Ecosystem effects of ocean acidification include those on metazoan life. However, while ecosystem effects of warming trends have clearly been identified, those of ocean acidification are still equivocal. Within the next decades, elevated CO2 levels are expected to affect marine water breathing animals directly through effects on the physiology and performance of the individual organism and indirectly through changes in food web structure. Emerging knowledge indicates that sensitivity to elevated CO2 levels differs between animal phyla and species. It may also differ depending on geographical latitude and associated climate conditions. Effects may be large and potentially detrimental especially in life forms with a low metabolic rate, for examples among calcifying benthic macroorganisms (Wood et al., 2008) or in the deep sea. This hypothesis is in line with recent observations in habitats contaminated by natural CO2 emissions, e.g. in volcanic areas around Ischia (Hall-Spencer et al., 2008). Initial findings suggest decreased growth and enhanced mortality of sensitive species such as among molluscs or echinoderms in response to a doubling of CO2 from pre-industrial levels to 560 ppm (Shirayama and Thornton 2005), a value which is likely surpassed during this century. As effects of ocean acidification are expected on top of those of ocean warming, studying ecosystem effects of OA will thus need to consider both, the direct influence of ocean physicochemistry on individual organisms and species, and also the CO2 dependent modulation of responses to temperature in particular.
For an in-depth cause and effect understanding, it is essential to unravel the physiological mechanisms that define whole organism sensitivity to ocean acidification (e.g. Pörtner et al. 2004, Fabry et al. 2008) and especially those, which synergistically interact with temperature effects on marine organisms (cf. Pörtner et al. 2005). A current hypothesis emphasizes a key role for the capacity of acid-base regulation in defining sensitivity (Pörtner, 2008 for review). Available data indicate that deviations of extracellular pH from its setpoint mediate several of the observed whole organism effects. Theme 2 will investigate how and to what extent these disturbances affect whole animal performance and how acclimation to various CO2 levels can alleviate some of these effects. It will also address to what extent sensitivity to ocean acidification interacts with thermal stresses and is shaped by the specialization of organisms on ambient climate conditions according to latitude.
Effects of anthropogenic ocean acidification on animal communities are expected on medium to long time scales, due to the progressive accumulation of CO2 and due to long generation times. In variable environments (e.g. upwelling areas, Feely et al., 2008) not only the drift in mean physicochemical parameters but also enhanced amplitudes will require consideration in analyses of CO2 effects. For an analysis of the interaction between specialization on various climates on the one hand and sensitivity to ocean acidification on the other hand, physiological studies (of e.g. performance and acid-base regulation) as well as investigations of gene expression patterns and population structure will be carried out in species and populations living in a climate gradient across latitudinal clines. Comparisons of fertilized eggs, juvenile and adult life stages will be essential to identify the bottlenecks of sensitivity throughout ontogeny as well as their physiological background. The physiological principles shaping performance may also control calcification, and thus the shell growth of bivalves and other calcifiers over time. Performance has been shown to link climate change to ecosystem effects of warming (Pörtner and Knust 2007). Performance characters like growth or foraging capacity are likely also involved in multistep processes affecting marine food webs. Here, species-specific responses and sensitivities cause various species of an ecosystem to be affected differently, resulting in changes in species interactions, population adaptability, food web structure and associated carbon fluxes. The work will include species of coastal areas and relevant to fisheries and other marine services. Theme 2 also includes approaches to test and further develop mechanistic concepts and models of effects of ocean acidification. This includes kinetic modelling of the mechanisms of ion and acid-base regulation for an improved quantitative understanding of effects, to generate a basis for a more comprehensive, mechanism based modelling approach.