Climate Change and the European Water Dimension - Agri ...

CO2 2- ). In turn, the alkalinity is a key element controlling the CaCO3 saturation state
of marine waters, through the chemical reaction:
Ca 2+ + 2HCO3- → CaCO3 + CO2 + H2O (1)
The formation of calcium carbonate can also be mediated through the metabolism of
various organisms as they build their exoskeletons and shells (e.g. corals,
coccolithophorids). Note that, in reaction 1, the production of one mole of carbonate
releases one mole of CO2 in marine waters, 60% of which is out-gassed back to the
atmosphere (Ware et al. 1991). Accordingly, the contribution of coral reefs to
atmospheric CO2 has been estimated to be 0.02 – 0.08 Gt C.yr -1 (i.e. 0.4 to 1.4% of
the rate of anthropogenic production from fossil fuel combustion; Ware et al. 1991).
Nevertheless, the calcite-carbon production is estimated at 0.4 to 1.4 Gt CaCO3-C as
an annual sediment flux from the mixed layer (Sundquist 1985). This flux is highly
depending on temperature and pressure, and, as a result, coastal tropical waters
represent the largest pool of CaCO3-C. Occasionally, a significant carbonate flux can
be observed in northern latitudes following blooms of coccolithophorids (Brown and
Yoder 1994). In that case, the calcification process is directly coupled with the
production of organic matter through photosynthesis. The resulting effect on the
carbon flux and the drawdown of atmospheric CO2 depends on the ratio of both
processes. In the North Sea, blooms of Emiliana huxleyi have shown to represent a
net sink of carbon (Buitenhuis et al. 2001).
The short-term-cycling production of organic carbon, so-called primary production,
derives from the light-driven fixation of inorganic carbon, i.e. photosynthesis, by the
phytoplankton cells and other marine plants such as seaweeds and seagrasses
following the reaction:
nCO2 + 2nH2O → (CH2O)n + nH2O + O2 (2)
A global estimate of the coastal primary production may be averaged to 8 GtCyr -1 ,
which represents ca. 20% of the total oceanic primary production (Liu et al. 2000).
Satellite ocean colour provides slightly higher estimates, ca. 12.4 GtCyr -1
corresponding to 24% of the global ocean production (Mélin and Hoepffner 2004).
Per unit area, however, the coastal systems generate at least twice as much
photosynthetic carbon than the open ocean systems, ca. 0.8-0.85 gC.m -2 .d -1 (Mélin
and Hoepffner 2004), with maxima observed within upwelling systems, 1.0 to 1.5
gC.m -2 .d -1 (ca. 0.04-0.06 GtC.yr -1 ) in the Moroccan and Mauritanian shelf waters
(Hoepffner et al. 1999), and in the vicinity of river plumes. Primary production in the
ocean is by far the major process controlling the flux of atmospheric CO2, contributing
to a net sink of 2 PgC.yr -1 .
Both inorganic and organic carbon cycles are affected by environmental forcing,
hence climate change. Future climate scenario (IPCC 2001) predicts an increase of
water pCO2, in response to an increase in atmospheric CO2. As a result, the pH of
surface water will decrease, acting directly on a decrease of calcification rate
(reaction 1). With expected conditions by the year 2100, the ratio of calcification:
photosynthesis for coccolithophores species could be reduced by 23 to 50%
(Riebesell et al. 2000). On the other hand, observations have shown an increasing
abundance of coccolithophore blooms in specific areas, e.g. the Bering Sea (Napp
and Hunt 2001) presumably reflecting changes in water physics and chemistry
(increasing temperature and alkalinity) due to climate anomalies. Such a climate link
has been recently challenged (Merico et al. 2003). Coccolihtophorids, as other
species belonging to the Haptophytes group (e.g. Phaeocystis) are important for their
role in the sulfur cycle and the production of dimethyl sulphide (DMS) which is
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