Sediment Processes and Mangrove-Habitat Expansion on a Rapidly-Prograding Muddy Coast, New Zealand

Mangrove-habitat expansion has occurred rapidly over the last 50 years in the 800 km² Firth-of-Thames estuary (New Zealand). Mangrove forest now extends 1-km seaward of the 1952 shoreline. The geomorphic development of this muddy coast was reconstructed using dated cores (²¹⁰Pb, ¹³⁷Cs, ⁷Be), historical-aerial photographs and field observations to explore the interaction between sediment processes and mangrove ecology. Catchment deforestation (1850s–1920s) delivered millions of m³ of mud to the Firth, with the intertidal flats accreting at 20 mm yr^–1 before mangrove colonization began (mid-1950s) and sedimentation rates increased to ≤ 100 mm yr^–1. ²¹⁰Pb data show that the mangrove forest is a major long-term sink for mud. Seedling recruitment on the mudflat is controlled by wave-driven erosion. Mangrove-habitat expansion has occurred episodically and likely coincides with calm weather. The fate of this mangrove ecosystem will depend on vertical accretion at a rate equal to or exceeding sea level rise.