| | | State of the art of research regarding atmospheric deposition of nutrients |
The processes governing the atmospheric transport, transformation and deposition of nutrients to coastal ecosystems is an important area of research within the atmospheric, marine and terrestrial sciences (see also atmospheric deposition section). The combustion of fossil fuels and the emissions from agriculture into the atmosphere constitutes a highly significant and growing percentage of total N-loading into estuarine and coastal waters [138]. In fact, rainfall associated deposition events are known to stimulate primary production in N-limited coastal and offshore waters [361]. Estimates of atmospheric inputs to the Baltic Sea [227], based on studies of the Group of Experts on Airborne Pollution of the Baltic Sea Area (EGAP), constitutes a considerable fraction of the total nutrient load, although much uncertainty surrounds these estimates [22]. However, only little effort has been put into experimental investigations of the atmospheric load of nitrogen in the coastal zone.
The processes controlling the atmospheric deposition loads are not well understood. For the dry deposition of the gaseous species the horizontal gradients are of major importance. This is in particular true for NH3, which can also be emitted from the water since, unlike HNO3, it does not fully dissociate. Emission and deposition of NH3 is influenced by chemical and biological gradients in the water and fluxes of both HNO3 and NH3 are influenced by the physical gradients caused by change in the wave field. Also horizontal changes due to atmospheric internal boundary layers influence the deposition of both gases. The nitrogen gases HNO3 and NH3 are both chemically reactive and soluble. Therefore it has been hypothesised that uptake of the gases by sea spray can influence the deposition of the gases. This has been tested in a series of field experiments and the results supported the hypothesis [170].
In order to develop models, which can calculate the nitrogen load to the coastal waters with sufficiently high accuracy (time resolution equal to or smaller than a week and grid size capturing the horizontal inhomogeneity) the air-sea gas fluxes dependencies of the horizontal and vertical inhomogeneity have to be well known. This knowledge is rather limited to non-existent in Scandinavia for the time being.
| | | State of the art of research regarding atmospheric deposition of nutrients |