Abstract
Ocean warming and increases in ultraviolet radiation over the past century have resulted in important changes in marine planktonic coastal communities around the world, that include the reductions of primary and secondary productivity. To examine the potential effects of multiple environmental stressors on secondary production, this PhD research uses the New Zealand krill, Nyctiphanes australis as a model species to understand how life-history traits, physiology and the energetic balance of krill may respond to changes in temperature and elevated ultraviolet radiation levels. There are 85 euphausiid krill species globally distributed, all important components of marine food webs, both ecologically and economically. N. australis is the dominant species in New Zealand waters, especially during summer, a period of warm waters, strong thermoclines, variable primary productivity levels and when ultraviolet levels are at their highest.
Using N. australis as a model zooplankter, the aim of this PhD is to investigate the responses to environmental change in New Zealand waters using a dynamic energy budget (DEB) model approach that quantifies changes in fatty acids and amino acids composition to track the usage of these compounds, and predicts the energetic costs associated with living under different temperatures and ultraviolet radiation doses normally present in the coastal environment around the Otago Harbour where N .australis is commonly found.
Fatty acids are known to play an important role in many metabolic processes, including the maintenance of metabolic homeostasis, a process that maintains a physiological optimum despite changes in environmental conditions. Variations in fatty acids abundance and composition are intrinsically connected to amino acids abundance, and both compounds are known to be used as a source of metabolic energy under stressful environmental conditions or during periods of starvation, hence these two bio-molecules can be used to reflect the energetic condition of an individual.
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I use a DEB model, a known tool that has helped to mechanistically describe the processes of how an organism assimilates and utilizes energy for maintenance, growth and reproduction purposes as a function of the state of the organism and the environment it is living in. The DEB can be influenced by changes in the environment and exposure to adverse conditions that result in trade-offs between stress responses and growth/reproduction. Therefore, it can then be used to quantify and predict how species may respond to environmental change, and help identify environmental thresholds associated with such changes.
To assess the impact of temperature and ultraviolet radiation, individuals of N. australis were reared in the laboratory using techniques described for the species and exposed to controlled increases in temperature and UVR under laboratory conditions. Respiration rates, fatty acids, amino acid content were measured following known protocols, and a DEB model was developed to examine the environmental influence on the energy budget and life-history traits of the species.
In summary, the first chapter of this thesis encapsulates all available information on the genus Nyctiphanes to date, aiming to identify potential areas of future research and delineate guidelines for environmental change research including the use of DEB models. In the second chapter, I use static modelling, based on surface and underwater measurements of ultraviolet radiation, to understand the potential impacts of solar ultraviolet radiation and other environmental variables on marine planktonic communities in Otago, New Zealand and understand how the depth to which UVR penetrates in coastal areas influence local productivity. Then I explore the effect of temperature and food availability under a DEB context for the first time for a Nyctiphanes species, aiming to mechanistically quantify processes such as growth and reproduction under variable environmental conditions, thereby connecting the interactive effects of environmental variability to functional traits.
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Finally, the last two chapters expand on the previously described DEB model application, measuring and using fatty acids and amino acids data to parameterize a DEB model. The model is aimed to study and quantify the effect of temperature and UV light on krill metabolism, and quantify changes in the energy utilization of krill exposed to several temperatures and ultraviolet radiation treatments.