Nitrogen concentration of the aquatic plant species in relation to land cover type and other variables of the environment

Nitrogen (N) deposition data, together with inland water parameters, provide evidence that N load may affect the vegetation of Baltic States. There is much concern about eutrophication of the rivers, although information about physiological parameters of riparian plant species is still poor. The present study is aimed at comparison of leaf N concentration among populations of aquatic plant species of Lithuania, relating N concentration data to the type of land cover (classification system of COoRdinate Information on the Environment, CORINE) in the neighbouring areas, river state and size, intensity of agriculture in 1991–1996, and natural vice versa regulated fragments of the riverbed. The leaf N concentrations of widely spread native and invasive species (5 riparian and 2 water plants) were estimated for 241 sites (collection time 1^st ten-day period of August, 2015) of the main river catchments of Lithuania. Only leaf blades were used for analyses and N concentrations were determined by the Kjeldahl method. The biggest (1.7 times) variation (p < 0.05) in leaf N concentration among populations was documented for Lythrum salicaria. According to the mean values (N % of dry mass, DM) of the leaf N concentration, species could be arranged into following order: Lythrum salicaria (3.0) < Stuckenia pectinata (3.1) < Phalaris arundinacea (3.5) < Bidens frondosa (3.8) < Phragmites australis (4.0) < Nuphar lutea (4.1) < Echinocystis lobata (4.2). Significantly higher (p < 0.05) concentrations of leaf N were found for L. salicaria populations growing near the small rivers (3.4% N DM) compared to the large ones (2.8% N DM). Other selected river and its environment parameters in most cases did not have significant effect on leaf N concentrations of aquatic plants of Lithuania. Leaf N concentration of the aquatic species was not influenced by land cover type (2000 and 2006 year data of database) and did not depend on the river size or state, based on the Water Directive guidance (EU, 2000). Among selected species the most nitrophylic was invasive for Lithuania species E. lobata, which is currently spreading along larger size (>1000 km²) rivers. The present levels of N entering riparian ecosystems are causing spread of the macrophyte species consuming relatively high amounts of N, although the main sources of macrophyte N remains to be examined.