Improving the design of environmental monitoring networks. Case study on the heavy metals in mosses survey in Germany

Germany participated in the European heavy metals in mosses surveys in 1990, 1995, and 2000. The goal was to map the spatial patterns of the metal bioaccumulation as a comparative measure for the atmospheric deposition of metals in terrestrial ecosystems. In 2000, the density of the German moss network exceeded the recommendation of the European monitoring manual by almost 100%. Therefore, the first objective in the campaign 2005 was to reduce the monitoring network from 1028 to 720 sites, without changing significantly in terms the ecological and land use coverage of the monitoring network, as well as geostatistical and descriptive statistical measures. In addition, the efficiency of the moss survey was to be improved by spatially linking it to other environmental monitoring programmes. The reorganisation of the monitoring network was based on the following data: measurements of up to 40 elements from 592 (1990), 1026 (1995), and 1028 (2000) German moss monitoring sites, surface maps on the surroundings of the monitoring sites with regard to the ecological coverage, land use, and modelled nitrogen deposition, as well as data from other environmental monitoring networks. These data sets were analysed in a four-step procedure comprising geostatistics, inference, and percentile statistics, as well as GIS techniques. As a result, the moss monitoring network could be reduced by 308 monitoring sites without significant change of the ecological coverage of the monitoring sites, their average neighbourhood regarding land use categories related to nitrogen and metal emissions, the geostatistical representativity of the measurement values or chosen descriptive statistical measures. The latter applies not only to the German territory but for all federal states and for 21 ecoregions as well. The methodology presented assists the restructuring of environmental monitoring networks und enables the quantitative description of possible effects on the information quality. The site optimisation approach could be applied to other environmental monitoring programmes for sampling efficiency.