European Context of Nutrient Dynamics

Nutrient budgets and fluxes have been established at the local (major rivers) and regional (coastal seas) scales across Europe. Two examples are provided below: (1) a N budget of the continental inputs to the North Sea^[1] and (2) a N budget of major riverine inputs and transformations along the Western Scheldt river-estuarine system^[2] . Other nutrient budgets have been established, among others, for the Western shelf of the Black Sea ^[3] and the Baltic Sea^[4] . At a smaller scale, detailed estimates of the nutrient sources, transport and transformations are also available for the Seine and Humber continuums^[5] ,^[6] .

North Sea

Figure 2. Total atmospheric nitrogen deposition to North Sea, 1999, in ton N per km2(source: Hertel et al, 2002)^[7]

The continental inputs of nitrogen to the North Sea originate from rivers, atmospheric inputs and, to a much smaller extent, direct discharges and dumping^[1]. The riverine contributions are summarized in Table 1 and, collectively, amount to almost twice that of atmospheric inputs^[8]. Figure 2 shows the spatial distribution of the total atmospheric N deposition to the North Sea. The spatial pattern results from the distribution of the source areas and precipitation rates. On average, deposition amounts to 0.9 ton N per km2, with deposition up to 50% higher than average around territorial waters of Belgium, the Netherlands and Germany. Approximately 60% of total atmospheric N deposition results from combustion (nitrogen oxides) and approximately 40% from agricultural activities (ammonia) ^[7] .

Table 1: Annual river inputs (ton per year) of nitrogen for all relevant rivers around the North Sea (source: Radach and Lenhart, 1995) ^[9] . P and S fluxes are also shown
River	N	P	Si
Firth of Forth	20	186	11
Tyne/Tees	14735	593	9309
Humber	60636	5891	17928
Thames	26214	3786	14931
Ems	25736	614	6805
Noordzeekanaal	10877	1767	3912
Lauwer	333	143	25
Lake IJssel/Kornwerderzand	12320	461	3588
Lake IJssel/Den Oever	21232	80	5170
Meuse	91159	4400	34402
Rhine	191543	14194	69623
Scheldt	31670	2116	15077
Yzer	267	109	37
Elbe	126314	3822	34520
Jade	8	3	2
Schleswig-Holstein river	8	3	2
Weser	52862	3420	18470
Danish rivers	1227	513	136

The Western Scheldt Estuary

Figure 3. Mass budget for (A) ammonium and (B) nitrate in the tidal rivers (right) and in the saline estuary of the Western Scheldt (left) in the summer of 1990, 2002 and 2010. Processes: resp=aerobic respiration; nitrif=nitrification; denit=denitrification; npp=net primary production. Transport fluxes are positive seawards. All fluxes are given in kmol day^-1. Top arrow: Riverine and lateral inputs; Left arrow: export to the coastal zone. (source: ^[2] Vanderborght et al., 2007

The Scheldt River and its tributaries drain 21,580 km² in northwestern France, northern Belgium and southwestern Netherlands^[10] . The Scheldt estuary is a a macrotidal system, with an average residence time in brackish waters of 1 to 3 months. The mixing zone of fresh and salt waters extends over a distance of 70 to 100 km. The area of tidal influence goes up to 160 km from the river mouth and includes the major ^[11] .

The hydrographical basin includes one of the most heavily populated regions of Europe, where highly diversified industrial activity has developed. As a consequence, the whole catchment was heavily polluted until the mid 1970s, when water degradation culminated due to the continuous increase of nutrient and organic mater inputs. The level of wastewater treatment, especially in the upstream zones, was an important factor contributing to this degradation. The estuary was particularly affected by domestic and industrial inputs from the great Brussels, Antwerp and Gent areas^[2] . Since then, better management of industrial and domestic wastewater point sources has led to a progressive improvement of the environmental conditions in the estuary. Billen et al. (2005) ^[12] and Soetaert et al. (2006) ^[13] provide two recent comprehensive reviews of this long term evolution.

A mass budget for nitrogen has been established for the saline estuary (km 0 to100) and for the tidal river network (km 100 to 160) of the Western Scheldt for the summer months^[2] . Three periods have been analyzed (1990, 2002 and 2010). This allows for the assessment of the influence the secondary and tertiary wastewater treatment in the catchment on the N dynamics. Figure 3 shows that the tidal river and the estuary contribute almost equally to the overall biogeochemical cycling of N, despite the very different volumes involved. For the simulated periods, the large decrease in N input (> 55 %) expected between 1990 and 2010 will not lead to a significant decrease of N export to the coastal zone during the summer period.

References

↑ ^1.0 ^1.1 Galloway, J., W. Chlesinger, H. Levy, A. Michaels, and J. Schnoor (1995), Nitrogen fixaton: Anthropogenic enhancement and environmental response, Global Biogeochemical Cycles 9, 235-252.
↑ ^2.0 ^2.1 ^2.2 ^2.3 Vanderborght, J-P, I. Folmer, D. Rodriguez Aguilera, T. Uhrenholt, and P. Regnier (2007), Reactive-transport modelling of a river-estuarine coastal zone system: application to the Western Scheldt, Marine Chemistry 106, 92-110.
↑ Gregoire, M., and J. Friedrich (2004), Nitrogen budget of the northwestern Black Sea shelf inferred from modeling studies and in situ benthic measurements, Marine Ecology Progress Series 270, 15-39.
↑ Wulff, F., L. Rahm, A. - K. Hallin, and J. Sandberg (2001), A nutrient budget model of the Baltic Sea. Chapter 13 A systems analysis of the Baltic Sea. Ecological Studies, F. Wulff, L. Rahm, and P. Larsson Eds., (Springer Verlag) Vol 148, pp 353-372.
↑ Garnier, J., G. Billen, and M. Coste (1995), Seasonal succession of diatoms and Chlorophyceae in the drainage network of the Seine River: observations and modeling, Limnology and Oceanography 40, 750-765.
↑ Tappin, A.D., J.R.W. Harris and R.J. Uncles (2003) The fluxes and transformations of suspended particles, carbon and nitrogen in the Humber Estuary (UK) from 1994 to 1996 : results from an integrated observation and modelling study. The Science of the Total Environment 314/316, 665-713.
↑ ^7.0 ^7.1 Hertel, O., C. Ambelas Skjøth, L.M. Frohn, E. Vignati, J. Frydendall, G. de Leeuw, U. Schwarz, and S. Reiset (2002), Assessment of the atmospheric nitrogen and sulphur inputs into the North Sea using a Lagrangian model, Physics and Chemistry of the Earth 27 ,1507 – 1515.
↑ Jickells T.D. (1998), Nutrient Biogeochemistry of the Coastal Zone, Science, 281 217 – 222
↑ Radach, G., and H.J. Lenhart (1995), Nutrient dynamics in the North Sea: Fluxes and budgets in the water derived from ERSEM, Netherlands Journal of Sea Research 33, 301-335.
↑ Wollast, R. (1988), The Scheldt estuary. Pollution of the North Sea: An Assessment, W. Salamons, B.L. Bayne, E.K. Duursma, and U. Forstner Eds., (Springer-Verlag, Berlin) pp. 183-193.
↑ Regnier, P., R. Wollast, and C.I. Steefel (1997), Long-Term Fluxes of Reactive Species in Macrotidal Estuaries. Estimates from a Fully Transient, Multi Component Reaction-Transport Model, Marine Chemistry 58, 127-145.
↑ Billen, G., J. Garnier, and V. Rousseau (2005), Nutrient fluxes and water quality in the drainage network of the Scheldt basin over the last 50 years. Ecological structures and functions in the Scheldt Estuary: from past to future. P. Meire, and S. Van Damme Eds. Hydrobiologia 540(1-3), 46-67.
↑ Soetaert, K. J.J. Middelburg, C. Heip, P. Meire, S. Van Damme, and T. Maris (2006), Long-term change in dissolved inorganic nutrients in the heterotrophic Scheldt estuary (Belgium, The Netherlands), Limnology and Oceanography 51, 409-423.