Effects of catastrophic coastal landslides on the Te Angiangi Marine Reserve, Hawke's Bay, New Zealand
Macpherson, D. J. (2013). Effects of catastrophic coastal landslides on the Te Angiangi Marine Reserve, Hawke’s Bay, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/7916
Permanent Research Commons link: https://hdl.handle.net/10289/7916
The Te Angiangi Marine Reserve protects 446 hectares of coastline considered to be representative of the nearshore marine environment of southern Hawke’s Bay. It was established in 1997, and since then has been an area where human foraging and fishing has been discouraged through legislation, mindful of the fact that it is known that poaching does occur. The aim of protection in a general sense is to facilitate natural ecosystem functioning through protection of species that can influence habitat character with the desired outcome being a hypothesised return to a more robust and ecologically natural state. However, the main purpose of a marine reserve under current law is for scientific study so that the response and recovery of important marine ecosystems and their component species’ can be studied and monitored after human predation stress has eased. The Te Angiangi Marine Reserve was subjected to a large-scale sedimentation event in April 2011, when 650 mm of intense rain fell over a four day period in the Hawke's Bay region, which resulted in a significant amount of sediment being delivered to the coast through catastrophic coastal landslides. An accompanying trigger was a M4.5 earthquake centred only 10 km offshore from Pourerere at a depth of 20 km. The bounding hills in which the landslides occurred consist of soft, jointed, smectite clay-rich mudstone of the late Miocene Mapiri Formation. The joints enhance water penetration and the swelling (wetting) and shrinkage (drying) of the expandable smectite clay component. Spheroidal weathering releases variably sized joint blocks of mudstone which are very easily and effectively eroded further by the coastal hydrodynamic forces. In particular, persistent wave action at the coastline and over the intertidal platform releases the mainly fine and very fine sand, silt and clay sized particles which are readily dispersed offshore across and beyond the reserve. A subtidal sediment survey shows that the seabed in the reserve is dominated by fine and very fine sand and occasional reefs of bedrock mudstone with pronounced mud deposition occurring seaward of the marine reserve boundary at about 40 m depth. The debris from the coastal landslides inundated the immediate intertidal platform adjacent to the hill side, which posed a serious threat to marine life both within and outside of the reserve. There was evidence of seagrass and marine organism mortality, especially in the upper intertidal zone. The occurrence of catastrophic scale landslide sedimentation across the interface of a coastal ecosystem comprising both protected and non protected habitats has provided a rare opportunity to examine the response and potential resilience of a marine reserve to a substantial physical disturbance event. Internationally, empirical evidence for marine reserve resilience in the face of any form of disturbance is rare, particularly because most studies lack information prior to the event. Here, relevant intertidal data is available for the coastal region of interest, covering a number of years prior to the April 2011 storm events. The study is important since there are very few investigations which focus on the resilience of protected organisms to a physical disturbance that is relevant to examining likely increases in marine ecosystem stress associated with a changing climate. More extreme storms predicted under Climate Change modelling equate to more coastal sedimentation events. With the hypothesis that protection offered by a reserve allows biological interactions within the ecosystem to return to more balanced and natural states, the expectation is that an area under protection will have a better chance of recovery than one which may have important ecological imbalance. Hence the ecosystem within the Te Angiangi Marine Reserve will have a ‘stronger’ starting point for recovery. Results from a preliminary intertidal survey of reserve and non-reserve organisms has indicated that reserve organisms are indeed showing hints of a resilience trend and reserve effect. This was unexpected given the relatively short time scale for this study (post sediment inundation) and also because the marine reserve covered a relatively small coastal area. Intertidal populations of paua (Haliotis spp.), kina (Evechinus chloroticus) and seagrass (Zostera capricorni) have generally indicated greater abundance and larger size in protected populations at Te Angiangi and adjacent areas, and a generally healthier reef platform compared with the non-reserve locations. The survey results provide an important contribution to the wider understanding of whether marine reserves increase the resilience of protected populations although the author hastens to add that further work is needed. The current study attempts to interrelate both Earth science and Biological science components of investigation for the purpose of more comprehensively examining the response of a marine reserve to sedimentation. Ideas and interrelationships between a large-scale sedimentation event and the observed response of the intertidal paua, kina and seagrass populations within and outside of a marine reserve are postulated.
University of Waikato
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