mangrove growth and distributions (McLeod and Salm 2006), as these are closely associated with sea level, temperature (Tomlinson 1994; Specht and Specht 1999), precipitation (Pareira et al. 2006; Viner et al. 2006), atmospheric CO 2 concentration (Solomon and Cramer 1993; Stiling et al. 2002; Hughes 2003; Ziska and Bunce 2006), the magnitude and frequency of extreme events (Cahoon et al. 2006), and the timing of the seasons (Viner et al. 2006). The absolute response of mangroves to climate change remains unknown, as the predictions of climate change are themselves still uncertain (Cahoon et al. 2006). However, there are some general conclusions that can be made.

Any alterations to the availability of resources (such as space) will be likely to increase competition within mangrove communities, and result in the domination of individual species that are advantaged by the changes (for example, pioneering species) (Korner 2006; Ziska and Bunce 2006; Lovelock and Ellison 2007). Increases in extreme events

and decreases in precipitation are both expected to prevail globally throughout the century (Solomon et al. 2007b), which could result in losses of mangrove coverage and diversity (Gilman et al. 2006; McLeod and Salm 2006). The 21 genera of angiosperms found in mangroves (Duke 2006) could be adversely affected by relative changes in phenology with their associated pollinators (Menzel and Sparks 2006; Morecroft and Paterson 2006). Several indirect climate change effects have also been predicted; for instance, loss of protective barrier reefs through coral bleaching would expose mangroves to more powerful wave energy (Hoegh-Guldberg 1999). However, of the many and varied possible climate change impacts, it is generally considered that sea level rise will present one of the greatest challenges to mangrove communities (Ellison 1994; Field 1995; Ellison and Farnsworth 1997; McLeod and Salm 2006; Gilman et al. 2008). Furthermore, sea level rise is likely to have the greatest impact on the distribution of mangroves at a local scale (Tomlinson 1994; McLeod and Salm 2006).

1.4 Response to Sea Level Rise Sea levels are rising globally (Solomon et al. 2007b; WMO 2009) and there is general agreement amongst climate scientists that global warming will cause significant increases in sea levels by the end of the century, with local variations (Pachauri and Reisinger 2007a; Rahmstorf et al. 2007; Siddall et al. 2009). Any alteration to sea levels will disturb the distribution of mangroves, and slight increases in local tide levels can have a disproportionate impact on mangrove communities. Figure 1 illustrates three different scenarios of the impact of sea level rise on mangrove coverage (Duke et. al 2003). The three scenarios illustrate how the impact of sea level rise on mangrove coverage is dependent on local topography. Green lines indicate mangrove coverage, blue lines indicate mean sea levels and highest astronomical tides, and the dashed and full lines indicate the before and after situations respectively. Despite having equal rises in sea level, scenario 1 has no net change in mangrove coverage, scenario 2 has a net gain in mangrove coverage, and scenario 3 has a net loss of mangrove coverage. Mangrove forests must adapt to elevated sea levels, in order to avoid reductions in area and local extinctions (Field 1995; Duke et al. 1998).

SCENARIO 1 NEW MANGOVE ZONE

INITIAL MANGROVE ZONE

E L E V A HAT T I O N MSL

SCENARIO 2 NEW MANGROVE ZONE

INITIAL MANGROVE ZONE E L E V A HAT T I O N MSL

SCENARIO 3 NEW MANGROVE ZONE

E INITIAL MANGROVE ZONE L E V A HAT T I O N MSL

Figure 1. Impact of sea level rise on mangrove coverage dependent on local topography (adapted from Duke et al. 2003).

Mangrove plants can respond to sea level rise by: increasing their elevation through vertical accretion, vertically extending their pneumatophores (aerial roots), or landward migration to higher elevations (Orson et al. 1985; Vanderzee 1988; McLeod and Salm 2006; Gilman et al. 2008). However, the ability of mangrove communities to respond to sea level rise will depend upon a number of factors. For example, mangroves will only be able to adapt by increasing their elevation if rates of sea level rise do not exceed the combined effects of accretion and autocompaction (Ellison and Stoddart 1991; Rogers et al. 2005; K Rogers 2010, pers. comm., 29 March). High rates and magnitudes of sea level rise are likely to force mangroves to respond by migrating landward (UNEP 2006; Gilman et al. 2008).

Several studies have found that mangroves have already begun migrating landward into saltmarsh environments across Australia (Bayliss et al. 1997; Saintilan and Hashimoto 1999; Saintilan and Williams 1999), though this has been associated with a number of causes such as increased precipitation and human disturbances (Woodroffe and Mulrennan 1993; Hughes 2003). Nevertheless, sea level rise is expected to augment this trend. Facilitating the further encroachment of mangroves is essential if we are to preserve what remains of these valuable ecosystems.

The capacity of mangroves to migrate landward depends on their physiographic setting. Landward migration will be prevented in locations with extensive development barriers (such as sea walls, roads and infrastructure). Barrier to migration may result in the permanent submersion (and subsequent demise) of mangrove forests (Woodroffe 1990; Field 1995; Kennedy et al. 2002; Gilman et al. 2008). Furthermore, the extent of landward colonisation is limited to the point at which inland slopes become steeper than those at the seaward edge (Woodroffe 1990; Gilman et al. 2007). At these points, it is hypothesized that landward migration is exceeded by erosion at the seaward edge and mangrove communities are progressively compressed (Pernetta 1993). In contrast, sea level rise also has the potential to create new zones into which mangroves can migrate, especially in undeveloped regions or gradually sloped lands (Hughes 2003; McLeod and Salm 2006).

Ensuring that mangroves have adjacent landward areas to migrate to is an important management strategy that can increase their natural resilience to sea level rise. This can be achieved by identifying and preserving existing suitable lands, or by establishing sufficiently large buffer zones to accommodate the imminent colonisation (McLeod and Salm 2006).

1.5 Research Questions, Aims, and Objectives The research project aimed to investigate the capacity of mangroves within the Coomera River estuary of southeast Queensland, Australia, to respond to sea level rise by landward migration. The specific objectives were to: a) establish the present distribution of mangrove zones within the Coomera River estuary; b) predict the redistribution of mangrove zones in relation to projected sea level rise; c) identify topographical barriers to mangrove establishment within these new mangrove zones; d) quantify the percentage of the mangrove zone surface area lost to these topographical barriers; and

e) identify areas that could serve as future mangrove refuges in the future.

It is acknowledged that the ability of mangroves to react and adapt to sea level rise will be more complex than landward migration alone (Cahoon et al. 2006; Gilman et al. 2008) due to the additional biophysical, geomorphological, and ecological factors that contribute to their realised niche. This study is designed to provide a first approximation of the expected affects of sea level rise on the Coomera River estuary mangroves. These data can then be used in conjunction with further research and testing to assist the management and mitigation of climate change impacts on the mangroves of the Coomera River estuary. It is therefore hoped that this study will highlight the vulnerability of southeast Queensland mangroves to sea level rise, and encourage similar research throughout the region.

2. Method

2.1 Study Site Southeast Queensland, Australia, is a heavily built-up region, which combined with ongoing developments, puts its local mangroves at high risk due to sea level rise. The Coomera River estuary was selected as a study site as there are diverse land uses and environments within the catchment that are typical of the region (Gold Coast City Council 2007), including channels, low-lying islands, canal estates, and causeways.

Coomera is one of the fastest growing regions in the Gold Coast City (Gold Coast City Council 2009). The estuary has been extensively modified and suffered critical habitat loss due to urban developments (Australian Government 2002). Local communities still benefit from the diverse environmental values of the Coomera River estuary, including aquatic ecosystems, primary recreation, visual recreation, cultural and spiritual values, aquaculture, oystering, and seagrass ( 2007).

The estuary is located in the tide-dominated delta of the Coomera River (153°22’E, 27°51’S), and as such has naturally high turbidity and low sediment trapping efficiency (Australian Government 2002). It contains approximately 4.58 km 2 of mangroves and 0.76 km 2 of saltmarsh saltflats (Australian Government 2002), which are protected by the state government under the Coomera Fish Habitat Area 033-012A (Queensland Government 1999). Prominent species of mangroves in the Coomera River estuary include the grey mangrove ( Avicennia marina ), long-style stilt mangrove ( Rhizophora stylosa ), river mangrove ( Aegiceras cornicultatum ) (Batton and Johnston 2006), and there are also a few stands of the milky mangrove (Excoecaria agallocha ) (Derbyshire and Batton 2006).

2.2 Compilation of Data To carry out the study it was necessary to acquire geographical information on the distribution of mangroves, infrastructure, surface elevations, and tides of the Coomera River estuary. Sea level rise predictions for southeast Queensland were also compiled.

2.2.1 Mangrove Distribution The Coastal Wetlands of SE Queensland data set (DSIN 10458) was provided by the Environmental Protection Agency of Queensland, which contained digital maps and geographical data of mangrove communities in the Coomera River estuary.

2.2.2 Infrastructure Cadastral maps and aerial photography were obtained from the University of Queensland School of Geography, Environmental Planning and Management. These tools were used to map the distribution of roads, buildings, and other infrastructure in the study area.

2.2.3 Refuge Sites Potential refuge sites were defined as greenspace areas that are currently protected by Local, State and Federal Governments. Maps and statistical data of protected areas within Coomera were sourced from the Collaborative Australian Protected Area Database (Australian Government 2009).

2.2.4 Tidal Elevations Semidiurnal tidal plane data was obtained from Maritime Safety Queensland. Two tide- gauging stations were used to estimate the present tidal elevations within the Coomera River estuary, the details of which are listed below in Table 1.

Table 1. MSL and HAT of tide gauges in metres above the Australian Height Datum (Maritime Safety Queensland 2009).

Location Geographical Coordinates MSL (m) HAT (m) Sanctuary Cove 27°51’S, 153°22’E +0.11 +1.09 Paradise Point 27°53’S, 153°24’E +0.03 +1.05

Averaging the above data yields a mean MSL of 0.07 m and a mean HAT of 1.07 m (above the Australian Height Datum) for the Coomera River estuary.

2.2.5 Sea Level Rise Predictions Globally averaged sea level rise predictions were taken from CSIRO Marine and Atmospheric Research (CMAR) data, which amalgamated Intergovernmental Panel on Climate Change (IPCC) projections from the third and fourth assessment reports (White 2008b). As current trends indicate oceans are rising at the upper limit of projections from the third assessment report (Rahmstorf et al. 2007), maximum values of the CMAR global sea level rise predictions were used for this study (Table 2).

Table 2. Maximum globally averaged sea level rise projections (in mm) according to various IPCC emission scenarios (White 2008b).

IPCC Emission Scenarios Year A1B A1T A1FI A2 B1 B2 1990 0 0 0 0 0 0 2010 59 59 60 60 56 58 2030 143 149 146 139 132 142 2070 413 413 471 401 333 369 2100 649 611 819 692 496 576

CMAR have further estimated Australian regional variations to global mean sea level rise under the SRES A1B, for the years 2030 and 2070. The adjustments for the southeast Queensland region are tabled below in Table 3.

Table 3: Regional variations to global mean sea level rise (in mm) in southeast Queensland (White 2008a).

Year Mean Regional Variation 2030 + 23 2070 + 31

The global average sea level rise projections under the A1B scenario were combined with the above regional adjustments for southeast Queensland to project mean sea level rise values in the Coomera River estuary in 2030 and 2070. It was thus estimated that Coomera River estuary sea levels would rise above 0.11m above present sea levels by 2030 and 0.39m by 2070.

As yet there are no such CMAR projections for regional departures to global mean sea level rise beyond the year 2070. Therefore the global mean sea level rise under the A1FI scenario was used to estimate sea level rise in the Coomera Estuary in 2100, which predicts a 0.82 m increase. The A1FI scenario was selected as the most appropriate prediction for the 2100 sea level rise because several studies have indicated that global sea level trends have been tracking A1FI predictions for the past two decades (Rahmstorf et al. 2007; White 2008a). When making these sea level rise predictions, it was assumed that vertical displacement through tectonic movement would be negligible in southeast Queensland throughout the twenty-first century. This assumption was confirmed by local geologists (G Rosenbaum 2009, pers. comm., 12 September; M Gasparon 2009, pers. comm., 14 September). Due to time and data-access constraints, it was further assumed that precipitation, sedimentation, accretion and autocompaction of sediments would remain constant throughout these sea level rise events. These assumptions however are likely to create a margin of error within the results (K Rogers 2010, pers. comm., 29 March).

2.2.6 Surface Elevation A digital elevation model (DEM) of the Coomera River estuary with a 1 m resolution was supplied by the Queensland Government Department of Environment and Resource Management. The DEM was used to generate topographical contours of the Coomera River estuary, which were then used to map present tidal contours and the redistribution of these tidal contours under each sea level rise projection. It is acknowledged that there may be large errors associated with use of a DEM with 1 m resolution. This was an unavoidable limitation, as the 1 m DEM was the best available data at the time of publication.

2.3 Projecting Future Mangrove Zones As mangroves are commonly distributed in zones between MSL and HAT (Tomlinson 1994), these intertidal zones were used as proxies for future mangrove zones. The sea level rise predictions were added to present day tidal elevations to estimate future intertidal zones in the Coomera River estuary for the years 2030, 2070 and 2100 (Table

4). It was assumed that the present interval between MSL and HAT of 1 m would remain constant throughout each sea level rise event.

Table 4. MSL and HAT above AHD in the Coomera River estuary at present and under various sea level rise predictions.

Year MSL (m) HAT (m) 2009 0.07 1.07 2030 0.18 1.18 2070 0.46 1.46 2100 0.89 1.89

2.4 Identifying Layers within Future Mangrove Zones The series of maps containing future mangrove zones were then overlain on images of other geographical features to identify the overall redistribution of mangroves. Present day distributions of mangroves were overlayed on future mangrove zones to map the areas of mangrove that will be unchanged by 2030 and 2070. Overlaying infrastructure layers on future mangrove zones enabled the identification of barriers to mangrove migration. Potential refuge sites were then mapped by overlaying protected greenspaces on future mangrove zones.

3. Results

3.1 Present Distributions of Mangrove Zones, Mangrove Communities, Infrastructure, and Protected Greenspaces The mangrove zone currently spans a total area of approximately 15.1 km 2 within the study site, and mangroves presently occupy 11.2 km 2 of land in total (Figure 3). The mangrove zone contains 85 per cent of these mangroves. Infrastructure covers an area of 25.4 km 2, with 2 per cent of this acting as barriers within the mangrove zone. Local, State and Federal Governments have reserved 9.7 km 2 of land in the Coomera River estuary as greenspace (Australian Government 2009).

2009 Key

Extent of Site Area Protected Greenspaces Present Mangrove Distribution Parks and Golf Courses

Infrastructure

0 1,000 2,000 3,000 4,000

Meters

Figure 3. Present day distribution of mangroves, infrastructure, protected and unprotected greenspaces.

3.2 Future Distributions of Mangrove Zones, Unchanged Mangrove Communities, Barriers and Refuge Sites within the Study Site Maps of mangrove distribution for each sea level rise projection are displayed in Figure 4, and accompanying data in Table 5. The presence of barriers to mangrove migration combined with loss of mangroves through inundation will result in a net reduction of the total inhabitable mangrove zone area over the twenty-first century. The maps also indicate that most of the new inhabitable land created through sea level rise will develop in the upstream parts of the estuary.

Table 5. Current mangrove areas and mangrove areas in 2030, 2070 and 2100 under sea level rise projections.

Year Total Unobstructed Loss of 2009 Protected Total mangrove mangrove mangrove greenspace protected zone area zone area* communities within inhabitable (km 2) (km 2) through sea mangrove mangrove level rise zone (km 2) area ** (km 2) (km 2) 2009 15.1 14.8 N/A 2.2 N/A 2030 12.2 12.2 3.8 2.2 9.0 2070 16.4 15.7 4.2 3.5 9.9 2100 14.0 9.6 8.6 3.7 6.0 * Unobstructed mangrove zone area = Total mangrove zone area – Total barrier area ** Total protected inhabitable mangrove area = Unchanged mangrove area + additional gains in protected greenspace within mangrove zone

The total area of unobstructed mangrove zone will decrease to 64 per cent of the present day value by 2100 (Table 5). Major losses of existing mangroves through permanent inundation are likely. The total area of protected greenspace within the mangrove zone is expected to increase over time, though by 2100 protected refuge sites will only be found on Coomera and South Stradbroke Islands. The total area of inhabitable refuge sites that is protected from development will decrease to approximately 6 km 2 by the end of the 21 st century. However, additional areas will need to be protected if there is to be no net loss of mangrove coverage throughout the 21 st century (Table 6).

Table 6. Maximum changes to mangrove coverage compared to 2009 are shown for each sea level rise projection, as well as the minimum area needed for protection to have no net loss of mangrove coverage.

Year Maximum change in Minimum area to be mangrove coverage since additionally protected to have 2009 no net loss of mangrove coverage (km2) 2030 - 14% 2.3 2070 - 29% 1.4 2090 - 58% 5.3

2009 2030

2009: 2030: 34% of present mangrove coverage lost to rising seas 2% of Mangrove Zone lost to barriers 0.7% of Mangrove Zone lost to barriers 16 % of Mangrove Zone protected 18% of Mangrove Zone protected

2070 2100

2070: 2100: 37% of present mangrove coverage lost to sea level rise 77% of present mangrove coverage lost to rising seas 4% of Mangrove Zone lost to barriers 12% of Mangrove Zone lost to barriers 21% of Mangrove Zone protected 26% of Mangrove Zone protected

Key

Unchanged Mangrove Coverage Gain in Protected Mangrove Zone ± Gain in Unprotected Mangrove Zone 0 500 1,000 2,000 3,000 4,000 Mangrove Zone Lost to Barriers Meters

Figure 4. Current mangrove distribution and projected mangrove distributions in 2030, 2070 and 2100 under sea level rise projections.

4. Discussion

4.1 Impact of Sea Level Rise on Mangrove Coverage The future of mangroves in the Coomera River estuary will largely depend upon whether more developments will take place. If no further developments are constructed in the future mangrove zones, it will be possible to see a gain in mangrove coverage up to the year 2070. This is because the total unobstructed mangrove zone area up to that point in time will be greater than the present day coverage of mangroves (Table 5). Any gains in mangrove coverage would be likely to come at the expense of other wetland vegetation communities, such as saltmarshes and Melaleuca swamps.

While it may be possible to see an increase in mangrove coverage up to the year 2070, a minimum loss of 15 per cent of the present day mangrove coverage will occur by 2100 due to reductions to the total area of the inhabitable mangrove zone. Removing barriers to mangrove migration (that is infrastructure within mangrove zones) may reduce this loss (Figure 6). However, if such barriers are not removed a net loss of 15 per cent of the current mangrove coverage will occur by 2100. Greater losses of mangrove habitat can be expected if developments are permitted to take place on the unprotected areas of mangrove zones.

To ensure that the total area of mangrove coverage does not decrease over time, certain areas of suitable land will need to be preserved for mangroves to migrate to. If additional land is protected for this purpose, it will be possible to achieve no net losses of mangrove coverage up until 2070. However, it is likely that an even greater loss may occur, as not all the land made available through sea level rise will be inhabitable for mangroves. The recruitment and establishment of mangroves in refuge sites will depend upon additional factors such as hydrology, sediment composition, competition, and dispersal ability (Krauss et al. 2008). In any case, it is best to protect as much land as possible for mangrove migration so as to maximise the availability and diversity of refuge sites.

4.2 Species-Specific Effects of Sea Level Rise Changes in distribution of individual mangrove species are likely to vary, with differential losses of species with varying tolerances of salinity or inundation (Pernetta 1993). The grey mangrove is generally regarded as a pioneer species of the mangrove system (Duke 2006), as they are tolerant of wide ranges of salinity, substrate types, precipitation levels, and tidal zones. Grey mangroves are therefore likely to be the first species to colonise new areas as they become available through sea level rise (Lovelock and Ellison 2007).

As most of the new mangrove zone areas created through sea level rise will form in the upstream and intermediate zones of the estuary (Figure 4), an increase in the population of mangroves that exist in these estuarine positions may arise (Table 7). Conversely, the downstream-dwelling long-style stilt mangrove may decrease in population due to loss of land through permanent inundation.

Table 7. Intertidal and topographical preferences of mangrove species in the Coomera River estuary (Duke 2006).

Species Intertidal Position Estuarine Position Grey mangrove High, middle and low Intermediate Avicennia marina Long-style stilt mangrove Middle and low Downstream Rhizophora stylosa River mangrove Low Intermediate and upstream Aegiceras cornicultatum Milky mangrove High and middle Downstream, intermediate Excoecaria agallocha and upstream

Mangrove forests of the Coomera River estuary are likely to experience erosion at the seaward edge and encroachment at the landward edge during sea level rise (Pernetta 1993). However, if rates of landward migration do not keep up with the loss of seaward mangroves it may result in coastal squeeze. This would see mangroves of the lower intertidal positions become inundated before any new land was made available for them, which could result in significant losses of the river mangrove and possibly also the long-style stilt mangrove (Table 7).

4.3 Environmental Management Strategies There are a number of ways in which managers can mitigate the impacts of climate change on mangroves (UNEP 2006). McLeod and Salm (2006) outline strategies to increase the natural resilience of mangroves to climate change, such as reducing anthropogenic stresses, restoration of degraded areas, and increased public awareness and activity. These management strategies would be likely to increase the natural resilience of mangroves in the Coomera River estuary. However, the most pressing issue in the Coomera River estuary is that of further developments.

The results of this study indicate that without further protection of land, there could be major losses of mangrove coverage in the estuary. Limiting further development within future mangrove zones may avoid significant reductions to the total area of mangrove habitat. Protection of mangrove zones across all estuarine and intertidal positions may ensure that specialist mangrove species do not become locally extinct.

Present day distributions of mangroves indicate that certain communities are capable of existing outside the mangrove zone. Further research is needed to understand why these communities are able to survive in such locations, as they could also serve as refuge sites for the future.

5. Conclusion The mangrove communities of Coomera River estuary are at high risk from climate change as they exist in a microtidal setting, and are predicted to be subject to high sea level rises, increased extreme weather events, and decreased precipitation. In addition, much of the estuary has been developed for housing and marine activities, which further increases its exposure to mangrove loss. The results of this study predicted a loss of 77 per cent of the existing mangrove forests due to sea level rise alone. Although

mangroves will gain new land as it becomes available through sea level rise, there may not be enough gains to balance the expected losses. A minimum of 15 per cent reduction to the total area of the current mangrove coverage will occur by 2100, though the net loss of mangrove coverage could be much greater if appropriate management strategies are not put into place.

The study was limited by several constraining factors, including a lack of available data for sea level rise and other climate change predictions in southeast Queensland, the use of a DEM with a 1 m resolution, and the exclusion of additional biophysical and ecological influences on future mangrove zones. Further research will be needed to investigate the suitability of identified refuge sites for mangrove migration. It would be advisable to restrict all further developments within future mangrove zone areas until such information is known.

References

Australian Government 2002, Estuary Assessment Framework For Non-Pristine Estuaries: Estuary 785 (Coomera River) , National Land & Water Resources Audit, Canberra.

Australian Government 2009, ‘Protected Area Information’, Department of the Environment, Heritage, Water and the Arts, Australian Government, viewed 10 September 2009, available at: http://www.environment.gov.au/parks/nrs/science/capad/index.html .

Badola, R. and Hussain, S.A. 2005, ‘Valuing ecosystem functions: an empirical study on the storm protection function of Bhitarkanika mangrove ecosystem, India’, Environmental Conservation , vol. 32, no. 1, pp. 85-92.

Batton, R. and Johnston, M. 2006, ‘Boardwalks – QLD, Coombabah Lakelands Conservation Area’, Mangrove Watch , Department of Primary Industries and Fisheries, Queensland.

Bayliss B., Brennan K., Eliot I., Finlayson, M., Hall, R., House, T., Pidgeon, B., Walden, D. and Walterman, P. 1997, Vulnerability Assessment of Predicted Climate Change and Sea Level Rise in the Alligator Rivers Region, Northern Territory Australia , Supervising Scientist Report 123, Supervising Scientist, Canberra.

Bridgewater, P.B. and Cresswell , I.D. 1999, ‘Biogeography of mangrove and saltmarsh vegetation: implications for conservation and management in Australia’, Mangroves and Salt Marhses , vol. 3, no. 2, pp. 117-125.

Bunt, J.S., Williams, W.T. and Duke, N.C. 1982, ‘Mangrove Distributions in North- East Australia’, Journal of Biogeography , vol. 9, no. 2, pp. 111-120.

Cahoon, D.R., Hensel, P.F., Spencer, T., Reed, D.J., McKee, K.L. and Saintilan, N. 2006, ‘Coastal Wetland Vulnerability to Relative Sea-Level Rise: Wetland Elevation Trends and Process Controls’, in J.T.A. Verhoeven, B. Beltman, R. Bobbink and D.F. Whigham (eds), Wetlands and Natural Resource Management , Ecological Studies vol. 190, Springer, Berlin.

Chapman, V.J. 1976, Mangrove Vegetation , J. Cramer, Vaduz.

Clarke P.J. and Myerscough P.J. 1993, ‘The intertidal distribution of the grey mangrove (Avicennia marina ) in southeastern Australia: The effects of physical conditions, interspecific competition, and predation on propagule establishment and survival’, Australian Journal of Ecology , vol. 18, no. 3, pp. 307-315.

CSIRO and Australian Bureau of Meteorology 2007, Climate change in Australia: technical report 2007 , CSIRO.

Derbyshire, K. and Batton, R. 2006, ‘Boardwalks – QLD, Paradise Point, Phil Hill Environmental Park’, Mangrove Watch , Department of Primary Industries and Fisheries, Queensland.

Duke, N.C. 2006, Australia’s Mangroves. The authoritative guide to Australia’s mangrove plants , University of Queensland, Brisbane.

Duke, N.C., Ball, M.C. and Ellison, J.C. 1998, ‘Factors Influencing Biodiversity and Distributional Gradients in Mangroves’, Global Ecology and Biogeography Letters , vol. 7, no. 1, pp. 27-47.

Ellison, J.C. 1994, ‘Climate change and sea level rise impacts on mangrove systems’, in JC Pernetta, R Leemans, D Elder and S Humphrey (eds), Impacts of Climate Change on Ecosystems and Species: Marine and Coastal Ecosystems , IUCN, Gland, Switzerland.

Ellison, J.C. and Stoddart, D.R. 1991, ‘Mangrove ecosystem collapse during predicted sea-level rise: Holocene analogues and implications’, Journal of Coastal Research , vol. 7, no. 1, pp. 151-165.

Ellison, A.M. and Farnsworth, E.J. 1997, ‘Simulated sea level change alters anatomy, physiology, growth, and reproduction of red mangrove ( Rhizophora mangle L.)’, Oecologia , vol. 112, no. 4, pp. 435-446.

Ewel, K.C., Twilley, R.R. and Ong, J.E. 1998, ‘Different Kinds of Mangrove Forests Provide Different Goods and Services’, Global Ecology and Biogeography Letters , vol. 7, no. 1, pp. 83-94.

Field, C.D. 1995, ‘Impact of expected climate change on mangroves’, Hydrobiologia , vol. 295, no. 1-3, pp. 75-81.

Gilman, E., Ellison, J. and Coleman, R. 2007, ‘Assessment of mangrove response to projected relative sea-level rise and recent historical reconstruction of shoreline position’, Environmental Monitoring and Assessment , vol. 124, no. 1-3, pp. 112-134.

Gilman, E.L., Ellison, J., Duke, N.C. and Field, C. 2008, ‘Threats to mangroves from climate change and adaptation options: A review’, Aquatic Botany , vol. 89, no. 2, pp. 237-250.

Gilman, E., Van Lavieren, H., Ellison, J., Jungblut, V., Adler, E., Wilson, L., Areki, F., Brighouse, G., Bungitak, J., Dus, E., Henry, M., Kilman, M., Matthews, E., Sauni Jr., I., Teariki-Ruatu, N., Tukia, S. and Yuknavage, K. 2006, Living with Pacific Island mangrove responses to a changing climate and rising sea level , United Nations Environment Programme, UNEP Regional Seas Reports and Studies.

Gold Coast City Council 2007, Our Living City Report January 2007: A sustainability update , Gold Coast City Council, Queensland.

Gold Coast City Council 2009, Coomera Town Centre Structure Plan - Final - January 2009 , Gold Coast City Council, Queensland.

Hennessy, K., Fawcett, R., Kirono, D., Mpelasoka, F., Jones, D., Bathols, J., Whetton, P., Stafford Smith, M., Howden, M., Mitchell, C. and Plummber, N. 2008, Drought: exceptional circumstances: An assessment of the impact of climate change on the nature and frequency of exceptional climatic events , report prepared for the Australian Government by the CSIRO and Bureau of Meteorology, Australia.

Hoegh-Guldberg, O. 1999, ‘Climate change, coral bleaching and the future of the world’s coral reefs’, Marine and Freshwater Research , vol. 50, no. 8, pp. 839-866.

Hughes, L. 2003, ‘Climate change and Australia: Trends, projections and impacts’, Austral Ecology , vol. 28, no. 44, pp. 423-443.

Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (eds) 2007b, Climate Change 2007: The physical science basis , contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge.

Jones, M. 1988, Country of Five Rivers: Albert Shire 1788-1988 , Albert Shire Council, Allen and Unwin.

Joyce, K. 2006, Wetland Management Profile: Mangrove Wetlands , Ecosystem Conservation Branch, Environmental Protection Agency, Queensland Government, Queensland.

Kennedy, V.S., Twilley, R.R., Kleypas, J.A., Cowan, J.H. Jr, and Hare, S.R. 2002, Coastal and Marine Ecosystems: Potential Effects on U.S. Resources and Global Climate Change , Pew Center on Global Climate Change.

Korner, C. 2006, ‘Significance of temperature in plant life’, in JIL Morison and MD Morecroft, Plant Growth and Climate Change , Blackwell Publishing, Oxford.

Krauss, K.W., Lovelock, C.E., McKee, K.L., Lopez-Hoffman, L., Ewe, S.M.L. and Sousa, W.P. 2008, ‘Environmental drivers in mangrove establishment and early development: A review’, Aquatic Botany , vol. 89, no. 2, pp. 105-127.

Ley, J.A., Halliday, I.A., Tobin, A.J., Garrett, R.N. and Gribble, N.A. 2002, ‘Ecosystem effects of fishing closures in mangrove estuaries of tropical Australia’, Marine Ecology Progress Series , vol. 245, pp. 223-238.

Lovelock, C. 1993, Field guide to the Mangroves of Queensland , Australian Institute of Marine Science, Townsville, Australia.

Lovelock, C.E. and Ellison, J.C. 2007, ‘Vulnerability of mangroves and tidal wetlands of the Great Barrier Reef to climate change’, in J.E. Johnson and P.A. Marshall (eds), Climate Change and the Great Barrier Reef: A Vulnerability Assessment , Great Barrier Reef Marine Park Authority and Australian Greenhouse Office, Australia.

Maritime Safety Queensland 2009, Semidiurnal tidal planes 2010 , Department of Transport and Main Roads, Queensland Government, Queensland.

Marshall, N 2004, ‘Mangrove conservation in relation to overall environmental considerations’, Hydrobiologia , vol. 285, no. 1-3, pp. 303-309.

McLeod, E. and Salm, R.V. 2006, Managing Mangroves for Resilience to Climate Change , IUCN, Switzerland.

Menzel, A. and Sparks, T. 2006, ‘Temperature and plant development: phenology and seasonality’, in J.I.L. Morison and M.D. Morecroft, Plant Growth and Climate Change , Blackwell Publishing, Oxford.

Morecroft, M.D. and Paterson, JS 2006, ‘Effects of temperature and precipitation changes on plant communities’, in J.I.L. Morison and M.D. Morecroft, Plant Growth and Climate Change , Blackwell Publishing, Oxford.

Orson, R.A., Panageotou, W. and Leatherman, S.P. 1985, ‘Response of tidal salt marshes of the U.S. Atlantic and Gulf Coasts to rising sea levels’, Journal of Coastal Research , vol. 1, no. 1, pp. 29-38.

Pachauri, R.K. and Reisinger, A. (eds) 2007a, Climate Change 2007: Synthesis Report , contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, Geneva.

Pareira, J.S., Chaves, M.M., Calderia, M.C. and Correia, V. 2006, ‘Water availability and productivity’, in J.I.L. Morison and M.D. Morecroft, Plant Growth and Climate Change , Blackwell Publishing, Oxford.

Pernetta, J.C. 1993, Mangrove Forests, Climate Change and Sea Level Rise: Hydrological Influences on Community Structure and Survival, with examples from the Indo-West Pacific , Marine Conservation and Development Report, IUCN, Gland, Switzerland.

Queensland Government 1999, Declared Fish Habitat Area summary , Department of Primary Industries and Fisheries, Queensland Government.

Queensland Government 2007, Coomera River Environmental Values and Water Quality Objectives: Basin No. 146 (part) Including all tributaries of the Coomera River , Environmental Protection Agency, Queensland Government.

Rabinowitz, D. 1978, ‘Early growth of mangrove seedlings in Panama, and an hypothesis concerning the relationship of dispersal and zonation’, Journal of Biogeography , vol. 5, no. 2, pp. 113-133.

Rahmstorf, S., Cazenave, A., Church, J.A., Hansen, J., Keeling, R.F., Parker, D.E., Somerville, R.C.J. 2007, ‘Recent Climate Observations Compared to Projections’, Science , vol. 318, no. 5825, p 709.

Rogers, K. 2004, ‘Mangrove and saltmarsh surface elevation dynamics in relation to environmental variables in Southeastern Australia’, PhD thesis, University of Wollongong, Wollongong, Australia.

Rogers, K., Saintilan N. and Cahoon, D.R. 2005, ‘Surface elevation dynamics in a regenerating mangrove forest at Homebush Bay, Australia’, Wetlands Ecology and Management , vol. 13, pp. 587–598.

Saenger, P., Specht, M.M., Specht, R.L. and Chapman, V.J. 1977, ‘Mangal and coastal salt marsh communities in Australasia’, in V.J. Chapman (ed.), Wet coastal ecosystems , Elsevier Scientific Publishing Company, Amsterdam.

Saintilan, N. and Hashimoto, T.R. 1999, ‘Mangrove-saltmarsh dynamics on a bay-head delta in the Hawkesbury River estuary, , Australia’, Hydrobiologia , vol. 413, pp. 95-102.

Saintilan, N. and Williams, R.J. 1999, ‘Mangrove transgression into saltmarsh environments in south-east Australia’, Global Ecology and Biogeography , vol. 8, no. 2, pp. 117-124.

Saintilan, N. and Wilton, K. 2001, ‘Changes in the distribution of mangroves and saltmarshes in Jervis Bay, Australia’, Wetlands Ecology and Management , vol. 9, no. 5, pp. 207-221.

Siddall, M., Stocker, T.F. and Clark, P.U. 2009, ‘Constraints on future sea-level rise from past sea-level change’, Nature Geoscience , vol. 2, no. 8, pp. 571-575.

Simas, T., Nunes, J.P. and Ferreria, J.G. 2001, ‘Effects of global climate change on coastal salt marshes’, Ecological Modelling , vol. 139, no. 1, pp. 1-15.

Smith, T.J. III and Duke, N.C. 1987, ‘Physical determinants of inter-estuary variation in mangrove species richness around the tropical coastline of Australia’, Journal of Biogeography , vol. 14, no. 1, pp. 9-19.

Solomon, A.M. and Cramer, W. 1993, ‘Biospheric Implications of Global Environmental Change’, in A.M. Solomon and H.H. Shugart (eds), Vegetation Dynamics and Global Change , Chapman and Hall, New York.

Specht, R.L. and Specht, A. 1999, Australian Plant Communities: Dynamics of Structure, Growth and Biodiversity , Oxford University Press, Melbourne, Australia.

Spenceley, A.P. 1982, The Geomorphological and Zonational Development of Mangrove Swamps in the Townsville Area, , Monograph Series No 11, James Cook University of North Queensland, Australia.

Stiling, P., Cattell, M., Moon, D.C., Rossi, A., Hungate, B.A., Hymus, G. and Drake, B.G. 2002, ‘Elevated atmospheric CO 2 lower herbivore abundance, but increases leaf abscission rates’, Global Change Biology , vol. 8, no. 7, pp. 658-667.

Tomlinson, P.B. 1994, The Botany of Mangroves , Cambridge Tropical Biology series, Cambridge University Press, Cambridge.

UNEP 2006, Pacific Island Mangroves in a Changing Climate and Rising Sea , United Nations Environmental Programme, Regional Seas Reports and Studies, Report no. 179.

Valiela, I., Bowen, J.L. and York, J.K. 2001, ‘Mangrove Forests: One of the World’s Threatened Major Tropical Environments’, Bioscience , vol. 51, no. 10, pp. 807-815.

Vanderzee, M.P. 1988, ‘Changes in saltmarsh vegetation as an early indicator of sea- level rise’, in G.I. Pearmen (ed.), Greenhouse: Planning for climate change , CSIRO, Melbourne, Australia.

Victor, S., Golbuu, Y., Wolanski, E. and Richmond, R.H. 2004, ‘Fine sediment trapping in two mangrove-fringed estuaries exposed to contrasting land-use intensity, Palau, Micronesia’, Wetlands Ecology and Management , vol. 12, no. 4, pp. 277-283.

Viner, D., Morison, J.I.L. and Wallace, C. 2006, ‘Recent and future climate change and their implications for plant growth’, in J.I.L. Morison and M.D. Morecroft, Plant Growth and Climate Change , Blackwell Publishing, Oxford.

White, N. 2008a, ‘Global Projections’, CSIRO Marine and Atmospheric Research , viewed 11 September 2009, available at: http://www.cmar.csiro.au/sealevel/sl_proj_regional.html .

White, N. 2008b, ‘Sea Level Projections’, CSIRO Marine and Atmospheric Research , viewed 11 September 2009, available at: http://www.cmar.csiro.au/sealevel/sl_proj_21st.html#21C_ts .

Wilton, K. 2002, ‘Coastal wetland habitat dynamics in selected New South Wales estuaries’, PhD thesis, Australian Catholic University, Fitzroy, Australia.

WMO 2009, WMO statement on the status of the global climate in 2009 , report by World Meteorological Organization, WMO- No, 1039.

Woodroffe, C.D. 1983, ‘Development of mangrove forests from a geological perspective’, in H.J. Teas (ed.), Biology and ecology of mangroves , Tasks for vegetation science 8, Dr W. Junk Publishers, Boston.

Woodroffe, CD 1990, ‘The impact of sea-level rise on mangrove shorelines’, Progress in Physical Geography , vol. 14, no. 4, pp. 483-520.

Woodroffe, C.D. 1992, ‘Mangrove sediments and geomorphology’, in A.I. Robertson and D.M. Alongi (eds), Tropical mangrove ecosystems , American Geophysical Union, Washington DC.

Woodroffe, C.D. and Mulrennan, M.E. 1993, Geomorphology of the Lower Plains, Northern Territory , Australian National University and the Conservation Commission of the Northern Territory, Darwin.

Ziska, H.L. and Bunce, .J.A 2006, ‘Plant responses to rising atmospheric carbon dioxide’, in J.I.L. Morison and M.D. Morecroft, Plant Growth and Climate Change , Blackwell Publishing, Oxford.