Effects of ocean acidification on pelagic organisms and ecosystems - Ocean Acidification - page 116

CO 2 -dependent increase in nitrogen i xation), com-
munity-level responses may be signii cantly differ-
ent from those observed in pure culture. Such
discrepancies could result, for example, from 'eco-
logical buffering' processes, whereby species
replacement and selection leads to functional stabil-
ity and resilience. These processes have not been
studied with respect to ocean acidii cation but have
been documented experimentally in terrestrial eco-
systems (Tilman and Downing 1996).
A full understanding of ecosystem responses will
require knowledge of how multiple trophic levels
respond. For example, hypothetical CO 2 -dependent
increases in the growth rates and productivity of
picoeukaryotes could be counteracted by a com-
pensatory increase in microzooplankton grazing
rates. For this reason, it is important to consider not
only the temporal scale of the experiments, but also
the spatial scale. Typical ship-board manipulation
experiments use incubation bottles ranging in vol-
ume from about 100 ml to 4 l, which may be too
small to adequately sample larger zooplankton spe-
cies. In this regard, mesocosm studies provide a
much better ecological approximation of natural
plankton assemblages. Yet the results of even these
large and logistically challenging experiments may
not scale up easily to a basin scale. For example,
enclosed mesocosms do not allow for the horizontal
advection of organisms, which may be better
adapted to newly created ecophysiological niches.
Given all the uncertainties in the ecological and
biogeochemical responses of marine systems to
increased p CO 2 and decreased pH, it may seem
premature to begin parameterizing quantitative
models. Yet these models can provide signii cant
insight into the potential sensitivities of biogeo-
chemical systems to various perturbations, for
example a change in the C:N stoichiometry of sink-
ing POC. In addition, models provide help in deter-
mining the magnitude of potential physical and
chemical changes used in manipulative experi-
ments (Boyd and Doney 2003; Sarmiento et al.
2004). Through a concerted and collaborative effort
over the coming decade, it should be possible for
ocean modellers and experimentalists to begin
understanding some of the consequences of
increased oceanic CO 2 on marine ecosystems and
biogeochemical cycles. This effort will require a

sophisticated approach which takes into account
the tremendous complexity of marine ecosystems
and the interaction of multiple driving forces which
shape biological responses over a range of spatial
and temporal scales.

6.7 Acknowledgements

U.R. gratefully acknowledges funding received as
part of the EU integrated project EPOCA (European
Project on OCean Acidii cation) and the coordinated
project BIOACID (Biological Impacts of Ocean
ACIDii caton) by the German Ministry for Education
and Research (BMBF).

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