To secure ecosystem properties and services in the future ocean we need to improve our understanding on response of pelagic communities to climate change, in particular to the emerging threat of ocean acidification.
In the recent years, a large effort has been set on elucidating responses to ocean acidification. Studies addressed responses from single organisms to plankton communities in experimental settings and in the ocean. These studies often included additional environmental factors, such as temperature or light.
A direct functional effect of ocean acidification was found for calcification which seems to be the most pH-sensitive process with decline by 25 per cent in projected acidification scenarios of the 21st century. Likewise, reproduction in decapod crustaceans, oyster and bryozoa seemed to be directly harmed when water pH drops below certain thresholds. Similarly, increased growth- and photosynthetic rates have been documented for elevated ocean acidification, most likely promoting photoautotrophic processes in algae with higher carbon assimilation rates in response to increased levels of dissolved CO2. Impacts of ocean acidification on traits such as elemental stoichiometry were also identified.
Ocean acidification and warming as combined stressors caused significant negative effects on calcification, reproduction, and survival of early life history stages of e.g. spider crabs and a significantly positive effect on photosynthesis.
It has to be clarified to what extend observations from experiments can be extrapolated to natural planktonic food webs and large biogeochemical cycles. Further uncertainties derive from the differing experimental set-ups. These have to be quantified in order to evaluate the significance of individual results.
To draw a more holistic picture, an integrative assessment of ocean acidification effects in combination with co-varying factors is urgently needed. Based on a review of empirical insights, more mechanistic process need to be described and substantiate model-based projections of how ocean acidification will impact on pelagic ecosystems and the services these provide.
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