As a result of climate change, the sea ice is shrinking in the Arctic, which in turn has an impact on the plankton community that forms the foundation of the entire marine food web.
Researchers from Aarhus University have found that that a potentially toxic species of plankton algae that survives through a combination of photosynthesis and food absorption may play a key role in the future of the Arctic Ocean.
Microscopic plankton algae need light and nutrients to produce food by photosynthesis. The once thick sea ice manages this by reducing the amount of sunlight that penetrates into the water, and so stops the algae getting enough light. However, as the sea ice thins, more light is reaching the plankton algae.
However, this will only result in a higher production of plankton algae if they also have enough nutrients. Due to the increase in freshwater melt from Arctic glaciers into the Arctic Ocean, there is an increasing amount of fresh and nutrient-depleted water flowing into the sea. The fresher water sits on top of the saltier ocean water and hinders nutrients from the deeper layers from mixing up towards the surface where there is light. Plankton algae can only be active near the surface.
The new study focuses on how mixotrophic plankton algae may play a crucial role in the production of food in the Arctic Sea. Mixotrophic algae are small, single-celled plankton algae that can perform photosynthesis but also obtain energy by eating other algae and bacteria. This allows them to stay alive and grow even when their photosynthesis does not have enough light and nutrients in the water.
“We showed that the plankton algae under the sea ice actually produced up to half of the total annual plankton production in the fjord,” says Dorte H. Søgaard from the Greenland Climate Research Centre, Greenland Institute of Natural Resources and the Arctic Research Centre, Aarhus University, who led the study. “Mixotrophic plankton algae have the advantage that they can sustain themselves by eating other algae and bacteria as a supplement to photosynthesis when there isn’t enough light. This means that they are ready to perform photosynthesis even when very little light penetrates into the sea. In addition, many mixotrophic algae can live in relatively fresh water and at very low concentrations of nutrients – conditions that often prevail in the water layers under the sea ice in the spring when the ice melts.”
The research team analysed an algal bloom driven by mixotrophic algae occurring under thick but melting sea ice in Young Sound during the Arctic spring in July. During this time, the sun gained strength and more melt ponds spread across the sea ice, which gradually let through more light.
The algae belong to a group called haptophytes. Many of these algae are toxic, and in this study they bloomed in quantities similar to those previously observed in the Skagerrak near southern Norway. In this case, the toxic plankton algae killed large amounts of salmon in Norwegian fish farms. They highlight the fact that it is the first time that a bloom of mixotrophic algae has been recorded under the sea ice in the Arctic.
“We know that haptophytes often appear in areas with low salinity – as seen in the Baltic Sea, for example,” added Søgaard. “It is therefore very probable that these mixotrophic-driven algae blooms will appear more frequently in a more freshwater-influenced future Arctic Ocean and that this shift in dominant algae to a mixotrophic algae species might have a large ecological and socio-economic impact.”
For information regarding co-authors and to read the full study, click here.
Photography by Lars Chresten Lund Hansen and Dorte H. Søgaard.
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