Toxic plankton algae could change the fate of the Arctic food web

A potentially toxic species of plankton algae may take over a critical role in the future of the Arctic food chain, according to a new study from Aarhus University. These algae survive through photosynthesis, but also by eating other algae and bacteria, which gives them a major advantage in the rapidly changing environment of the Arctic.

The plankton community that produces food for the entire Arctic food chain is being transformed by sea ice decline. The base of this food web is formed of tiny plankton algae, which use light and nutrients to produce food through photosynthesis.

As sea ice becomes thinner in the Arctic, plankton algae are exposed to more light. While it seems that this would boost algae numbers and provide more food for fish, the effects are much more complicated. 

The experts explain that more sunlight in the sea will only lead to a higher production of plankton algae if they also have enough nutrients, and this is often not the case. 

Freshwater that is entering the Arctic Ocean from melting glaciers is lacking in nutrients, and this water is now spreading out further across fjords and into the sea. The fresh water sits on top of the denser saltwater, preventing nutrients from the deeper layers from mixing up towards the surface where plankton algae are active. 

This is where the so-called mixotrophic plankton algae come in, because they do not rely entirely on sunlight and nutrients for their survival. The fact that these algae can also obtain energy by eating other algae and bacteria allows them to stay alive and grow without adequate light for photosynthesis. 

In northeast Greenland, a team of researchers measured the production of plankton algae under the sea ice in the high-Arctic fjord Young Sound.

“We showed that the plankton algae under the sea ice actually produced up to half of the total annual plankton production in the fjord,” said study lead author Dorte H. Søgaard.

“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 researchers measured an algal bloom driven by mixotrophic algae in Young Sound for nine days. The algae belong to a group called haptophytes, many of which are toxic. The team found that these algae bloomed in quantities similar to those previously observed in the Skagerrak, where they killed large amounts of salmon in Norwegian fish farms.

“We know that haptophytes often appear in areas with low salinity – as seen in the Baltic Sea, for example. 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,” said Søgaard.

According to the researchers, this is the first time that a bloom of mixotrophic algae has been recorded under the sea ice in the Arctic.

The study is published in the journal Nature: Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer