Abstract
Ocean warming is a key driver of kelp forest decline globally. Restoration efforts have been made to secure these important ecosystems as sea surface temperatures (SST) are projected to continue to increase in concert with more frequent, intense and longer marine heatwaves. The giant kelp, Macrocystis pyrifera, is sensitive to warming and marine heatwaves but little is known about how increased 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.
To develop an experimental foundation for this work, the current knowledge around cultivation of M. pyrifera was synthesised with a focus on approaches to stock collection and preservation of diversity. It is crucial to preserve the current genetic diversity of this species immediately and long-term culture approaches such as germplasm banking and cryopreservation provide the tools to allow this. A concerted effort is also needed to better understand the physiological attributes of M. pyrifera in order to select strains for aquaculture and restorative applications that may provide resilience to future environmental stressors. Finally, attention must be given to developing effective in situ restoration approaches whereby large-scale stock production can be optimized and out-planting strategies developed to ensure restoration success.
Investigation on the effects of increasing temperature on sporogenesis, germination and germ tube growth was conducted on cultured stocks of the giant kelp, 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. Spores were then incubated at these temperatures under a 20.3±1.7 µmol photons m-2 s-1, 16L:8D photoperiod for 5 days. Results indicate that spore release was positively correlated with increasing temperature, whereas an inverse trend was observed between temperature and germ tube growth. The thermal threshold for spore and germling development for the Otago population was determined to be between 21.7 °C and 23.8 °C. Among the four observed stages, the settlement of spores was the most drastically effected by increasing temperature. This study highlights the vulnerability of early life stages of M. pyrifera development to rising ocean temperature and has implications for modelling the future distribution of this valuable ecosystem engineer in a changing ocean.
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 temperature 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. Male gametophytes were less tolerant to higher temperatures than female counterparts in all populations.
To provide perspective of the genetic structure of New Zealand’s M. pyrifera populations and place the learnings regarding thermal tolerance in context, seven microsatellite markers were used to analyse 389 individuals from eight distinct M. pyrifera beds in New Zealand. The data from this work suggest that M. pyrifera can be grouped into three main biogeographic clusters: northward and islands (Wellington, Marlborough, Stewart Island, and Chatham Island), southeast coast (Canterbury, Otago, and Southland), and west coast (Fiordland). The greatest genetic diversity was seen in the southeast cluster, whereas the northeast had the lowest diversity. There may be a link between temperature and genetic diversity among populations, in which genetic diversity is higher in low annual SST regions with the exception of the Stewart Island population. There is evidence that ocean circulation currents may play an important role maintaining high genetic connectivity among populations in the east coast. However, it remains unclear why there was a genetic discontinuity between Fiordland and other populations. Further studies, which focus on isolation by environment is needed to encompass this matter.
In summary, results from this thesis indicate that the thermal threshold of different life stages of M. pyrifera vary over microscopic haploid stages. Evidence provided from this study suggests that sporophyte reproduction is a population bottleneck of this species. Also, a genetic basis could be used to distinguish geographically distinct populations since genetic differentiation was significantly different among and within populations, and genetic diversity was lower in areas with higher annual SST.