Benjamin M. Williams1,2, Christopher D. Hepburn1,2, Daniel W. Pritchard2, Robert O. Smith1,2, Matthew J. Desmond1,2
1Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand
2Coastal People Southern Skies Centre of Research Excellence
The giant kelp Macrocystis pyrifera, is an ecosystem engineer, forming kelp forests that provide valuable ecosystem services. Since the 1940’s when records began, M. pyrifera kelp forests have been declining in some regions of southern New Zealand. Changes in average sea surface temperature alongside more frequent and severe marine heatwaves are breaching the thermal limitations of local M. pyrifera. This research determined the current distribution of M. pyrifera in the Marlborough Sounds (41ºS), near the northern limit of M. pyrifera in New Zealand, and a more southerly population in Stewart Island (47ºS). Video transects were utilised to rapidly survey the two regions and 49 sites from the Marlborough Sounds and 28 from Stewart Island were compared with previous records of distribution in these regions from 1942 and 1988. Satellite data of sea surface temperatures indicate the warmest water originates in the north-west of both regions, also breaching the thermal thresholds of M. pyrifera in these areas. M. pyrifera was found to be receding from the north-west of both regions. Therefore, warming sea surface temperature appears to be a primary driver of this decline. The implications of these losses will likely have flow on effects for local fisheries and impact the ecosystem services which kelp forests provide to an extent which is not yet fully understood.