Surface elevation changes and sediment characteristics of intertidal surfaces undergoing mangrove expansion and mangrove removal, Waikaraka Estuary, Tauranga Harbour, New Zealand

Abstract

Since the 1940s mangroves have expanded their areal coverage in many estuaries in the northern half of the North Island of New Zealand. The extent of mangrove colonization in Waikaraka Estuary, Tauranga Harbour, has been documented using photogrammetric analysis, and the impacts of subsequent mangrove removal are analysed. Surface elevation changes in response to mangrove removal are measured using RSETs (Rod Surface Elevation Table) and erosion pins, and sediment accumulation rates were calculated from sediment trap results. Temporal changes to surface sediment texture are analysed. Mangrove physiognomy is described from analysis of mean plant height, plant density and pneumatophore density. Plant heights reflect the shrubby growth form of mangroves growing toward their southern climatic limit, with mean plant heights under 1.5 m. Mangrove coverage has increased from approximately 16,000 m2 in 1943 to 115,000 m2 in 2003. Since May 2005, 9,600 m2 of mangrove vegetation has been removed from the estuary. For the monitoring period of March 2006 to March 2007, rates of surface elevation change in cleared areas ranged from - 9 mm to - 38 mm yr (mean -14 mm yr). Conversely, surface elevation under mangrove forest varied between sites, ranging from -5 mm to 14 mm yr (mean 3 mm yr). Results from RSETs and erosion pins and a coarsening of surface sediment texture at the cleared sites, is consistent with sediment release after mangrove removal.