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

suggesting that climate rather than change in land-use is a major forcing factor in
dust transport (Moulin et al. 1997).
In Asia, after a long period of stability or negative trend in the dust event frequencies,
recent observations showed an upsurge of heavy dust events during the last 3-4
years, with a monthly peak in March-April (Kurosaki and Mikami 2003). The reason
for such an outbreak in dust storms is still under investigation, although greater
importance is given to a change in climate (Sugimoto et al. 2003).
Atmospheric inputs of nutrients to the coastal system and the ocean at large can take
place through dry and wet (i.e. rain) deposition, and is not restricted to iron and other
trace elements. As for river runoff, the atmospheric input of macro-nutrients have
increased by a factor of at least two above natural levels due to human activity and
climate change (Cornell et al. 1995). According to Guerzoni et al (1999), the
atmospheric input of inorganic nitrogen represents 60% of the total nitrogen entering
the Mediterranean from continental origin, 66% of that flux is through wet deposition.
Bio-available dissolved organic nitrogen (e.g. urea) is also an important constituent
observed in marine rainwater samples and contributes to enhanced marine primary
production (Cornell et al. 1995, 2001; Bishop et al. 2002).
Transport via the atmosphere is also recognized as an important route for reactive P
(Garrison et al. 2003). Unlike N compounds which have dominant anthropogenic
sources (urban pollution), the aerosol P content is of continental / natural origin (e.g
rock and soil) as it often correlates with the calcite content (Herut et al. 1999).
Regional differences in the aerosol content of bio-available macro-nutrients
contribute to a change in the N:P ratio in coastal waters with possible shift from a
nitrogen-limited ecosystem to a phosphorus-limited one, such as is observed in the
Eastern Mediterranean (Herut et al. 1999, Kouvarakis et al. 2001).
IV.C.5. Upwelling
Sites of coastal upwellings are of economical importance due to high productivity
sustaining large biodiversity and valuable fish resources. Upwelling events are
driven by along shore wind stress inducing offshore transport of surface waters,
replaced then by deeper cold water enriched in macronutrients, iron and inorganic
carbon. Coastal upwellings are especially intense along the eastern boundary
currents. Four main systems are commonly differentiated, contributing to 80-90% of
the total new production: Northwest Africa-Iberia, Southwest Africa-Benguela, Peru-
Chile, and California-Oregon. A direct link to meteorological forcing would expose
upwellings to higher sensitivity to climate disturbances. Warming is associated with a
stronger pressure gradient between land and ocean, which in turn, reinforce
alongshore geostrophic wind, hence coastal upwelling. Bakun (1990) highlighted a
possible mechanism whereby global greenhouse warming could accelerate coastal
upwelling through multi-decadal intensification of the alongshore wind stress
observed at all upwelling sites (Figure IV.C.7).
Several lines of evidence (Schwing and Mendelssohn 1997; Snyder et al. 2003) have
now confirmed Bakun’s hypothesis showing an increasing strength of the seasonal
upwelling in the California Current system over the last 30 years, and an extension of
the phenomena through late fall. The ecological consequences of higher upwelling
strength are still uncertain. Primary productivity would be significantly stimulated,
galvanizing then the rest of the food chain. In the South African Benguela system,
Verheye (2000) measured a 100-fold increase in zooplankton numerical abundance
as a long-term biological response to intensified coastal upwelling. Higher
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