Will less sea ice mean hungry krill?

Sea ice provides the productivity engine for the ice-affected regions of the Southern Ocean. The ice effectively blocks out the light to the underlying water column. So plant life, in the form of microscopic algae called diatoms, is limited to living on and within the sea ice. These plants are the only food available for the entire ecosystem over the long winter months. Thus they control the abundance of key grazers such as krill and other zooplankton. They in turn provide the main food source for all other animal life.

Each year sea ice extends to cover over 10% of the world’s oceans, which around Antarctica is 20 million km2. The sea ice microalgae are thought to contribute between 25% and 50% of the total primary productivity. With global warming reducing the extent of Antarctic sea ice, it is likely that the capacity of the Southern Ocean to support its large biomass of whales, seals and seabirds will reduce.

A team of researchers from the Institute of Antarctic and Southern Ocean Studies (IASOS) at the University of Tasmania have developed innovative methods of measuring photosynthesis to quantify sea ice productivity. Previous techniques have involved removal of samples, melting of the ice and hence destruction of the intricate micro habitats. The new methods use state of the art technology to measure sea ice photosynthesis in situ, using imunoassays, micro electrodes, micro manipulators and fibre optic fluorometers. Equipment is deployed through holes in the ice. Arms move the equipment away from the hole to remove the sensors from the effect of light coming through the hole. The equipment moves up to the under surface of the ice and the microsensors are moved rapidly up and down in the 1–2mm beneath the ice taking measurements at 10µm (ie one hundredth of a mm) intervals. The electronic equipment was operated from a small tent on the ice. These measurements allow the oxygen flux, and hence photosynthesis, to be measured in situ and in real time. Measurements are made throughout the day and night so the effects of changing daylight can be assessed.

Productivity studies were carried out in the vicinity of the Mertz Polyna, eastern Antarctica, in October and November 2002. Conditions were mostly fine and there was a mix of young ice < 20cm thick and older ice 60–100cm thick. Many measurements of productivity were successfully taken and these are contributing to building up a picture of the role sea ice production in Eastern Antarctica. Productivity levels in the area were high although the biomass was generally quite low. We also found that the microalgae were able to rapidly adjust to the changing light and salinity conditions associated with the ice melting and that they were well suited to their changing environment and were very efficient photosynthesisers. These measurements will contribute to providing the basis for determining the effects of future climate change. A reduction in quantity of ice algae or a change in the species composition is likely to lead a reduction in the amount of food available for krill and other grazers. This hypothesis will form a major element of the research program of the new ACE CRC.

Andrew McMinn
University of Tasmania & Antarctic CRC