Ecological Effectiveness of Four Common Coastal Defence Structures Used on New Zealand Coastlines

Abstract

With rapid coastal development and the increasing threat of sea level rise as a result of climate change, defence structures are fast becoming ubiquitous features of coastal landscapes. Because the primary objective is to protect assets on land, the ecological impacts associated with modifying the coastal landscape were not taken into consideration in the early widespread integration of seawalls. Research in this area has identified coastal defence structures (CDSs) as systems that do not generally support abundant and diverse assemblages of species when compared with natural rocky reefs. However, much of this research has been done internationally and focuses on incorporating habitat complexity to existing structures for benthic communities. It is difficult to extrapolate overseas results to New Zealand, as the most suitable substrate to incorporate into coastal defence structures during development depends on how local species interact with locally available substrate types. For this reason, four common coastal defence structures used on Auckland City’s east coast were quantified and replicated in order to identify their capacity to support native biota and enrich community assemblages as a measure of ecological effectiveness. Part one surveyed control and modified plots of removed biofouling on mudcrete, concrete, masonry and revetment structures already deployed in the marine environment. Part two experimentally added tiles of each substrate type to four locations to overcome potential confounding between substrate type and environmental conditions in the existing structures survey. This study confirmed the underlying principle that habitat heterogeneity supports greater species diversity and abundances in a New Zealand context and identified revetment as the most ecologically effective coastal defence method of the four types surveyed. It also showed how CDSs are not comparable with naturally occurring substrates as mudcrete concrete and masonry lacked diversity and abundance, while revetment supported different assemblages of species. Further study of the interactions CDS assemblages have with the environment and what their contributions to ecosystem functions are, would provide more deterministic results as to their level of impact on the marine environment.