By the end of this century important krill habitats in the Southern Ocean off East Antarctica are likely to be unsuitable for krill reproduction, if carbon dioxide (CO2) emissions continue unabated.
Worse still, the entire Southern Ocean krill population could collapse by 2300 if ocean acidification, caused by increasing amounts of CO2 dissolving in seawater, extends throughout the crustaceans’ habitat.
These grim projections, along with risk maps showing krill reproductive success under different CO2 emission scenarios, were published in Nature Climate Change in July by Australian Antarctic Division scientists and their Japanese colleagues.
The findings are not only disastrous for krill; they have negative implications for the survival of marine mammals and seabirds that rely on them for food.
For more than five years the Antarctic Division’s krill biologists, So Kawaguchi and Rob King, have been studying the effect of ocean acidification and other climate-change related stressors on Antarctic krill (Euphausia superba) reproduction and development.
For this study they exposed Antarctic krill eggs to six different CO2 levels (equivalent to 380, 1000, 1250, 1500, 1750, 2000 parts per million or ppm*) until they hatched.
The current atmospheric CO2 concentration is about 400 ppm, but in some regions of the Southern Ocean the current maximum CO2 concentration below 200m depth reaches 550 ppm. CO2 levels in deep water will continue to increase as atmospheric CO2 increases, changing the chemistry of the water and increasing its acidity.
‘We found egg hatch rates significantly decreased at CO2 levels of or above 1250 ppm, with almost no hatching at 1750 and 2000 ppm,’ Dr Kawaguchi said.
‘We also found that embryonic development was significantly impaired if the eggs were exposed to 1750 ppm CO2 during the first three days following spawning, even if the eggs were returned to lower CO2 conditions after three days.’
This finding is particularly critical as krill eggs sink from the surface and hatch at 700–1000m, where CO2 levels are higher than surface waters and current atmospheric levels. The larvae then have to swim back to the surface to feed. A delay in their development would compromise their ability to do this or even prevent them from reaching the surface before their energy reserves were exhausted.
To find out what sort of ocean CO2 profile we can expect in the future, Dr Kawaguchi’s Japanese colleagues used a three-dimensional ocean circulation model to project CO2 levels in the Southern Ocean under the Intergovernmental Panel on Climate Change’s four Representative Concentration Pathway (RCP) atmospheric CO2 scenarios (RCP 8.5, 6.0, 4.5 and 3.0 — from no emission mitigation to strong mitigation).
‘The resulting risk maps showed that much of the present habitat for krill will be at damagingly high CO2 levels of above 1000 ppm by 2100 under RCP 8.5, or by 2300 under RCP 6.0,’ Dr Kawaguchi said.
‘Under RCP 4.5 and 3.0 the risks were minimal, with a reduction in hatching success of less than 10% by 2300.’
Dr Kawaguchi said the results of the study should be incorporated into existing models of the dynamics of krill populations, to allow assessment of the regional impacts of ocean acidification on the reproductive success of these populations. These results can then be used to inform ecosystem and krill fishery management models under different carbon emission scenarios.
However, Dr Kawaguchi warned that the findings might well be conservative due to the cumulative effects of increased CO2 across life stages and generations, and other stressors such as increasing seawater temperature and sea ice decline in some parts of the Southern Ocean.
Corporate Communications, Australian Antarctic Division
*In seawater CO2 is measured in micro-atmospheres (µatm). When seawater is bubbled by air containing a certain concentration of CO2 in parts per million (e.g. 400 ppm) the seawater bubbled by that air will equate to the same value expressed in micro-atmospheres (i.e. 400 µatm).
Kawaguchi S., Ishida A., King R. et al. Risk maps for Antarctic krill under projected Southern Ocean acidification. Nature Climate Change 7 July 2013 (doi:10.1038/nclimate1937).