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

and organic material more rapidly to the coastal waters. Accordingly, the total
dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP)
loadings can be scaled directly to functions of the population density and runoff
magnitude (Smith et al. 2003). This provides global estimates of the present DIN and
DIP fluxes to coastal waters that have doubled or even tripled during the past several
decades (Mackenzie et al. 2002, Smith et al. 2003). Future projection adopting
business-as-usual scenarios with sustained perturbation on land from human
activities and from increasing global temperature will maintain the trends in the same
direction with increasing delivery of reactive nitrogen and phosphorus to the global
coastal margins (Mackenzie et al. 2002). At regional scales, observations are giving
some credit to the global model results. The export of nitrate by the Mississippi river
to the Gulf of Mexico has tripled since the 1950s, owing to an increase in agricultural
fertilizer application and hydrological changes (Donner et al. 2002, Justic et al. 2003).
The combination of meteorological forcing and human activities has not only changed
the load of material to the coastal systems, but also the chemical speciation and
elemental stoichiometry of the water. Unlike N and P, silica (Si) is little affected by
human pollution and results mostly from the weathering or dissolution of silicate
minerals. Consequently, the Si to N and Si to P ratios are significantly decreasing in
coastal waters, even though higher precipitation and runoff must enhance the erosion
of rock minerals and Si loads in the aquatic system. In coastal waters, the lack of Si
with respect to other macronutrients would affect the phytoplankton assemblage and
the entire ecosystem.
On the other hand, the loading ratio of DIN and DIP remain relatively stable over time
in spite of having very different chemical transformation pathways. Using a data set
of 165 globally-distributed sites, Smith et al. (2003) describe the tight coupling
between DIN and DIP with a loading ratio of 18:1, very close to Meybeck’s (1982)
observations made several decades earlier on a much smaller data set, although the
concentration of these nutrients were three times less. Note that the DIN:DIP ratio is
also very close to the well-known Redfield ratio of 16:1 for oceanic waters published
earlier (Redfield 1958). Anomalies from this ratio occur systematically on regional
and temporal scales according to the dominant chemical and ecological processes
(nitrification, denitrification, mineralization, uptake) controlling the nutrient cycling.
For example, the riverine N:P ratio for the East China Sea is 111 (Chen et al. 2003),
much different than the average ratio of 16. This ‘excess nitrate’ still remains,
although at lower values, in the adjacent coastal waters, making the ecosystem Plimited
rather N-limited. Inversely, higher runoff increases both the delivery of
nutrients in coastal waters and the water column stratification, thereby increasing the
probability of a sub-surface oxygen-depleted layer and favoring denitrification
processes with the release of N2O into the atmosphere and a decrease in the N:P
ratio. Over an extended area on the Indian continental shelf, Naqvi et al. (2000)
observed an intensification of the denitrification process resulting from increasing
runoff of nutrients from land and water stratification, producing anoxic conditions in
coastal sub-layers, and reducing considerably the N:P ratio when compared with the
value of 16 (i.e. Redfield ratio) observed in oxygenated waters. Although
anthropogenic nutrient inputs are mostly responsible for this situation, the influence
of climatic factors such as an intensification of the summer monsoon due to global
warming can be significant (Naqvi et al. 2000).
According to Seitzinger and Kroese (1998), ca. 21 TgN.yr -1 is exported from rivers to
estuaries and coastal zones, i.e. twice the value estimated earlier by Meybeck
(1982). Inputs to the enclosed and semi-enclosed European Seas account for 12%
(2.5TgN.yr -1 ) of the world river N export. A large part (75%) of this N input results
from anthropogenic activities and as expected displays a significant north-south
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