Holocene Habitat Analysis and Organism-Sediment Interactions at Mangawhai Estuary, North Island, New Zealand

Abstract

Mangawhai estuary is a small (4.6 km2), mesotidal estuarine lagoon with an active dune system that forms its coastal barrier and a catchment that is roughly 73 km2. The expansion and growth of mangroves (Avicennia marina var australasica) has been recorded within the estuary since the early 1950s based on aerial photographs. Mangrove expansion is a result of an increase in suitable habitat. Within the Mangawhai estuary, methods were implemented to delineate the current habitat distribution, ecology, and sediment characteristics prevalent across the estuary. These methods included habitat mapping, shallow coring, species identification and counting, and numerous laboratory analyses. Results of this data were used to produce a modern habitat map illustrating the locations and area encompassed by the 13 identified habitats. A series of vibracores were extracted from various locations (primarily the upper estuary) to supplement the existing collection of vibracores from the lower and middle estuary (Low, 2006) and to provide a more detailed stratigraphic profile across the estuary. Depositional environments observed throughout the estuary were correlated to similar environments represented throughout the stratigraphy of the vibracores based on neoichnological and ichnological comparisons, grain size distributions, percent organic matter (%OM), percent total organic carbon (%TOC), and carbon to nitrogen ratios (TOC:TN), to illustrate habitat shifts over time. The surficial sediments (<30 cm depth) sampled across the estuary indicate a general fining upward trend throughout the estuary with the highest percentages of clays and silts (between 20 and 53%) in the upper 20 cm of the areas with established mangrove stands. Investigation of sediment characteristics and accumulation suggest that sediment infill rates have increased due to anthropomorphic influences (especially the construction of the causeways) from the surrounding catchment area. The elevated surfaces resulting from increased infill rates have contributed to the rapid growth and expansion of mangroves. This recent mangrove expansion plus the catchment modifications, have increased, and continue to increase, the overall surficial silt content in varying percentages throughout the study area. Variations in these processes therefore, will significantly affect the rate of expansion of mangrove stands within the Mangawhai estuary.