Blue mussels are a keystone species in the Baltic Sea.
Mytilus mussels are keystone species in the Baltic Sea: they build reefs which provide more complex habitats for various species (macrofauna, macroalgae and meiofauna), filter water (linking the pelagic and benthic systems) and are a staple food source for numerous organisms (eider ducks, flounder, crabs, starfish and the larval stages feed herring larvae and other carnivorous zooplankton). They can also be extremely numerous – forming up to 90% of the animal biomass in some shallow waters.
A recent study by Sanders, Thomsen, Müller, Rehder and Melzner (2021) investigates why these important animals show decreased growth and reduced calcification (shell building) in low salinity areas. These Mytilus mussels use calcium ions dissolved in the seawater to build their shells but increased rainfall (due to climate change) and resulting river discharge into the Baltic Sea dilutes the concentration of calcium ions in some areas as well as lowering the salinity. At the same time ocean acidification lowers the pH of the water. These changes to the chemistry of the water make conditions difficult for mussels – low concentrations of calcium ion and lower salinity mean that the mussels need to spend more energy in building their shells while acidification affects larval soft tissue development, shell construction and the maintenance of a healthy, robust shell. While the Mytilus mussels in coastal areas were better able to cope with these conditions thanks to more food being available and more alkaline fresh water mitigating the impact of acidification, below a certain salinity (<5) these Mytilus mussels are unable to survive.
With climate change driving predicted acidification and desalination in areas of the Baltic it is expected that Mytilus mussels may be displaced by freshwater and brackish species such as the invasive Dreissena mussels. Though Sanders et al. note that further work is needed to assess the ability of Mytilus mussels to adapt to these changing conditions.
The paper “Decoupling salinity and carbonate chemistry: low calcium ion concentration rather than salinity limits calcification in Baltic Sea mussels” by Trystan Sanders, Jörn Thomsen, Jens Daniel Müller, Gregor Rehder, and Frank Melzner, 22 April 2021 is published in Biogeosciences.
by Andrew Pollard