Duong M. Le1,2, Mathew J. Desmond1,2, Daniel W. Pritchard1,2, and Christopher D. Hepburn1,2
1Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand
2Coastal People Southern Skies Centre of Research Excellence, Department of Marine Science, University of Otago, New Zealand
The giant kelp, Macrocystis pyrifera, is sensitive to warming and marine heatwaves but little is known about how increased sea surface temperature (SST) and heatwave events will influence microscopic life stages of this species. The main objective of this work was to determine the thermal threshold of microscopic haploid stages of M. pyrifera and identify whether the differences in populations’ susceptibility to warming in New Zealand is reflected in genetic differences. Investigation on the effects of increasing temperature on sporogenesis was conducted on cultured stocks of M. pyrifera from a population in Otago, New Zealand. Sporulation was carried out across 10 temperature treatments from 9.5 to 26.2 °C ± 0.2 °C at approximately 2 °C intervals. Results from this experiment indicate that spore release was positively correlated with increasing temperature. Gametophytes of M. pyrifera from six sites in three regions at different latitudes across the South Island of New Zealand were exposed to nine temperature levels from 10.5 to 23.8 °C for a duration of twenty days. This experiment was conducted to determine the effects of elevated temperature and the effect of population location on gametophytic development. The thermal threshold for successful fertilisation was between 18.8 – 20.2 °C for the southern (coolest annual SST) population and 21.8 – 23.6 °C for the mid-latitude and northern (warmest annual SST) populations. In addition, over 30% of gametophytes survived under the maximum treatment temperature of 23.6 °C, suggesting a higher upper thermal threshold for this haploid stage.