Krill at risk in a warming, acid ocean

Krill face an uncertain future with new research showing that ocean acidification adversely affects larval development and survival.

The research follows a recent discovery that ocean acidification – caused by increasing amounts of atmospheric carbon dioxide (CO2) dissolving into the ocean – could kill krill embryos.

Like the embryo experiment, PhD student James P. Robinson, of the Institute for Marine and Antarctic Studies and Australian Antarctic Division, has found that as the amount of CO2 in sea water increases, the survival and successful development of larval krill decreases.

Mr Robinson exposed the early larval stages of krill (nauplius I-II, metanauplius and calyptopis I) to four concentrations of CO2 – 380, 700, 950 and 2000 parts per million (ppm). According to projections based on the Intergovernmental Panel on Climate Change emissions scenarios, CO2 concentrations could rise to between 1000 and 2000 ppm, at the depths inhabited by developing larval krill (between 0–1000 m), within this century.

‘Between 380 and 2000 ppm there was a 54% increase in mortality, a 44% decrease in the proportion of larvae that successfully developed to the calyptopis I stage and a 72% reduction in the number of actively swimming larvae. Mortality also increased by 39% between 950 and 2000 ppm,’ Mr Robinson said.

Krill larvae hatch at a depth of between 700 and 1000 m and gradually ascend to the surface over 3–4 weeks as they develop through the various larval stages. In the nauplius and metanauplius stages the larvae rely on yolk reserves to grow. By the calyptopis I stage they have developed mouth parts which allow them to feed on phytoplankton at the ocean’s surface. Mr Robinson’s experiments showed that the proportion of larvae which survived to the calyptopis I stage, with sufficient swimming ability to complete the developmental ascent, decreased by about 35% at 950 ppm and 66% at 2000 ppm.

‘These results suggest that larval recruitment to the adult population may be significantly reduced by ocean acidification, with the greatest impact observed somewhere between 950 and 2000 ppm of CO2,’ Mr Robinson said.

‘As krill are a keystone species, this would have major implications for Southern Ocean and Antarctic ecosystems and for the sustainable management of the Antarctic krill fishery.’

Mr Robinson will now try to identify the precise ‘tipping point’ (between 1000 and 2000 ppm) at which CO2 will negatively affect krill’s lifecycle and development.

He will also look at the combined effects of rising CO2 and water temperatures on larval development. Krill generally occur in 0.5°C waters, but some sectors of the Southern Ocean are already experiencing higher water temperatures.

In a recent pilot study Mr Robinson found that larvae exposed to 2.5°C and 380 ppm CO2 developed faster and were larger than larvae raised in colder water (0.5oC and 380 ppm), but their numbers were greatly reduced. At 2.5°C and 950 ppm, hardly any larvae survived.

‘As the Southern Ocean is simultaneously expected to warm and become more acidic, this result is quite concerning,’ he said.

Mr Robinson will now investigate the effects of different CO2 concentrations between 950 and 2000 ppm, and an increased temperature of 4.5°C, on the development of embryonic and larval krill. He will also begin examining the potential long-term effects on subsequent generations of krill raised under these conditions.

‘One of the long-term goals of the research is to identify the underlying physiological, biochemical and genetic mechanisms behind the observed responses of krill to ocean acidification and increasing temperatures. It will also be very interesting to see whether krill have the potential to adapt to these conditions over many generations,’ he said.

Throughout his different experiments Mr Robinson will look for unusual morphological (physical appearance) changes in larvae due to environmental effects. He aims to create a detailed guide and description of normal larval krill development using photographic and scanning electron microscope images of the different larval stages. His studies will use a combination of wild krill collected from the Southern Ocean and laboratory-raised krill.

Australian Antarctic Division, 9 November 2010. Article.

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