Mangrove-Habitat Expansion in the Southern Firth of Thames Sedimentation Processes and Coastal-Hazards Mitigation

The Waikato Regional Council (then Environment Waikato) commissioned NIWA to undertake a study of mangrove-habitat expansion and sedimentation processes in the southern Firth, to: • Reconstruct the historical sequence mangrove-habitat expansion based on historical aerial photography (1944–2005) and dated sediment cores. • Quantify sediment accumulation rates (SAR) and changes in SAR resulting from mangrove colonisation and habitat expansion. • Determine the role of mangrove habitat in mitigating coastal erosion and inundation hazards in the southern Firth. The Grey mangrove (Avicennia marina subsp australasica) or Manawa has rapidly colonised intertidal areas of the southern Firth of Thames during the last 50 years or so. Today, mangrove habitat occupies some 7 km2 of former intertidal flat between the Piako and Waitakaruru Rivers and 11 km2 in the southern Firth as a whole (section 1). Grey-mangrove seedlings can colonise intertidal areas down to about mean sea level (MSL), where they are submerged for < six hours per tide. Mangrove-habitat expansion has occurred in many North Island estuaries in recent decades as sediments delivered by rivers has built intertidal habitat suitable for mangrove colonisation. Aerial photographs show that mangrove stands were restricted to delta deposits at river mouths in the mid-1940s. The mangrove forest that has developed over the last 50 years is the result of only 4–5 major seedling-recruitment events that occurred in the early-1950s, mid-1960s, mid-1980s and early-1990s. No major seedling recruitment has occurred in the last decade. Seedling mortality is primarily controlled by episodic wave-driven erosion of the mudflat and recruitment events are likely to coincide with extended periods of calm weather. This indicates a direct link between climate and mangrove-habitat expansion in the southern Firth of Thames. The fate of mangrove-forests primarily depends on sediment-surface elevation increasing at a rate equal to or exceeding SLR. This has previously been assessed by comparing sedimentation and SLR rates. However, this approach is does not take into account the potential effects of sediment compaction and biotic factors, which also influence surface elevation. Mangrove systems are sensitive to changes in sea level because they depend on the maintenance of suitable upper-intertidal habitat. Mangroves can respond to rising sea levels by either maintaining sediment-surface elevation and/or retreating from the sea. In many estuaries, shore-protection structures preclude the retreat of mangrove forest from the rising sea. In the southern Firth, the mangrove forest can only respond to rising sea levels by maintaining surface elevation through sedimentation. This sedimentation process has implications for the efficacy of the stopbanks and maintenance of drainage. There is some uncertainty in how the mangrove forests will respond to climate change and rising sea levels in the future due to the complex feedbacks between the physical and biological processes controlling substrate elevation. Future climate warming is likely to influence mangrove growth and productivity, rainfall patterns, catchment runoff, sediment loads, wave activity and storm-tide frequency. Sediment Elevation Tables (SET) have been widely used to monitor surface-elevation changes in mangrove forests and salt-marshes due to shallow subsidence and sedimentation. SET have been installed in the mangrove forest by NIWA and Environment Waikato to provide basic information for research and management of mangrove forests.

Analysis of and mangrove expansion from aerial photography. Shore normal transects of sediment elevation. Radioisotope and pollen analysis of sediment cores from transects. There is ongoing data collection by NIWA and supported by the Waikato Regional Council.

Southern Firth of Thames

Initial data collection 2004-2005. Some data collection continues (funded by NIWA)

Mangrove expansion mapped using historical air photography: 1944, 1952, 1963, 1977, 1987, 1996, 2002, 2006. Shore normal elevation profiles were measured through the mangrove forest (Geodimeter Model 464 total station) to provide information on large scale coastal morphology, relate bed elevations to mean sea level and to provide information on coastal inundation hazard. Sediment cores were taken along transects for radioisotope and pollen dating. Full details in report.

Various - see NIWA and EW. Aerial photography held in digital form by EW - .jpg and .tiff formats.

Waikato Regional Council technical report. Swales, Andrew - PhD (in prep.)

Contact NIWA and EW.

Complete. Some ongoing data collection continues for related studies.