Element concentrations and enrichment ratios in the aquatic moss Rhynchostegium riparioides along the River Iregua (La Rioja, Northern Spain)

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. locat ed along the course of the Iregua River (La Rioja, northern Spain). A princ ipal components analysis (PCA) revealed two key factors ordinating the samp ling sites on the basis of their physicochemical data 1) substrate litholog y since the river's upper course is predominantly siliceous whereas the mid dle and lower courses are influenced by CaCO3 and 2) human impact, especial ly observed irt the irrigation channels of the lower course. The element co ncentrations in the tissues of R. riparioides, generally comparable to thos e cited for other aquatic bryophytes, were strongly and significantly corre lated with the respective element concentrations in the surrounding water. Therefore, the PCA ordinating the mass populations on the basis of their el ement concentrations was highly coincident with the physicochemical PCA: th e headwater populations had the lowest element concentrations, the middle c ourse populations were notably Ca-enriched, and the lower course population s showed the highest concentrations of N, P, K and Na. The significant corr elations between the element concentrations differentiated two groups Of el ements: mainly intracellular ones (N, P, and K), and mainly exchangeable on es (Ca and Mg). This dichotomy was also noticed in other respects 1) the re lationship between the water and the tissue element concentrations was line ar 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 C a and Mg Mere correlated-negatively-with their respective ERs. Na showed so mewhat peculiar characteristics, probably reflecting weak uptake efficiency and metabolic irrelevancy. ERs were strongly dependent on the respective e lement 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 e xpected in such art euryecious species as R. riparioides, would permit the acclimation of the different populations to changing water chemical conditi ons in order to avoid nutrient deficiencies.