Ecological interactions modulate responses of two intertidal mussel species to changes in temperature and pH

Predicted climate change scenarios for the end of this century suggest a substantial warming and ocean acidification of sea surface waters. Physiological performance of marine coastal organisms will be affected leading to changes in whole communities. In particular, ocean acidification poses an important threat for shelled molluscs. Nevertheless, climate change impacts are species-specific and responses may even differ among species that play a similar role in the ecosystems. Currently, two mussel species, the native Mytilus galloprovincialis Lamarck 1819 and the non-indigenous Xenostrobus securis Lamarck 1819, share the same habitat along shores in the inner part of the Galician Rias Baixas (NW Spain) forming mixed patchy clumps. Here, we used an experimental approach in mesocosms to evaluate the effects of increasing temperature and acidification on mussel clumps of different composition (monospecific and mixed clumps). Our model systems were artificially constructed clumps resembling those found on rocky shores. Responses to environmental stressors were measured on several functioning variables at the individual (i.e. survivorship, growth, condition index and composition of shell) and clump level (i.e. respiration, ammonia and phosphate excretion rates). The decrease of pH had a lethal effect and reduced growth on the native mussels. The invader was more resilient although individual responses were very often shaped by the composition of the clump. The reduced pH and, especially the high temperature conditions, tended to cause an increase in respiration rates of all types of clumps. Interactions between the two species seemed to modulate many effects at both individual and clump levels highlighting the complexity of responses when considering multiple stressors.