| Summary: | Marine mesozooplankton (zooplankton >0.2 mm in size) play critical roles in ocean food webs and biogeochemical cycles. Because mesozooplankton are taxonomically and functionally diverse, community composition is an important factor that influences zooplankton-mediated ecological functions. In particular, food web processes such as grazing and trophic transfer within diverse plankton communities set the foundation for the flow of energy and important biomolecules such as fatty acids throughout marine ecosystems. However, our understanding is limited regarding how these complex processes will respond to climate change as the oceans become warmer and more acidic due to increased anthropogenic carbon dioxide being released into the atmosphere. The present thesis aims to explore the roles of zooplankton in New Zealand marine ecosystems through the lens of a changing ocean. Specifically, this thesis 1) describes zooplankton community compositions and associations with water masses and seasons during a year when there was a summer marine heatwave event off the south-eastern coast of New Zealand, and 2) investigates the responses of two important zooplankton-mediated processes within marine food webs – grazing behaviour and fatty acid trophic transfer – to the combined climate stressors of ocean warming and acidification. To characterise zooplankton communities off the south-eastern coast of New Zealand, mesozooplankton were collected along an oceanographic transect passing through neritic, modified subtropical, and subantarctic surface waters from November 2017 to September 2018 (Chapter 2). During the study period, some samples were coincidently collected when there was a summer marine heatwave event. Multivariate analyses revealed that zooplankton community compositions differed significantly among water masses and were characterised by seasonal community shifts involving copepods, pelagic tunicates, chaetognaths, and thecosome pteropods. These findings improve our limited understanding of the present-day biology off the Otago coast and provide first insight into seasonal differences in zooplankton composition in the different water masses. The findings also provide a possible window into how zooplankton communities could be affected by future heatwave events, but more importantly serve as a baseline dataset for future Munida transect zooplankton studies. To investigate effects of climate change on zooplankton grazing dynamics, mesozooplankton grazing incubation experiments (24 h) were carried out under present-day temperature and pH conditions and those predicted for the year 2100 in the form of single and combined stressors. The incubations used either isolated copepods of a single species or a mixed mesozooplankton community incubated with their natural prey communities. The incubations took place during short-term mesocosm studies (20-22 days) in two consecutive years (Mesocosm Experiment 3 in 2017 and Mesocosm Experiment 4 in 2018), which were part of a larger project (CARIM – Coastal Acidification: Rates, Impacts, and Management) designed to assess effects of climate stressors on New Zealand coastal marine ecosystems (Chapters 3 and 4). Results demonstrate the complexities and variable interactions within natural plankton communities and suggest that climate change may alter zooplankton grazing dynamics under combined (Chapter 3) or single stressors (Chapter 4) at both the community (Chapter 3) and species level (Chapter 4). Results from both chapters also contained some negative grazing rates, which, while biologically impossible, are not uncommon in zooplankton grazing experiments using disappearance-based methods and bottle incubations. This indicates that, although the methods used were the best currently available, they were not robust enough to fully resolve all trophic interactions within the experimental system. To investigate the effects of climate change on plankton fatty acid content and trophic transfer, the relative fatty acid compositions of the mesozooplankton and prey community within one of the coastal climate change mesocosm experiments (Mesocosm Experiment 4) were analysed (Chapter 5). Results show that neither acidification alone nor acidification in combination with warming significantly affected the close correlation between diet and consumer fatty acid composition. However, polyunsaturated fatty acids, particularly the relative concentration of C18:4n-3, decreased in zooplankton under combined acidification and warming but not in the diet, which suggests combined stressors may have directly affected zooplankton physiology regardless of the fatty acid composition of the diet. Overall, the present thesis improves our understanding of how ocean warming and acidification may affect zooplankton community dynamics, particularly in New Zealand waters. The main findings 1) provide an important baseline of zooplankton distributions and community compositions against which future studies can be compared, and 2) demonstrate the complexities of zooplankton trophic interactions, suggesting their responses to climate stressors will also be complex and variable. Specifically, results from mesocosm experiments imply that effects of climate stressors on copepod grazing are likely species-specific, are different across levels of biological organization, and are influenced by variability in the prey field, including internal microzooplankton dynamics. Mesocosm results also suggest climate stressors do not affect the overall trophic transfer of fatty acids from diet to zooplankton consumers at the community level but may still directly affect some zooplankton polyunsaturated fatty acids. The present thesis also highlights the need for continued and long-term monitoring efforts of zooplankton off New Zealand’s coasts, and the development of methods that are conducive for accurately estimating grazing rates of copepods within climate manipulation experiments in productive coastal systems. Findings from this thesis add to our understanding of present-day zooplankton-driven ecological functions in New Zealand waters and have implications for improving predictions of future climate change effects on marine ecosystems.
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