Effects of ocean acidification on the nutritional quality of Antarctic phytoplankton as food for Euphausia suberda larvae

• Main Author: Wynn-Edwards, CA
• Format: Thesis
• Language: English
• Published: 2014
• Subjects: ocean acidification; Southern Ocean; phytoplankton; Euphausia superba; nutritional quality; Antarctic; The Antarctic; Antarc*; Antarctic Krill; Antarctica; Ocean acidification
• Online Access: https://eprints.utas.edu.au/20835/; https://eprints.utas.edu.au/20835/1/Whole-Wynn-Edwards-thesis-2014.pdf

As a consequence of human activity carbon dioxide (CO2) concentration has risen from ~280 parts per million (ppm) pre-industrial to ~397ppm today. About 30% of anthropogenic CO2 emissions have been taken up by the oceans, raising the seawater partial pressure of CO2 (pCO2) while lowering seawater pH. This change in carbonate chemistry has the potential to alter phytoplankton biochemistry, physiology and species composition, thereby affecting the quality and quantity of the basis of the food web. Little is known about how this may flow on to affect dependent grazers, particularly in polar regions. In the Southern Ocean, Antarctic krill, Euphausia superba, is the key species that supports many predators. Negative impacts on the abundance and quality of this species could have far reaching consequences for the Antarctic food web. To address this knowledge gap, the aim for this research project was to assess the potential of ocean acidification to affect Antarctic krill through impacts on Antarctic phytoplankton as their food. To investigate the effects of ocean acidification on phytoplankton as food for krill, a culture system was designed and developed that could maintain phytoplankton growing exponentially for extended periods at different pCO2. The phytoplankton could then be fed to krill over a time period sufficient to detect physiological changes in the krill. As a first step, a semi-continuous culture system was developed and used. Furthermore, only phytoplankton cultures were exposed to elevated pCO2, while krill larvae were maintained at ambient pCO2 in order to ascertain the direct effects of feeding on phytoplankton grown at elevated pCO2. A pilot study using the diatom Synedropsis hyperborea did not reveal strong CO2 perturbations in either the alga or the krill feeding on it. A follow-up experiment with the ubiquitous diatom Pseudo-nitzschia subcurvata cultured at elevated pCO2 showed subtle changes which included a decrease in the nutritionally important long-chain (>C20) polyunsaturated fatty acids (PUFA) (22:6Ѡ3, DHA and 20:5Ѡ3, EPA) and a small increase in cellular carbohydrate concentrations. Krill larvae fed Pseudo-nitzschia subcurvata cells grown at 896μatm CO2 had significantly higher mortality rates than those larvae fed algae grown at ambient CO2 concentrations and this was attributed to the CO2-induced changes in the nutritional quality of Pseudo-nitzschia subcurvata cultured at elevated pCO2. During the course of early experiments, a number of limitations and scope for improvements of the semi-continuous culture system were identified and therefore a continuous culture method was developed. This system greatly reduced the maintenance time for phytoplankton cultures, and reduced variation in experimental pCO2 treatments over time. With this new culture system three more Antarctic phytoplankton species, Pyramimonas gelidicola, Phaeocystis antarctica and Gymnodinium sp. were assessed for their susceptibility to elevated pCO2. The observed changes were species-specific, variable and often subtle, and included changes in the fatty acid composition and cellular carbohydrate concentration. Exposing high CO2 grown phytoplankton cells for up to two days to ambient pCO2 seawater before being replaced by fresh culture was a limitation of the previously used semi-continuous culture system. A final experiment therefore aimed to establish whether exposing phytoplankton for two days to a different pCO2 environment altered the algal biochemistry and had thus potentially compromised the previous experimental design. I cautiously conclude that a rapid change in CO2 concentration did not affect the biochemistry of Pseudonitzschia subcurvata, however, this may be attributed to an unexpected depletion of nutrients during the experiment. In conclusion, the phytoplankton response to elevated CO2 concentrations was species-specific and mostly subtle. Based on the final experiment, it seems that the impacts of CO2 concentrations could be small when compared to the effects of other abiotic factors such as nutrient concentration. Future phytoplankton research should therefore focus on elucidating the combined effects of elevated CO2 concentrations and concurrent changes in environmental parameters such as, seawater temperature, light intensities and / or nutrient concentration as mediated by global climate change. The increase in krill mortality when feeding on high CO2 grown Pseudo-nitzschia subcurvata cells confirmed that ocean acidification can negatively impact Antarctic krill indirectly through its food source. However, to be able to extrapolate these results from the laboratory to nature, more Antarctic phytoplankton species must be tested and the combined effects of altered carbonate chemistry and food quality need to be further investigated. In addition to the changes occurring to individual phytoplankton species, changes to the community composition need to be understood as well as the possibility of selective feeding by krill which might allow them to compensate for possible changes in the quantity and quality of their food source.