Abstract
Driven by tidal, marine and atmospheric conditions, intertidal zones experience extreme spatial and temporal environmental variability. While intertidal species have evolved to live within this environment, climate change is altering the conditions with evidence for changes in distribution and physiology of intertidal species as a result. Species may, however, be able to persist within microclimates - fine-scale variations in environmental conditions - which can create refuges for individuals away from extreme conditions. Microclimates may also facilitate the establishment of novel species, in regions such as Antarctica where the conditions in the intertidal zone are rapidly changing. This thesis focuses on the microclimate created by the holdfasts of Durvillaea spp. (bull kelp, rimurapa) and the importance of this refuge for the invertebrate species found within the holdfast.
The first part of this thesis investigated the seasonal thermal environment within southern New Zealand Durvillaea holdfasts using temperature loggers to characterise the buffering capacity of the holdfasts relative to the adjacent bare rock in the intertidal zone. The results showed that the holdfast acts as a thermal refuge from extreme temperatures in both summer and winter, with the extreme temperatures outside the holdfast 4.4°C hotter in summer and 6.9°C colder in winter than temperatures recorded inside the holdfast refuge. Thermal variability was also reduced, with the average rate of temperature change being 0.16°C hr-1 inside the holdfast, compared to 0.45°C hr-1 outside. Air and sea surface temperature interacting with the tide were used to accurately model and predict the temperature inside and outside the holdfast. This allowed for end-of-century air and sea temperatures to be used in the model to forecast how the temperatures inside and outside the holdfast might change into the future under the assumption that daily variability will remain unchanged. Model predictions suggest that a larger increase will be seen inside the holdfast, with the average temperature increasing by 0.9°C to 2.9°C, compared to 0.8°C to 1.9°C outside the holdfast, with greater change under more extreme future climate scenarios.
The second part of the thesis looked at the thermal tolerance of two common Durvillaea-associated invertebrates, a snail Cantharidus antipodum and an amphipod Parawaldeckia kidderi, measuring their critical thermal limits and physiological response to warming and cooling in a thermal ramping experiment. The thermal range of C. antipodum (-1.95°C to 30.1°C) is wider than P. kidderi (-0.3°C to 27.68°C), which may be due to C. antipodum being less mobile and needing to tolerate a wider range of temperatures. Depression of the respiration rate in C. antipodum was identified at temperatures both above and below what they experience inside the holdfast, suggesting that they do not encounter thermal stress inside the holdfast but would be vulnerable outside the holdfast. There was no detectable response to temperature in the respiration rate of P. kidderi.
Although temperatures are projected to increase both inside and outside the holdfast in future, the holdfasts could continue to provide some thermal buffering for resident taxa, with extreme temperatures 4.6°C cooler within the holdfast than outside. Temperatures outside the holdfast are predicted to reach extremes of 27°C to 28.6°C (noting that this model was conservative with regard to changes in extreme temperatures) which is near or exceeds the critical thermal limits of C. antipodum and P. kidderi, respectively. The holdfast refuge will, therefore, become increasingly important for the survival of the holdfast-associated species and suggests that the holdfast-associated species will only continue to persist in their current distribution if Durvillaea, or another suitable refuge, remains. The presence of the holdfast microclimate may assist in the establishment of the invertebrate species in new areas, such as Antarctica where Durvillaea has been shown to be able to reach, where the holdfast refuge would need to be able to shelter the invertebrate community from the extreme cold.