Southern Ocean Ecosystem Key for Global Climate
16 Feb 2007 - Interviews, Water & Oceans, Antarctic
Christiane Lancelot
© Christiane Lancelot / Christiane Lancelot
Professor Christiane Lancelot is a marine ecological modeller whose work focuses on sea-ice extent and ecosystem dynamics in the Southern Ocean. She is the Principle Investigator of the Belgian research projects BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) and SIBClim (Sea Ice Biogeochemistry in a Climate change perspective). SciencePoles interviewed her on her work and the contribution of Southern Ocean processes to the Earth system and climate.
What do biological processes contribute to sea-ice-atmosphere-ocean interaction in the Southern Ocean, and why is it important?
The role of the Southern Ocean ecosystem is crucial to Earth's climate because of its capacity to absorb atmospheric CO2, a major greenhouse gas. We now know that the Southern Ocean (only about 10% of the Earth's ocean surface) is responsible for uptake of about a third of all anthropogenic CO2 absorbed by the Earth's oceans.
Biological processes play a significant role in modulating this uptake. Biological uptake of atmospheric CO2 depends on the seasonally varying balance between photosynthesis (CO2 absorption) and respiration (CO2 release). Less CO2 is absorbed in winter than at other times as during winter sea-ice formation, micro organisms are trapped in the ice for the lightless winter. When light returns in the spring, micro algae start photosynthesising again and allow development of a microbial food web. During summer, when the ice melts, the micro organisms are released back into the water, continuing to absorb and release CO2, until the following winter.
The interesting thing we've learned recently is that micro organisms have a higher photosynthesis potential in the ice during the spring, than they have in the water during summer. Hence a reduction in the sea-ice extent might result in a drop in CO2 uptake by the Southern Ocean, and this obviously has important implications for the Earth system as a whole.
The reason for the higher photosynthesis potential in the ice during the spring, we now believe, is that the process of sea ice formation means that iron, which is found in smaller quantities in the Southern Ocean than in other oceans, is concentrated in the ice.
Additionally the process of ice retreat releases iron rich fresh water from melting ice. This stratifies the upper layer of the water column and the sea ice algae released at the same time are able to grow in a shallow well-illuminated and iron-rich surface layer, enhancing biological production in the water.
And just how dependent is the Southern Ocean ecosystem on sea ice extent?
Significantly. First, sea ice provides a habitat for micro-organisms such as diatom, bacteria, or protozoa. Second, it also serves as a protective habitat for krill juveniles, which play a crucial role as the link between the lower trophic level (microorganisms) and the higher trophic level (fish, mammals, birds) of the Southern Ocean ecosystem. Krill juveniles find refuge in sea ice pockets, so that krill survival and population size is directly linked to sea ice cover, with direct repercussions all the way up the food chain.
So how and what do biological processes contribute to the Earth's Climate?
They contribute directly through the uptake and release of CO2. Amazingly, until very recently, scientists thought that sea ice was inert in terms of CO2 and other gas exchanges. But since then the porosity of sea ice regarding gas exchanges has been demonstrated as well as the role of sea ice as a carbon sink and source during the spring/summer and winter seasons respectively.
This is because photosynthesis stops during the dark while other "heterotrophic" ocean processes, which release CO2, proceed throughout the year.
And how do the Southern Ocean compare to the Arctic Ocean in this respect. Do both oceans undergo the same fluctuations and processes?
It is still too soon to give you an answer on this, but this is also something that is starting to be studied, especially within the context of the International Polar Year.
What I can tell you, however, is that the Arctic Ocean is different from the Southern Ocean in that it does have a lot of fresh water and material input from rivers across the Arctic Rim. This is something that is not at all the case with the Antarctic and which would normally increase the iron levels in the Arctic Ocean, with direct positive repercussions on biological production, as well as carbon uptake.
Iron belongs to elements essential for cellular machinery. When iron is abundant, the ecosystem is dominated by large species that pump atmospheric CO2. When there is limited iron, as in the remote Southern Ocean, a microbial network dominates, that keeps CO2 at the surface layer.
So what actually regulates the level and production of iron?
Iron can enter surface ocean waters from above (atmospheric dust), from below (upwelled iron-rich waters) or from rivers, such as in Arctic and other ocean. The Southern Ocean's remoteness explains its low iron content.
This is a unique situation, resulting from Antarctica being entirely frozen. Usually, sea water lying closer to the more temperate continents is richer in iron, due to river outflow.
Do you ever test the models?
Yes, we compare the model simulations with long term data collected at different stations across the Southern Ocean, or otherwise with observations made during campaigns and research cruises. Such a data base is crucial for evaluating models' performance and requires a huge international effort by ICED. This international database will archive data obtained in the Southern Ocean and will be shared between nations and research groups.
Finally, how sophisticated would you say are current Southern Ocean models, and what do they tell us about anthropogenic climate forcing and the future?
Models' complexity is really a big question. Any model which represents the environment must have two components: a physical one describing the habitat, and a biological one describing ecosystem evolution. Models' complexity levels must be adequate to represent correctly the ecosystem's functioning and constraints. Today we are working internationally to agree on the minimum complexity level needed to make accurate predictions for the future.
By: Jean de Pomereu
