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

atmosphere (Liu et al. 2000). With increasing atmospheric CO2, coastal waters, now
under-saturated with respect to CO2, has turned into a net carbon sink. In an attempt
to include the continental shelf pump into a global circulation model, Yool and
Fasham (2001) estimated a net uptake of 0.6Gt C.yr -1 imputable to the continental
shelf pump. Even in river-dominated coastal systems like the East China Sea,
Tsunogai et al. (1999) have estimated a mean air-to-sea flux per unit area of 35
gC.m -2 .yr -1 . In comparison, the Baltic Sea absorbs 11 gC.m -2 .yr -1 (Thomas and
Schneider 1999). High biological activity stimulated by increasing input of nutrients,
on the one hand, and the decoupling in time and space of the production and
respiration processes, on the other hand, have been highlighted by Thomas et al.
(2004) as main controlling factors of the water pCO2 in the North Sea, characterized
by a net sink of 8.5 TgC.yr -1 , most of it being exported to the North Atlantic Ocean.
Frankignoulle and Borges (2001) estimated an annual sink of ca. 90 to 170 Mt C for
the European continental shelves with, however, a large variability in time and space
depending on the physical conditions (temperature, stratification) and the duration of
the phytoplankton growing season. According to the authors, situations of pCO2
super-saturation in coastal waters (i.e. source of atmospheric CO2) can occur, for
example, in winter time with low productivity, in direct influence by rivers with
supersaturated water, in period and/or areas of intense vertical mixing and sediment
re-suspension. In the well-mixed English Channel, CO2 under-saturation only occurs
from May to July when light is optimal for primary production (Frankignoulle and
Borges 2001). Under global warming and increased stratification, the undersaturation
period may increase due to enhanced primary production in the upper
layer.
As important as the seasonal cycle to identify the continental margins as sink or
source of CO2, the spatial distance from the coast contributes to the debate on the
trophic status of the coastal system. According to Gattuso et al. (1998, also in
Frankignoulle and Borges 2001), proximal coastal areas directly under the influence
of terrestrial inputs may often be considered as net heterotrophic systems with a net
efflux of CO2 to the atmosphere. On the contrary, distal continental shelves are
autotrophic systems, acting as a net sink of CO2. The impact of climate change on
the carbon budget of coastal systems will thus depend on the regional manifestations
of the dominant forces (e.g. higher runoff, intensification of the wind field).
Large inputs of POC and high turbidity commonly set upper estuaries upstream of
the turbidity maximum zone (TMZ) where freshwater meets seawater, as net
heterotrophic ecosystems with mineralization of POC, resulting in CO2supersaturated
waters. Frankignoulle et al. (1998) showed that European estuaries
emit 30 to 60 MtC.yr -1 to the atmosphere, representing a CO2 source equivalent to 5-
10% of anthropogenic emissions for Western Europe. On the other hand, the outer
estuaries, downstream the TMZ, higher inorganic nutrients from POC mineralization
and less turbidity favour the production of organic matter through photosynthesis,
making the area as a net sink for atmospheric CO2. Körtzinger (2003) measured a
net sink of CO2 of 0.014 PgC.yr -1 in association with the most external zone of the
front created by the Amazon River plume in the Tropical Atlantic Ocean.
IV.C.7. Coastal biodiversity and Ecosystem Shifts
Since 1856, the global mean temperature has warmed by a mean of 0.6°C (IPCC
2001). There is new and stronger evidence that most of the warming observed over
the last 50 years is attributable to human activities. Key questions for biologists and
ecologists are:
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