Part II: Species and species groups Chapter 9: Vulnerability of mangroves and tidal wetlands of the Great Barrier Reef to climate change

Study Number:

62

Author:

C. E Lovelock and J. Ellison

Main Results and Conclusions:

• The introduction of this chapter discusses mangroves, salt marshes and salt flats, the role of mangroves in the Great Barrier Reef (e.g. physical structure, carbon and nutrient storage and cycling, fauna and dependencies, and fisheries), and critical factors for mangrove survival (e.g. physiological limits to tree growth and limits to faunal distribution) (242-246).
• Vulnerability of mangroves to climate change: changes in ocean circulation, temperature, atmospheric chemistry, UV, sea level rise, physical disturbances (tropical storms), and rainfall and river flood plumes are all discussed (246-258). **See table 9.1 for a complete summary
  • Changes in ocean circulation – may affect genetic structure of mangrove populations via dispersal and gene flow (247). (Extinction).
  • Changes in Temperature – may either increase or decrease mangrove growth depending on location of forest via processes such as respiration, photosynthesis and productivity (247).
  • Changes in atmospheric chemistry – continued increase of CO2 concentrations may increase fresh and brackish wetland mangrove species growth via photosynthesis and respiration as well as increase mangrove carbon: nitrogen ratio of tissues. Primary production may be enhanced, but this will not occur uniformly throughout mangroves of the Great Barrier Reef (248).
  • Changes in UV – increase in UV radiation my lead to damage in plant tissues, but effects will be minimal (248).
  • Sea level rise –
    • Impacts: “With similar bathymetry a greater proportion of mangrove forest will be lost in settings with low (microtidal) compared to high (macrotidal) tidal ranges” (Fig. 9.3, 251).
    • Adaptive capacity: “Mangrove forests and other intertidal wetlands may adapt to rising sea level and remain stable if the rate of vertical accretion of the soil surface of the wetland equals or exceeds the rate of sea level rise (Cahoon et al. 1999, Morris 2002)”(253).
    • Vulnerability and thresholds: northern areas of the Great Barrier Reef will be more vulnerable to sea level rise because of its low tidal range. Southern areas of the Great Barrier Reef have a higher tidal range and will therefore be less vulnerable (249, 254).
  • Physical disturbance – tropical storms – mangroves protect shorelines and coastal communities from waves and wind, but increases in storm intensity may have severe consequences in terms of mangrove habitat destruction (257). However, impacts of cyclone disturbances in Australia are not well documented (Bardsley 1985, Woodroffe & Grime 1999).
  • Rainfall and river flood plumes –
    • “Increases in frequency of intense rainfall events combined with land use change in catchments will increase sedimentation which will increase the availability of suitable mangrove habitat and enhance mangrove growth (Lovelock et al. 2007), however excessive sedimentation events could result in forest losses (Ellison 1998)” (257).
    • “…areas of the GBR with low tidal ranges, low rainfall and limited sediment supply are more likely to experience retreat of seaward fringing mangroves with sea level rise compared to those areas with high tidal range, high rainfall and an ample sediment supply, which are conditions where mangrove expansion is likely to occur” (258).
• Threats to resilience: “The largest threat to the resilience of intertidal wetlands with climate change is the presence of barriers that will prevent landward migration of intertidal wetland communities”(258).

Works Cited:

Bardsley K (1985) The effects of Cyclone Kathy on mangrove vegetation. In: K Bardsley, J Davie and C Woodroffe (eds) Coasts and tidal wetlands of the Australian monsoon region. Canberra, Australian National University North Australia Research Unit, pp. 167–185.

Cahoon DR, Day JW and Reed DJ (1999) The influence of surface and shallow subsurface soil processes on wetland elevation, a synthesis. Current Topics in Wetland Biogeochemistry 3, 72–88.

Ellison JC (1998) Impacts of sediment burial on mangroves. Marine Pollution Bulletin 37, 8–12.

Lovelock CE, Feller IC, Ellis J, Hancock N, Schwarz AM and Sorrell B (2007) Mangrove growth in New Zealand estuaries: The role of nutrient enrichment at sites with contrasting rates of sedimentation. Oecologia doi 10.1007/s00442-007-0750-y.

Morris JT, Sundareshwar PV, Nietch CT, Kierfve B and Cahoon DR (2002) Responses of coastal wetlands to rising sea level. Ecology 83, 286–287.

Woodroffe CD and Grime D (1999) Storm impact and evolution of a mangrove-fringed chenier plain, Shoal Bay, Darwin, Australia. Marine Geology 159, 303-321.