Cathryn Wynn-Edwards

Impacts of ocean acidification on juvenile Antarctic krill: Quantification of nutritional changes of dietary phytoplankton and sea ice bottom algae and krill

Supervisors: Dr Patti Virtue (IASOS), Prof. Andrew McMinn (IASOS), Dr Peter Nichols (CMAR), Dr So Kawaguchi (AAD)

Scientific background:
Ocean acidification is one facet of global change that has to date not received as much scientific attention as for example temperature or sea level rise. One issue raised recently is a possible change in the chemical composition of phytoplankton. Riebesell et al. (2007) reported an excess uptake of carbon with increasing CO2 concentrations, whereas nutrient uptake remained unaltered. Such carbon over-consumption (Toggweiler 1993) shifts the C:N:P ratio of primary producers towards C, with consequent deterioration of its nutritional value as food for grazers and microbial degraders, which rely on their prey as a source of N and P. Possible knock-on effects include reduced growth, fecundity and productivity of zooplankton feeding on algae (Urabe et al. 2003; Carotenuto et al. 2007).

At a base level of the food chain Euphausia superba, hereafter termed krill, represents a vital part of the diet for many animals in the Southern Ocean such as seals, seabirds and whales (Atkinson et al. 2004). By sustaining higher trophic levels, as well as being a target for fisheries itself, krill products are also of commercial interest for humans (Virtue et al. 1993; Martin et al. 2006; Kawaguchi and Nicol 2007; Simmons and Isaac 2007).As a predominantly herbivorous pelagic species, krill is largely dependent on primary producers as their food source. Sea ice algae on the bottom of the ice cover are a major food source, especially for juvenile krill, during winter.

Recent research has shown that development of juvenile krill is strongly dependent on its diet quality as well as quantity (pers. comment Kawaguchi). As for most living organisms, larval to juvenile stages are the most vulnerable ones to stressors (Kurihara et al., 2004). Therefore, it is hypothesized that larval and juvenile krill may suffer most under the future potential decrease of ocean pH. This is an issue of concern since survival rate during this period of life stage primary dictates the magnitude of annual recruits, which eventually is the major determining factor for the population size of krill.

Project outline and objectives:

1. to determine any change of the nutritional composition of bottom sea ice algae and phytoplankton species under ocean acidification conditions
2. to assess whether a change in the C:N:P ratio and lipid composition of its food source have negative repercussions on fitness, growth and lipid composition of krill
3. to design and utilize model for projecting future krill population size with changes in biomass and community structure of sea ice algae and phytoplankton communities in response to elevated CO2 concentrations. The model should be designed in such a way that the outcome of this study, regarding alterations in larval krill development due to nutritional changes in primary producers, can be incorporated.

Significance and innovation:
The significance of this study lies in improving predictions of krill annual biomass, which is vital for setting allowable catch rates (Quetin and Ross 1991). The innovation of this project lies in investigating krill growth and fitness  under the effect of a change in food source quality in a high CO2 ocean. Under the prediction of decreasing sea ice cover and duration, the integrated approach of testing bottom sea ice algae as well as water column phytoplankton species will help to better assess the possible outcomes of varying future CO2 emission scenarios for krill. This study also aims to assess whether nutritional changes in primary producers are a determining factor for krill population size.