Mangrove expansion and their human removal in Tauranga Harbour, New Zealand

Mangrove (Avicennia marina var. australasica) populations have expanded rapidly in recent years in northern parts of New Zealand. In some estuaries and harbours, an increase in coverage of 120% over the last 50 years has been recorded. Conflicting public perceptions of mangroves cause difficulty in designing agreeable management strategies. Perceived negative factors include: a hindrance to water access; the loss of habitat for culturally important benthic organisms; reduced foraging habitat for wading birds; and the muddy substrate is seen as aesthetically unpleasant. Others see the mangrove expansion as a natural process and believe it should be unimpeded. Tauranga Harbour, in the Bay of Plenty, is a large and populated estuary in the North Island. Geomorphologically it is a barrier-enclosed lagoon, with approximately 70% exposed intertidal flats at low tide. Rapid mangrove expansion means that this estuary provides an excellent case study to assess the geomorphological and ecological changes that occur in response to the expansion and any consequent mangrove removal. To this end a two-year intensive monitoring program has been developed to determine temporal and spatial changes to sediment particle size and substrate depth. Comparisons will be drawn between control sites and those where mangroves have been removed, and determine how these differences might reflect variation in the physical environment (wave, tidal currents, nutrient levels). These local results will provide the basis for a mangrove management policy for the estuary as a whole