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Published: April 1, 2009
First published in IGBP's Global Change Newsletter Issue 73, April 2009

Mechanisms linking climate to ecosystem change: physiological background and ecological implications

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The Ocean in a High-CO2 World: Science highlights from the symposium
Hans O. Pörtner
Alfred-Wegener-Institute for Polar and Marine Research
Integrative Ecophysiology,
Bremerhaven, Germany
hans.poertner@awi.de
Climate change causes ocean warming and acidification on global scales. In contrast to well established effects of warming, evidence for the effects of rising carbon dioxide (CO2) on marine ecosystems is only just emerging. However, future scenarios also indicate threats to marine life through combinations of rising CO2 levels, warming and more frequent oxygen depletion (hypoxia) in the ocean. There is a need to understand the causes and effects of realistic future climate scenarios on ecosystems. We need to identify key physiological mechanisms and their responses to combined effects of progressive acidification, warming and hypoxia. In the changing ocean, these are physiological mechanisms which define species performance, including their capacity to interact, e.g. in food webs [1]. Many current ecosystem changes likely occur when ambient temperature drifts beyond the species-specific temperature limits of survival (thermal tolerance window) and causes a shift in phenology resulting in the species no longer being able to survive in this location. High sensitivity to elevated CO2 levels may involve a low capacity for acid-base regulation, as seen in lower marine invertebrates [2]. The disturbed extracellular acid-base status affects processes involved in growth, calcification, neural functions, blood gas transport and behavioural capacities [2]. Metabolic pathways shift to new equilibria. Current evidence indicates elevated sensitivity to higher CO2 levels towards the extremes of thermal windows [3]. The ultimate consequence may be a narrowing of thermal tolerance windows and associated ranges of geographical distribution and of the performance at ecosystem level. Thus, CO2 may exacerbate warming effects on marine ecosystems. Future research will have to test these concepts under realistic climate and ocean acidification scenarios and in various marine ecosystems between the tropics and the poles.
Conceptual model of CO2 dependent effects on species interactions at ecosystem level (modified after [1]). Species differ in their thermal windows of performance and coexist where these windows overlap (left). Changes in species interactions are elicited by warming and also by the specific sensitivity of species to ocean acidification under elevated CO2 levels. The narrowing of thermal windows and differential loss of performance will affect coexistence ranges, relative performance, and thus the patterns of competition and susceptibility to predation (right).
References
1. Pörtner, H.O., and A.P. Farrell. 2008. Physiology and climate change. Science 322: 690-692.

2. Pörtner, H.O. 2008. Ecosystem effects of ocean acidification in times of ocean warming: a physiologist’s view. Marine Ecology Progress Series 373: 203-217.

3. Metzger, R., F.J. Sartoris, M. Langenbuch, and H.O. Pörtner. 2007. Influence of elevated CO2 concentrations on thermal tolerance of the edible crab Cancer pagurus. Journal of thermal Biology 32: 144-151.

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