The neglected role of environmental fluctuations and its implications for ocean acidification research

Chair: Gretchen Hofmann

Christian Pansch, Yvonne Sawall, Martin Wahl
Department of Marine Ecology, GEOMAR Helmholtz Centre for Ocean Research Kiel, 24105 Kiel, Germany

Background
Climate change will not only shift environmental means but will also increase the variability around means and specifically the intensity of extreme events, exerting additional stress to organisms and ecosystems. Environmental fluctuations could, however, also offer transient refuge from stress. While a substantial amount of studies now indicates that environmental fluctuations of carbonate chemistry and other variables can be severe at different temporal and spatial scales, experimental evidence on the effects of these fluctuations is limited. We argue that fluctuating pH/ pCO2 stress will very differently affect marine organisms and communities as compared to constant stress of the same mean.

Methods and Findings
We will demonstrate the relevance of environmental fluctuations and will review the ocean acidification literature addressing this topic. Our hypothesis will be underlined by own examples from 5 years of observational as well as experimental research on different Baltic Sea habitats and populations of invertebrate species and their responses to elevated pCO2. In macrophyte stands, carbonate chemistry varies severely with fluctuations of more than 1.5 pH unit (2500 µatm) on a daily basis during late summer. Seasonal natural acidification with annual mean values of 700 µatm pCO2 suggests adaptation of local populations. We demonstrate that for example barnacles from this naturally acidified and strongly fluctuating habitat display a much stronger resistance to elevated pCO2 than barnacles from a more stable habitat. We will also present data on the ability of macrophytes to buffer pH stress from ocean acidification.

Conclusions
By including the new aspect of environmental variability into experimental assessments, and by elucidating detrimental as well as beneficial effects of fluctuations, the ocean acidification community will be able to substantially improve predictions of future ecosystem shifts in response to global changes.