Recent tidal-flat evolution and mangrove-habitat expansion: Application of radioisotope dating to environmental reconstruction

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Mangrove-habitat expansion and tidal flat evolution in the Firth of Thames (North Island, New Zealand) is reconstructed using dated sediment cores ( 210Pb, 137Cs) and field observations to explore interactions between sediment processes and mangrove ecology. Mangrove habitat in the southern Firth has expanded rapidly over the last five decades, colonising prograding intertidal mudflats. The original lower-intertidal sand flat was transformed by the erosion of millions of m3 of mud following catchment deforestation (1850s-1920s). Sediment accumulation rates (SAR) on the tidal flats over the last 60 years have averaged approx. 20 mm year-1, outstripping sea level rise (1.3 mm year-1). Mangrove colonisation was delayed until the early 1950s when surface elevation reached approx. 0.5 m above mean sea level (MSL). Maximum SAR of approx. 100 mm year-1 occurs in the seaward edge of the mangrove forest. In contrast, SAR in the landward old-growth forest have averaged <10 mm year-1 since the 1970s as the forest was progressively isolated by distance and elevation from the tidal-flat. The forest today at 1.7 m above MSL is near the upper tidal limit and is infrequently inundated. Feedbacks between surface elevation, tidal inundation and sediment supply exert strong controls. The fate of the mangrove forest will depend on surface elevation increasing at a rate equal to or exceeding sea level rise. © 2008 IAHS Press.

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