Key site variables governing the functional characteristics of Dissolved Natural Organic Matter (DNOM) in Nordic forested catchments

The objective of this study was to extract and evaluate key catchment characteristics explaining the main variation in the properties of Dissolved Natural Organic Matter (DNOM) in the surface water draining the sites. A number of classical and sophisticated DNOM characterisation methodologies were used for this purpose. The explanatory capability of site characteristics on the variation in descriptors of DNOM, including elemental analysis (EA), DOC fractionation, SEC, CZE, CGE, UV/Vis- and FTIR spectra, FES, TLS, ESR, ESI/MS, 13C-CPMAS-NMR, potentiometric pH titration as well as functional characteristics as biodegradability, response of macrophyte and PAH partitioning coefficient were evaluated. The sampling sites are all natural, coniferous, heathery-forested sites with bogs forming a climatic, anthropogenic deposition and proportional biotope cover gradient. Correlations to the main principal components, as well as a correlation matrix, singled out total S-deposition as the key explanatory site parameters, accounting for most of the variation in the DNOM descriptors. Other significant factors were H+ concentration, length of growing season, biotope coverage (i.e. peatland/ or lake/watershed ratio), site elevation and the TOC concentration. The total S-deposition was strongly negatively correlated to the molecular weight, aromaticity, carboxylic acidity and sorption capacity of PAH. The greater the proportion of lake and histosol biotopes of the sampling sites the greater the aliphatic character of the DNOM. Clustering in PCA of samples from the same site sampled in spring and fall implied a basic site-specific DNOM quality and a weak season effect. The Hasse diagram technique (a method of partial order theory) is used to show, for instance, how the relative histosol coverage may determine NOM quality and, in turn, how this coincides with high macrophyte response.