Results presented in Figure 2.1(b) and Figure 2.1(c) (from 2. Characterisation of nutrient sources) show the variations in both the HNO₃ concentrations and their spatial gradients with fetch (de Leeuw et al., 2001) . The high sea spray concentrations over the surf zone cause an immediate effect when the air mass passes the coast line. The uptake by sea spray aerosols is most evident at lower altitude where the concentrations of the particles are highest. Diffusion distributes the particles homogeneously in the vertical, causing a homogeneous uptake in the whole column. At a fetch of 25km the profile becomes almost uniform except very close to the surface.

Large deposition of nitrogen may occur during short periods (de Leeuw et al,. 2003). This is illustrated with an example for the ANICE experiment in August 1999, when the southern North Sea experienced a short period of strong south-easterly flow. Mass concentrations of ammonium and nitrate aerosol in the fine and coarse fractions were measured at Weybourne during the period centred on 26 August 1999. During this time, the atmosphere provided enough nitrogen to fix 5.3 mmolC m ^-2 day ^-1.

Figure 1.3(a) below shows the spatial distribution of nitrogen deposition to the North Sea and the strong gradients near the source areas that result from the processes described above. They have the effect of focusing atmospheric deposition into coastal areas already stressed by various other anthropogenic inputs (Jickells, 1998).

In large-scale atmospheric transport models, such coastal chemical processes are generally not included. Without these, it is not possible to effectively manage nitrogen enrichment issues in coastal waters.

In the Kattegat Strait , the events of high atmospheric wet deposition could increase the growth of chlorophyll around 20% or more (Hasager et al., 2003). Similarly, results by Spokes et al. (2000) suggest that about 30% of the new production in eastern Atlantic surface waters off Ireland can be supported by atmospheric inputs in May 1997, and that most of the input occurs during short lived, high-concentration, south-easterly transport events.

These episodic fluxes largely determine the total primary productivity due to atmospheric nitrogen deposition in the area. For the southern North Sea, the atmospheric contribution is estimated at ca 5.5% of the total required new nitrogen. For the entire North Sea the atmospheric contribution is ca 3.2%. Although these numbers may not seem impressive, presented results show that most of the nitrogen is delivered during short episodes. One such episode resulted in an average deposition of 0.8 mmolNm ^-2 day ^-1 (de Leeuw et al., 2003) which has the potential to promote primary productivity of 5.3 mmolC m ^-2 day ^-1.