Eutrophication of aquatic ecosystems a new method for calculating the potential contributions of nitrogen and phosphorus
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- Seppälä, J., Knuuttila, S. & Silvo, K. Int J LCA (2004) 9: 90. doi:10.1007/BF02978568
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Aim, Scope and Background
Aquatic eutrophication is a widespread problem in inland and coastal waters around the world and it should therefore be one of the impact categories to be considered in LCA studies of products and services. In LCAs there are several impact assessment methods to determine characterisation factors for eutrophying nutrients, but few methods have been developed to model fate and spatial aspects. One such method was developed as part of an LCA application of the Finnish forest industry. The aim of this study was to present this characterisation method in which the potential contributions of nitrogen and phosphorus to eutrophication of aquatic ecosystems are calculated. The use of the method was demonstrated by producing site/sector-specific characterisation factors and by constructing a reference value of aquatic eutrophication for Finland. A discussion of sensitivity and uncertainty aspects related to input data is also presented.
The potential contribution to eutrophication from a product system is calculated as a result of the nutrient inputs causing increased production of biomass within aquatic systems. Accordingly, direct nitrogen and phosphorus emissions as well as nitrogen and phosphorus deposition into the watercourses can be included. In the method, characterisation factors for nitrogen and phosphorus emissions are generated by multiplying commonly used equivalency factors by transport and effect factors. Transport and effect factors of the nutrient sources are case-specific and can be determined for each substance individually on the basis of scientific models, empirical data or expert judgements. In this paper, transport and effect factors are determined for different sectors (forest industry, field cultivation, fish farming, etc.) in Finland. In addition, temporal aspects can be taken account of in the model by coefficients representing the proportion of nutrient load in the productive period of the total annual load. The model uncertainty was studied by using three different scenarios based on different input data and assumptions. Uncertainties within the input data were assessed as ranges and the effects of input data uncertainty on the results were studied by varying maximum and minimum values of each input variable in the same time.
Results and Discussion
The characterisation method developed was applied to provide characterisation factors of aquatic eutrophication for different sectors in Finland. The magnitude of these sector-specific characterisation factors varied greatly. For this reason, the method results were easily differentiated from those commonly used, site-generic characterisation factors produced in LCIAs. Furthermore, reference values of aquatic eutrophication for normalisation purposes in LCIA were gener-ated on the basis of sector-specific nutrient emissions and characterisation factors. Different scenarios produce alternative pictures of the contributions of various sectors to aquatic eutrophication. By examining the scores of different sectors, it can be concluded that uncertainties in the emission estimates of field cultivation and fate of nitrogen originating from deposition have the greatest effect on the results. In the average scenario, the uncertainty range for the reference value was estimated to be ± 40% due to all uncertainties in input variables.
The results of the work reveal the importance of site-specific characterisation approach in the context of aquatic eutrophication. Furthermore, differentiation of nutrient forms in various sectors means that the question of determination of characterisation factors is also related to sector-specific issues. The method demonstrated is flexible, and it can be applied to geographical areas rather than to Finland as a whole. The use of sensitivity and uncertainty analysis is recommended for the interpretation of results, because there is no empirical test applicable for evaluation of the validity of results. In order to reduce uncertainty in results, further research is needed, in particular on the roles of different nutrient forms as sources for aquatic biota, on spatial differentiation of nitrogen and phosphorus as production-limiting factors, and on fate of nitrogen in catchments.
The weakness of the method is related to the accessibility of input data, restricting to construct the characterisation model of aquatic eutrophication, for example, on the European level. However, it seems that legislative requirements of the European Union to study ‘target nutrient loads’ of aquatic eutrophication in the catchments of each Member State can improve the situation of the accessibility of input data in the near future.