Global warming causing rapid changes in the Arctic

Cogombre de mar menjant algues Melosira arctica

A study published in Science shows for the first time that global warming and its physical effects are causing alterations in the ecosystem of the Central Arctic, from surface to depth, more quickly than previously thought. The thinning and eventual melting of the ice cover leads to considerable growth of algae, which end up on the sea bed, where rapid biological processes reduce oxygen levels. A team of researchers led by the Alfred Wegener Institute (AWI, Germany), including two researchers from the Institute of Environmental Science and Technology (ICTA), discovered this in situ in the summer of 2012, when Arctic ice cover reached a record minimum.

14/02/2013

When the researchers arrived in the central Arctic in the summer of 2012 on board the German icebreaker Polarstern, they found huge quantities of the algae Melosira arctica, a type of alga that grows on the underside of ice, and determined that it was responsible for at least half the primary production in the area- the production of organic material from carbon dioxide. When the ice melted, these algae sank rapidly to the sea-bed, at over 4,000m., and attracted large numbers of sea cucumbers and brittle stars, which fed on them. What was left was then metabolised by bacteria, consuming the oxygen in the sea-bed. The article published in Science describes the changes taking place in the sea ice and their effects on ocean productivity and the Arctic ecosystem.

Researchers from 12 different countries took part in the expedition - among them the ICTA PhD students Montserrat Roca and Viena Puigcorbé - using a large number of ultra-modern research devices, such as camera-guided sampling devices and an under-ice remotely operating vehicle (ROV). The researchers were able to study the algal deposits, up to half a metre in diameter and covering up to 10% of the sea-bed, using an Ocean Floor Observation System (OFOS). For the first time in the ice-covered Arctic, bacterial and faunal oxygen consumption could be measured directly in the deep sea using micro-sensors, and considerable biological activity was found under the algae cover. The bacteria had begun to decompose the algae, as evident from a greatly reduced oxygen content in the sediment, while the algae-free areas were aerated down to a depth of 80 cm.

“It has been known for some time that diatoms of the type Melosira arctica can form long chains under the ice. However, such a massive occurrence has so far only been described for coastal regions and old, thick sea ice" say the researchers. When planning the expedition three years ago they proposed the hypothesis that this type of alga could grow faster under the thinning sea ice of the Central Arctic. The observations now published in Science confirm this: These algae were responsible for almost half of the primary production in the Central Arctic Basin. The rest is attributed to other diatoms and nanoplankton, which live in the upper layers of the ocean.

Normally, the small phytoplankton cells sink very slowly through the water column and are largely consumed when still within the ocean surface layer. By contrast, the long chains of algae formed by Melosira arctica are heavy and can quickly sink to the bottom of the sea. Just before the expedition, these algae exported more than 85% of the carbon fixed by primary production from the water surface to the deep sea. The researchers suppose that the algae had actually grown recently because they found only one-year old ice in the Central Arctic, and because the algae extracted from the guts of sea cucumbers were still able to photosynthesise when examined in the ship’s laboratory. The good nutritional state of the sea cucumbers was also evidence of the massive food supply. It was found that the animals were larger than normal and with highly developed reproductive organs – an indication that they had been eating abundantly for some two months.

The sea ice experts on board investigated why ice algae are able to thrive beneath the thinning Arctic sea ice, and how they may also lose their habitat quickly due to the increasing ice melt. They determined the ice thickness by ice drillings and with an electromagnetic probe dragged by a helicopter. They also used an underwater robot (ROV) to view the ice from below and to measure how much light penetrated through the ice. At the end of the summer they still found a lot of ice algae remains, and were able to quantify them. The increasing ice melt allows more light to permeate the ice, and makes the algae grow faster. However, since the ice has become much thinner in recent years, and the Arctic much warmer, the ice algae melt out more quickly from the ice and eventually sink.

The deep sea has so far been seen as a relatively inert system affected by global warming only with a considerable temporal delay. But thanks to this project “We were able to demonstrate for the first time that the warming and the associated physical changes in the Central Arctic cause fast reactions in the entire ecosystem down to the deep sea“, summarises Antje Boetius, of the AWI and the Max Planck Research Institute, the project leader.

The fact that microbial decomposition processes fuelled by the algal deposits can generate anoxic spots in the deep sea floor alarms the researchers. “We do not know whether we have observed a one-time phenomenon in this part of summer or whether it will continue in the coming years.“

Current predictions by climate models assume that an ice-free summer could occur in the Arctic in the next decades. Boetius and her colleagues warn: “We still understand far too little about the function of the Arctic ecosystem and its biodiversity and productivity, to be able to estimate the consequences of the rapid sea-ice decline.“

ICTA's research in the Arctic

Montserrat Roca i Viena Puigcorbé took part in the study published in Science, investigating the effects of the fluctuating central Arctic ice cover on the export of organic carbon from the surface of the water column to deep waters, and the efficiency of sea ice in intercepting and accumulating atmospheric flows, using radioactive sensors of natural origin. The research forms part of their Ph.D theses, directed by Professor Pere Masqué, of the UAB's Department of Physics and ICTA.

"Working with a multidisciplinary team of more than 50 researchers, over 10 weeks, and in the middle of the Arctic, has been a great experience. This study shows the importance of polar research in the present climatic context. We are convinced that such an ambitious project will produce more findings that will help to understand the complexity of the Arctic ecosystem and the consequences of the changes taking place there", concludes the ICTA PhD researcher Montserrat Roca.

Original publication:
http://www.sciencemag.org/lookup/doi/10.1126/science.1231346

Image:
Sea cucumber ingesting ice algae in the Central Arctic (AWI-Antje Boetius).