75 ACTA ZOOLOGICA BULGARICA Acta zool. bulg., Suppl. 7, 2014: 75-81 Introduction Besides sustaining biodiversity, wetlands provide numerous additional ecosystem services, such as nu- trient retention, drought and lood mitigation. These important ecological functions and the wetland eco- logical status as a whole seemed to be better deined by physical and chemical characteristics as shown by Peeters et al. (2009) for shallow lakes. Thus, depend- ing on environmental conditions and connectivity, inluenced by hydrology, the wetlands might act ei- ther as sources or sinks of nutrients (Hein et al. 2005, Bondar et al. 2007). According to saunders, Kalff (2001), nitrogen retention in wetlands is higher than in lakes and rivers due to denitriication, uptake by aquatic plants and sedimentation. The alternation of wet and dry phases in wetlands may favor either den- itriication or mineralisation of nitrogen (V enterinK et al. 2002). In contrast, phosphorus, may be either bound in organic or inorganic form to sediments and assimilated by plants, or released by microbial min- eralisation and redox-processes; such processes are controlled by the prevailing hydrological conditions (HillBricHt-ilKoVsKa 1999). In large rivers, in par- ticular the Middle and Lower Danube, the wetlands play an important role in nutrient cycling. In this pa- per we evaluate the main factors that inluence the chemical composition and nutrient retention potential as well as the trophic character of selected wetlands in the Middle and Lower Danube River. Factors Inluencing the Chemical Characteristics and Nutrient Retention / Release Potential of Wetlands in the Middle (Hungarian) and Lower (Bulgarian) Danube River Roumen Kalchev 1 , Maria Dinka 2 , Mihaela Beshkova 1 , Hristina Kalcheva 1 , Árpád Berczik 2 , Edit Ágoston-Szabó 2 1 Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, 1 Gagarin Str., 1113 Soia, Bulgaria; E-mail: rkalchev@zoology.bas.bg 2 Danube Research Institute, Centre for Ecological Research, Hungarian Academy of Sciences, Alkotmány u. 2-4, H-2163 Vácrátót, Hungary Abstract: Several physical and chemical variables including nutrients (N- and P- forms) of wetlands in the Middle and Lower Danube River, each presented by ive water bodies, were measured and analysed. The applica- tion of multivariate redundancy analysis (RDA) showed that variables relecting the degree of wetland connectivity (isolation) to the main river and the macrophyte development are among the main and com- mon factors explaining spatial variations of recorded variables, nutrient concentration ratios between wetlands and the river and ordination of wetland sampling sites. The morphological type of wetlands related to their connectivity to the river seems to inluence the nutrient dynamics (i.e. release/ retention) stronger in the Lower Danube River, while factors such as low availability and direction are of signiicant importance in the wetlands of the Middle Danube River. Keywords: Wetlands, connectivity, ecosystem services, nutrients, trophic status, multivariate analysis