Can seasonal variation in fen water chemistry influence the reliability of vegetation- environment analyses?

Temporal variation in fen water chemistry was studied in the Western Carpathian flysch zone (Czech Republic and Slovakia). Ten sites representing particular spring-fen types (tufa-forming fens, rich fens, spring-fen meadows, rich Sphagnum-fens, poor Sphagnum-fens) were studied.Water chemistry was determined three times a year (spring, summer, autumn) for 3 years. Water pH and conductivity were themost stable of the measured variables. Na+, K+, Ca2+ and SO42– were also relatively stable. In contrast, N-NO3–, Cl–, Fe, PO43– and redox-potential temporally varied. These fluctuating, unstable variables explained very little or insignificant amounts of the variation in plant species data in our study area, possibly because of their instability. Further, seasonal variation in physical-chemical properties of the water confounded associations with vegetation types when data from different seasons was used. The significance of the differences among vegetation types (between- subject effect in RM ANOVA) distinctly changed among seasons for temperature and Si, N-NO3 and Cl– concentrations and to a slight degree, for Fe, Mg and water redox-potential. The differences in Ca, Na and SO42– concentrations, pH and conductivity were highly significant in all three seasons. The first axis of the PCA of the chemical variables reflects the gradient from mineral-poor to mineral-rich fens in all the analyses, i.e. spring, summer and autumn. The separation of the sites along this axis is clearest in the ordination of the autumnal data. Major vegetation types were separated in PCA even when data from all three seasons were pooled. There is no major-nutrient that is characteristic of meadow- species rich and more productive fen habitats, even when repeated water samples are analyzed.