The Baltic Sea is a physically diverse habitat with a generally low species diversity. The blue mussel Mytilus edulis is widely distributed in the benthic macrozoobenthos of the Western Baltic, the main predators of which are the common sea star Asterias rubens and the shore crab Carcinus maenas. Any environmental stress influencing the predator-prey interactions between these species has the potential to shape the entire ecosystem. The current increase in atmospheric pCO2 causes a concurrent increase in the acidification of seawater and can thus pose such an environmental stress. In coastal habitats and specifically the Baltic Sea, the decrease of seawater pH can be much more pronounced than in the open ocean. In order to estimate possible interaction shifts in the macrozoobenthos under conditions of seawater acidification, this work investigates the effect of an increase in water pCO2 on the predators A. rubens and C. maenas and their consumption of M. edulis. The results of three different own studies show an impact of increased seawater pCO2 around 3500 μatm on growth and mussel consumption in adult sea stars and a seawater pCO2 of around 1200 μatm to impact growth, mussel consumption, scope for growth and righting response of juvenile A. rubens. Mussel sizes consumed, metabolism, NH4+-excretion and calcification were, however, not impacted by an increase in seawater acidification and coelomic pH not regulated by means of active bicarbonate accumulation. Crabs were impacted in metabolism, NH4+-excretion, O:N-ratio and metabolic energy loss at a seawater pCO2 of about 3500 μatm. Mussel consumption was only impacted at a pCO2 of 3500 μatm, over intermediate time spans (10 weeks), but not over a longer (six month) time span. Hemolyph pH was regulated to remain at control levels by active bicarbonate accumulation at intermediate (10 week) time spans over all levels of seawater acidification, while hemolymph pH followed the non-bicarbonate buffer line at the intermediate (around 1200 μatm) level and was only regulated at the high (around 3500 μatm) treatment level over the long (six month) time span. Moulting intervals, growth, mussel sizes consumed, carapace thickness, stability, dry weight and calcification were not influenced by seawater acidification These results indicate a change in feeding pressure on the blue mussel M. edulis under future high levels of seawater acidification. Further, A. rubens appears stronger impacted by seawater acidification than C. maenas and juvenile A. rubens even stronger than adult specimen. I conclude that seawater acidification has the potential to reshape the benthic ecosystem of the Western Baltic. This work therefore helps to understand ecosystem responses to environmental stress and contributes to making predictions on future species distributions in the Baltic Sea.
Appelhans Y. S., 2012. Benthic interactions in a high CO2-world. PhD thesis, Christian-Albrechts-Universität zu Kiel, 124 p. Thesis (restricted access).