Ohad Peleg1,2, Alexandra Northmore1,2, Maya Korth1, Philip Trompke1,3, Erik Krieger1,4, Valerio Micaroni1, Roberta D’Archino5, Neill Barr5, François Thoral5, Lee Rauhina-August6,7, Māia Holman-Wharehoka6,8, Zoe Studd9,8, Christopher Cornwall1,2
1Victoria University of Wellington, New Zealand - 2Coastal People: Southern Skies, Centre of Research Excellence, New Zealand - 3St Andrews University, Scotland - 4King Abdullah University of Science and Technology, Saudi Arabia - 5National Institute of Water and Atmospheric Research, New Zealand - 6Taranaki Whānui ki te Upoko o te Ika, New Zealand - 7Ahumai, New Zealand - 8Love Rimurimu, New Zealand - 9Mountains To Sea Wellington, New Zealand
The success of kelp forest restoration depends on targeting areas with suitable conditions and removing impediments to recovery. In Wellington Harbour, New Zealand, Macrocystis pyrifera forms small-scale forests (<1000 m²) at 1–7 m depths, mainly at outer Harbour sites. Anecdotal evidence indicates a decline in M. pyrifera, now absent from some inner Harbour sites where it was historically found.
We investigated barriers to M. pyrifera recovery through field surveys and an experiment across six sites spanning the inner to outer Harbour. We recorded the abundance of key macroalgal and invertebrate species, including the sea urchin Evechinus chloroticus, over three years during the Austral winter. Juvenile M. pyrifera plants were translocated out in the open and within sea urchin exclusion cages, and their survival, growth, and erosion rates were monitored over three weeks. We logged sea surface temperature (SST) and photosynthetically active radiation (PAR) in situ to assess their link with kelp growth and erosion.
Macroalgal and invertebrate communities varied among sites. M. pyrifera was absent from inner Harbour sites but present at some outer Harbour sites, including an existing forest at the outermost site. Large brown algae were found at all sites with varying abundances, particularly the invasive kelp Undaria pinnatifida found at all sites and the fucoid Carpophyllum flexuosum. Sea urchin densities were high at most sites but lowest at the outermost Harbour site, where the density of their predator, the red rock lobster Jasus edwardsii, was high. Kelp transplanted in the open had low survival (0–17%) at four of the six sites. Kelp survival was highest at the outermost Harbour site (88%), where grazing was minimal, perhaps due to predatory control, and at another site (66%), likely due to a prior sea urchin removal project, which resulted in increased macroalgal food abundance for sea urchins. Kelp exhibited net growth at all sites, indicating sufficient light for summer growth, but was higher at outer Harbour sites.
Overgrazing by sea urchins was the main impediment to M. pyrifera recovery in the Wellington Harbour. Restoration should initially focus on controlling sea urchin grazing at outer Harbour sites to facilitate recovery. This study will enable effective kelp forest restoration in Wellington Harbour.
Biography
Dr Ohad Peleg is a Coastal People: Southern Skies Postdoctoral Research Fellow and Royal Society of New Zealand Marsden Fund Grant recipient. Peleg’s field-based research focuses on Wellington’s shallow reef ecology, where he conducts large manipulative experiments to understand reef ecosystem resilience and stability. Partnering with coastal communities, his work aims to advance marine conservation and explore scalable kelp forest restoration solutions.