Reactive N in the global hydrologic system
Increased concentrations of nitrate have been observed in groundwater in many, primarily agricultural, regions, although the magnitude of this increase is difficult to determine outside a few well-characterised aquifers. High levels of nitrate in drinking water have given cause for concern in some industrialised regions, e.g. Western Europe. Recently, nitrate concentrations in groundwater have also been shown to increase in developing countries with intensive agricultural production.
Impacts
Nitrogen and phosphate concentrations in many rivers have increased over time. With increasing population densities and agricultural production riverine N and P fluxes are also increasing in many developing countries. The effect of increasing N fluxes depends on the availability of particularly phosphorous (P) and silicon (Si). N and P limit the growth of phytoplankton, macroalgae and vascular plants, and Si limits the growth of diatoms. In combination with increased N fluxes during the past decades, similar changes have occurred with P, while the Si loads have remained constant or even decreased in many rivers primarily as a result of dam construction. This has often altered the stoichiometric balance of N, P and Si which may not only affect the total production in freshwater and coastal marine systems, but also its quality. When diatom growth is compromised by Si limitation, non-diatoms may be competitively enabled, with dominance of flagellated algae including noxious bloom-forming communities. Thus food web dynamics leading to fisheries harvests are affected by shifts in the relative availability of N, P and Si.
In both freshwater and marine coastal aquatic systems typical consequences of the most serious manifestations of eutrophication with consequences for water quality and harvestable fisheries are:
- algal blooms;
- massive growth of submersed and floating macrophytes;
- changes in food webs and species composition;
- depletion of the oxygen reserve.
In freshwater systems often phosphate is the major cause of eutrophication, while in coastal marine systems generally nitrogen is the major element causing eutrophication.
PBL studies
In 2003 PBL (then RIVM) published a global model that describes the fate of nitrogen (N) from point and nonpoint sources in the hydrological system up to the river mouths (Van Drecht et al., 2003). This model has recently been used to assess the changes in river N export and the relative contributions from point and nonpoint sources for the 60-year period of 1970 to 2030 on the basis of the FAO Agriculture Towards 2030 study (Bouwman et al., 2005b). In a recent paper Beusen et al. (2005) analysed data on river sediment transport and export of particulate carbon, nitrogen and phosphate as a contribution to the global NEWS project.
Effects of increasing N and P fluxes in freshwater systems are being studied at PBL, and a presentation on this subject was recently presented at the European Conference on Ecological Modelling.
- presentation "GLOBIO-aquatic: A global model framework for the assessment of aquatic biodiversity" (PDF, 16.5 MB)
Regarding coastal marine ecosystems: PBL (through co-chairman A.F. Bouwman) is involved in the SCOR Working Group on Land-based Nutrient Pollution and the Relationship to Harmful Algal Blooms in Coastal Marine Systems. This project will advance predictive capability of the extent of blooms, the dominant harmful taxa involved, and our ability to manage these Harmful Algal Blooms (HABs) by an improved understanding of the impacts of nutrients on HABs.
