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

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Wynn-Edwards, CA (2014) Effects of ocean acidification on the nutritional quality of Antarctic phytoplankton as food for Euphausia suberda larvae. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: ocean acidification, Southern Ocean, phytoplankton, Euphausia superba, nutritional quality
Copyright Holders: The Author
Copyright Information:

Copyright 2014 the Author

Date Deposited: 03 Jun 2015 00:12
Last Modified: 15 Sep 2017 00:59
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