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Juveniles of Baltic isopds
Juvenile isopods from the Baltic Sea were exposed to low salinity in a study on climate change effects.
Photo: Luca Rugiu
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Desalination jeopardizes the future of key species in the Baltic Sea

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The marine isopod Idotea balthica is an important part of the Baltic coastal food chain. A new experimental study from the University of Gothenburg suggests that desalination due to climate change impose a severe challenge for this key species. However, genetic variation in tolerance allow for some isopods to cope better than others.

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Baltic isopod on bladder wrack
The Baltic isopod lives in shallow coastal waters and feeds on seaweed and small filamentous algae. The isopod is very common in the Baltic Sea as well as on the Swedish west coast.
Photo: Luca Rugiu

"The Baltic isopod is the most common herbivore in the inner Baltic Sea. It grazes on macrophytes such as bladderwrack and seagrass, and also on ephemeral algae growing on them, which is considered very good since it keeps seagrasses and seaweeds clean. It is also food for many coastal fish, so isopods have a very important role for the Baltic Sea foodweb", says Luca Rugiu at Tjärnö Marine Laboratory. 

Harsh environment for many species

The low salinity makes it difficult for both marine and fresh water organisms to live in the Baltic Sea. Only a handful of species have managed to adapt. The Baltic isopod Idotea balthica is one of them, and any change in its abundance or distribution might have consequences for the Baltic Sea ecosystem.

"Our study indicates that the future decline in salinity of the northern Baltic Sea will have a clear negative effect on the survival of Idotea, as the juvenile stages will cope poorly with such environmental change", says Luca Rugiu.

The experiment did not directly study effects on the food web. But loss of a very abundant species in this species poor environment could make a big difference.

"Sure enough, many migth starve", says Luca Rugiu.

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Luca Rugiu.
Luca Rugiu is postdoctor at Tjärnö Marine Laboratory.
Photo: Susanne Liljenström

Genetic variation gives hope

An interesting result is that the response to hyposalinity varied between families, implying there is a genetic variation in tolerance.

"This means that, within populations, some individuals will survive and potentially give birth to more survivors. Hopefully, this might help these population to adapt with the passing of many generations", says Luca Rugiu.

Another key species in the Baltic Sea ecosystem is the bladder wrack. This habitat forming species provides food and shelter to Idotea and other invertebrates, as well as cod and other fish. Research shows that bladder wrack too is challenged by the future desalination, but just like in the case of the isopods, genetic variation in tolerance might make some populations more fit than others.

"In my opinion, incorporating genetic diversity as a measure of the potential impact of climate change on marine species will give a much better resolution and tools for decision-makers. For instance, monitoring genetic variation allows to identify sensitive populations as well as more tolerant ones that can carry the species into the future.", says Luca Rugiu.

Text: Susanne Liljenström

Isopod facts

The Baltic isopod, Idotea balthica, lives among seaweed and seagrass in shallow water on both the Swedish west and east coasts. They graze on small, filamentous algae but also on the macrophytes. Sometimes the isopods become extremely abundant, resulting in the complete removal of wracks in that area. Isopods is an important food source for coastal fish.

Read the article
The study Climate change driven hyposalinity as a selective agent in the littoral mesoherbivore Idotea balthica, is a collaboration between University of Gothenburg and Turku University. Authors are Luca Rugiu and Pierre de Wit from University of Gothenburg, and IIita Kostian and Veijo Jormalainen from Turku University. Published in Marine Environmental Research, Volume 163, January 2021.