Abstract
A major impact of climate change is an increase in ocean temperatures. Both average ocean temperature and extreme warming events, such as marine heatwaves, have increased over at least the past three decades and are projected to increase further in the future. The subantarctic water (SAW) around New Zealand is an example of an area already experiencing increased marine heatwave strength and frequency. These temperature changes will have significant impacts on marine ecosystems. At the base of all these ecosystems are phytoplankton, photosynthetic microbes with important functions in marine food webs and biogeochemical cycles. Thermal performance curves delimit the temperature range that an organism can live in and can be used to illustrate the direct effects of warming on the metabolic processes of phytoplankton, such as growth. There is evidence that exposure to warmer conditions can lead to acclimation or adaptation of phytoplankton cells, resulting in increased optimum and maximum temperatures. By exposing a natural subantarctic phytoplankton community and also selected species to simulated heatwave conditions, this project aimed to investigate if different phytoplankton species can alter their thermal tolerances, and if so, how cellular characteristics, such as volume and chlorophyll content may be affected. Phytoplankton growth rates in both mixed communities and isolated cultures exhibited significant increases, up to 48% faster, when incubated under simulated marine heatwave conditions (15.8°C) compared to those incubated at ambient temperatures (12.2°C). Subsequent experiments to determine whether this led to permanently higher growth rates at elevated temperatures found indications of acclimation, but this was not sustained once conditions returned to ambient. Although absolute temperature affected growth rate, there were no significant differences between the growth rates of Control and heatwave-exposed populations. Two experiments showed greater variability in the growth rate response to temperature when multiple populations of the same species were used, compared to the response of replicate subcultures of one population. The findings of these experiments imply that the presence of phytoplankton in the SAW will not be directly impacted by projected increases in sea surface temperatures, as both the haptophyte Emiliania huxleyi and diatom Cylindrotheca sp. showed faster growth rates at higher temperatures. However, E. huxleyi cells exposed to higher temperatures also demonstrated a long-term cell volume reduction compared to those maintained at ambient conditions, which may impact their ecosystem role in the future.