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Vulnerability assessment of the effects of climate change on estuarine habitats in the lower Hawkesbury estuary

Details
Aims: 

i) Assess the vulnerability of estuarine habitats in the region covered by the Lower Hawkesbury Estuary Management Plan (Wisemans Ferry to Broken Bay) to the effects of climate change under a range of scenarios

ii) Map the projected vulnerabilities of each habitat in terms of loss, shift or gain of habitat

iii) Assess the level of risk of these vulnerabilities from climate change compared to other human activities within the Lower Hawkesbury estuary (LHE)

iv) Recommend appropriate adaptive management action to enhance the ecological resilience of vulnerable habitats

Time Scale: 

This project was undertaken during 1.5 years. A vulnerability assessment of the effects of climate change on the vegetated estuarine habitats of the Hawkesbury River (seagrass, mangroves, saltmarsh and floodplain forest) was undertaken.

Outputs: 

Seagrass and floodplain habitats had 100% of sites with the highest risk (moderate-high and high, respectively) of loss of habitat. This was based on their exposure and sensitivity to climate change variables only. Mangroves (represented by A. marina) and saltmarsh (represented by J. krausii) had moderate to low levels of risk of loss in both seasons. When the capacity of habitats to respond to the potential effects of climate change was added, mangroves had the highest levels of resilience of all habitats. This occurred for all scenarios and seasons. Seagrass had the next highest resilience followed by saltmarsh. Incorporation of small and large scale Non-climatic human stressors (NCHS) resulted in saltmarsh having the largest percentage of sites with the highest vulnerability under all scenarios and seasons. No habitat type had a large percentage of sites with the lowest vulnerability level. Mangroves had the largest percentage of sites with moderate vulnerability.

These results suggest that when non-climatic human stressors are incorporated with the potential impacts of the effects of climate change the outcomes for a habitat can be substantially altered. If the assessment was solely based on the exposure and sensitivity of habitats to climate change variables then it could have been concluded that management of seagrass beds would be of little value in the longer term. However, when the capacity to respond to changed conditions due to climate variables were added seagrass habitats showed consistently high levels of resilience. Saltmarsh, which had lower levels of risk of loss than seagrass, had lower resilience than seagrass at most sites, giving saltmarsh a higher priority for management. Finally, when non-climatic human stressors were incorporated all habitats had increased vulnerability to being negatively affected by climate change disturbances to varying extents. The four staged assessment used in this project also revealed windows of opportunity where management action might be effective in enabling habitats to improve their level of resilience. For example, rehabilitating potential areas for habitats to expand or move into or increasing the protection of seagrass beds at some sites.

Executive Summary: 

Hornsby Shire Council (HSC) has implemented their Lower Hawkesbury Estuary Management Plan (Hornsby Shire Council, 2008). It identified climate change as a high risk to the management of natural assets within the estuary including estuarine habitats. Climate change will affect all natural habitats and how people will use these natural resources. Management agencies need a means of assessing the extent to which climate changes will affect these habitats. Such assessments need to identify the issues management can address to best conserve natural habitats and also guide how they might prioritise these issues amidst many other responsibilities. Therefore, HSC commissioned the NSW Department of Primary Industry to undertake a vulnerability assessment of the effects of climate change on the vegetated estuarine habitats in this estuary to provide some of this information. It consisted of hydrological modelling of 15 different climate change scenarios and using the outputs of these scenarios in a four staged assessment process; risk, resilience, vulnerability and priority.

A three-dimensional hydrodynamic model ELCOM (Estuary, Lake and Coastal Ocean Model) was applied to the lower Hawkesbury-Nepean estuary. Baseline (1990) and 32 scenarios of maximum, mean and minimum projections of sea level, sea temperature and air temperature in 2030 and 2050 were then applied. Modelled water levels, salinity and temperature in the baseline and projections were outputted for 18 habitat locations. The results of the 2030 and 2050 scenarios indicate that habitat sites may experience increased frequencies of inundation. Water depth at submerged sites was increased by up to 0.5m, salinity was increased by up to 6 psu, and water temperature was increased by up to 1.0ºC. The locations that are likely to experience the greatest mean change in inundation, depth, temperature and salinity were:

  • Brooklyn Oval, which was dry at all times during baseline conditions, experienced inundation for 2% of time during summer and 4% of time during winter in maximum projections of sea level change.
  • Cowan Creek at Bobbin Head, Cowan Creek at Smiths Creek, Pumpkin Creek, Popran Creek and Gentlemans Halt experienced the greatest change in salinity. The salinity was increased by up to 6 psu during summer.
  • Crosslands and Calna Creek, Gentlemans Halt and Mullet Creek experienced greatest change in temperature of up to 1°C. The increase was greatest during summer.

Four different habitat types were assessed using the data produced from the modelled scenarios; seagrass, mangroves, saltmarsh and floodplain forest. These were assessed at sixteen different sites within the Lower Hawkesbury Estuary (LHE). Under the worst case scenarios of maximum sea level rise, maximum water and air temperature for 2030 and maximum sea level for 2050 seagrass was at moderate-high risk of loss at all sites during summer and at 60% and 40% of sites, respectively, in these scenarios during winter. Seagrass at the Cowan Creek site had moderate-high levels of resilience to the effects of climate change during summer and winter for the maximum sea level rise scenarios. The remaining sites all had moderate resilience during summer and moderatehigh during winter for these same scenarios. Seagrass at Cowan and Patonga had moderate-high levels of vulnerability from non-climatic human stressors under all maximum sea level rise scenarios. The remaining sites all had moderate vulnerability levels.

Mangrove habitats had the lowest level of risk of loss from the effects of climate change. Approximately 50% of sites had moderate levels of risk during summer and 80% of sites had low levels during winter under maximum sea level rise scenarios. Mangroves also had the highest levels of resilience with over 80% of sites having moderate-high levels in both summer and winter. However, when non-climatic human stressors were added, mangroves had moderate-high to moderate levels of vulnerability to climate change for both summer and winter at most sites.

Saltmarsh habitats had different levels of risk and resilience depending upon the species. Juncus krausii had moderate levels of risk of loss in approximately half the sites and low levels in the remaining sites during summer under maximum sea level rise scenarios. During winter risk levels for loss of habitat for this species dropped to low. By contrast, Sporobolus virginicus had high to moderate-high levels of risk of loss of habitat in all sites during summer and winter for all maximum sea level rise scenarios. A similar pattern for both species occurs for resilience. Juncus krausii had moderate levels of resilience in all sites under all scenarios in both summer and winter, except Popran Creek which had moderate-high resilience. Sporobolus virginicus had low resilience in all sites under all scenarios in both summer and winter, except Popran Creek which had moderate resilience. When non-climatic human stressors were added both species had moderate to moderate-high levels of vulnerability to climate change effects.

Floodplain forests were only analysed at the risk assessment stage due to a lack of information about the effects of climate change variables on these habitats. All sites had high levels of risk to being lost at under all maximum sea level rise scenarios in summer and 68% of sites in winter.

Seagrasses at Cowan Creek and Dangar Island sites had the highest priority for management based on the moderate to moderate-high levels of resilience and high concentrations of small and larger scale non-climatic human stressors. One Tree Reach and Farmland sites had the greatest potential for rehabilitation for mangrove and saltmarsh habitats due to the land available for expansion. Mangroves at these sites had moderate-high resilience but also had the greatest concentration of small scale and large scale non-climatic human stressors. Saltmarsh habitats at Courangra Point and Pumpkin Creek would be high priorities for protection due to large habitat proportion and low concentration of non-climatic human stressors.

Affiliations
Researcher(s): 
Karen Astles
Researcher(s): 
Alicia Loveless
Researcher(s): 
Peter Coad
Contact Address: 
Email contact form
Partner Agencies: 
Hornsby Shire Council
Funding or Commissioning Agencies: 
Hornsby Shire Council
Funding or Commissioning Agencies: 
DECC Department of Environment and Climate Change, NSW

Locations

Mullet Creek 33° 30' 29.4984" S, 151° 14' 8.6388" E
Cowan Creek 33° 35' 57.0444" S, 151° 14' 3.696" E
Berowra Creek 33° 37' 5.4948" S, 151° 7' 26.9508" E
Patonga Creek 33° 32' 58.38" S, 151° 16' 5.4372" E
Mooney Mooney Creek 33° 28' 45.3972" S, 151° 12' 30.9996" E
Broken Bay 33° 33' 57.6" S, 151° 17' 38.7528" E