2.3. Statistical methods
Cluster analysis was performed using Statistica version 13 (TIBCO
Software Inc.) on a selection of variables listing elemental, ionic and
DOC concentrations , with the selection criterion that only variables
with 33% or fewer values < LOQ (or below blank) were taken
into account to avoid artifacts in grouping. Prior to cluster analysis,
values <LOQ (or below blank) were replaced by half LOQ (or
blank, whichever was higher). All data was standardised (z-transformed)
within variables. Clustering was performed using Ward’s method and
squared Euclidean distance. This analysis may be cross-compared with the
principal component analysis presented in Suppl. Mat. 1 .
Enrichment factors (EFs) relative to average concentration of elements
in the suspended sediment of World rivers (after Viers et al. ,
2009 and Taylor, 1964) have also been calculated and are shown in Table
S4 (Suppl. Mat. 1 ).