The concentrations of seven elements (N, P, K, Ca, Mg, Fe, and Na), five of which are macroelements, were measured in 3-cm apices of 17 populations of the euryecious aquatic moss Rhynchostegium riparioides (Hedw.) Card. located along the course of the Iregua River (La Rioja, northern Spain). A principal components analysis (PCA) revealed two key factors ordinating the sampling sites on the basis of their physicochemical data 1) substrate lithology, since the river's upper course is predominantly siliceous whereas the middle and lower courses are influenced by CaCO3 and 2) human impact, especially observed in the irrigation channels of the lower course. The element concentrations in the tissues of R. riparioides, generally comparable to those cited for other aquatic bryophytes, were strongly and significantly correlated with the respective element concentrations in the surrounding water. Therefore, the PCA ordinating the moss populations on the basis of their element concentrations was highly coincident with the physicochemical PCA: the headwater populations had the lowest element concentrations, the middle course populations were notably Ca-enriched, and the lower course populations showed the highest concentrations of N, P, K and Na. The significant correlations between the element concentrations differentiated two groups of elements: mainly intracellular ones (N, P, and K), and mainly exchangeable ones (Ca and Mg). This dichotomy was also noticed in other respects 1) the relationship between the water and the tissue element concentrations was linear for N, P, and K, but similar to a Michaelis-Menten saturation-type curve in the case of Ca and Mg; 2) the quotients between the concentration of a given element in moss and in water (enrichment ratios, ER) were much higher for N, P, and K than for Ca and Mg; 3) only the tissue concentrations of Ca and Mg were correlated–negatively–with their respective ERs. Na showed somewhat peculiar characteristics, probably reflecting weak uptake efficiency and metabolic irrelevancy. ERs were strongly dependent on the respective element concentrations in water through hyperbolic relationships, suggesting that the uptake efficiency may be higher when elements are more diluted in water and decreases as the moss becomes saturated. This plasticity, to be expected in such an euryecious species as R. riparioides, would permit the acclimation of the different populations to changing water chemical conditions in order to avoid nutrient deficiencies.