Areas of the Arctic Ocean where phytoplankton blooms may occur. (Graphic: Christopher Horvat)
Read aloud Climate change gnaws on the Arctic sea ice. The once thick layers of ice are getting thinner and thinner. This trend is apparently paving the way for unexpected life: as more sunlight penetrates the frozen mass, phytoplankton is spreading beneath the ice. Model calculations show that such algal blooms may have been part of everyday life in the Arctic for years. Its role for the climate and the Arctic ecosystem could thus have been drastically underestimated.

Phytoplankton plays a crucial role in the oceanic ecosystem: the tiny algae and cyanobacteria live in the light-flooded zones of the oceans, where they drive biological, chemical and geological cycles. For example, the organisms absorb carbon dioxide for photosynthesis and release oxygen into the atmosphere. As soon as they die and sink to the bottom, they dump a good portion of the greenhouse gas taken into the depths of the ocean - and thus regulate the climate. In addition, plankton, with its growth and decay, forms the basis of the marine food web.

The multiplication of small organisms takes on an almost dramatic scale at regular intervals. The plankton can then be recognized as a green, blue or red algae bloom on the sea surface. Also in the Arctic mass growth occurs every year - always where the ice retreats. However, as long as ice sheets cover the Arctic Ocean, life dependent on photosynthesis is impossible. Because through the ice does not penetrate enough sunlight. That's why scientists would never have come up with the idea of ​​looking for an algae bloom under the ice. In July 2011, however, they discovered just that: quantities of phytoplankton in a fully ice-capped region of the Chukchi Sea. Since then, experts have been puzzling over the question: Was this observation an isolated case - or have conditions in the Arctic changed so much that such algal blooms become more common? For example, thinning ice could be one reason for the strange phenomenon.

Optimal lighting conditions

Christopher Horvat from Harvard University in Cambridge and his colleagues have now come to the search for an answer. Using model calculations, the researchers analyzed how light conditions under the Arctic ice have changed over the past three decades. In doing so, they also took into account the formation of so-called melt-pool - water accumulations, which are formed by melting snow and ice in warmer months on the ice surface and allow more light through. The result: Algae blooms, as in the Chukchi Sea, seem to be part of everyday life in today's Arctic - and have been for twenty years. The growth of phytoplankton could have been particularly strong during the last ten years. Every July, almost 30 percent of the Arctic region has optimal light conditions for algal blooms under the ice, the researchers report. That was not the case twenty years ago.

For Horvat's team, the driving force behind this development is clearly global warming, which is causing the ice in the Arctic to become increasingly thin. The occurrence of melting ponds still enhances this effect. Whether the altered light conditions have actually led to more phytoplankton, would now confirm further analysis, the researchers. For example, they did not take into account the distribution of nutrients under the ice during their calculations. But even this is an important factor for the multiplication of tiny organisms in addition to the light. However, if the results are confirmed, the role of the plankton in the climate and the Arctic ecosystem must be reassessed, including with a view to the future: "Forecasts assume that the Arctic sea ice will become even thinner in the future. This means that phytoplankton blooms could become even more common, "concludes the team. display

Original work of the researchers:

  • Science Advances, doi: 10.1126 / sciadv.1601191
© - Daniela Albat
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