EKOLOGIJA. 2007. No. 53. No. 2. P. 30–36 © Lietuvos mokslų akademija, 2007 © Lietuvos mokslų akademijos leidykla, 2007 Loading and retention of phosphorus in riverine systems e catchment of the Dovinė river (right tributary of the Šešupė river, the Nemunas catchment) is situated in the Žuvintas Biosphere Reserve. However, anthropogenic pressure due to agriculture, fishery and urbanization is observed in this catchment. ere are forests (33%), agricultural lands (34%), swamps (21%) and water bodies (8%) in the Žuvintas Biosphere Reserve. ere are about 1000 plant species (30 of them protected) and 2500 animal species (2200 invertebrates, 220 birds, 15 amphibians and reptiles, 21 fishes, and 43 mammals) in this reserve. e recent transformation of the hydrochemical and hydrophysical conditions, as well as of the former habitats is caused by the huge amounts of biogenes observed in the water bodies of the biosphere reserve and in the other lakes of the Dovinė river catchment. e ecological situation in the river catchment is investigated considering the distribution of phosphorus in the riverine system. According to the observation results, the annual inflow of phosphorus reaches 15200 kg P y –1 (78% of it is the an- thropogenic phosphorus) in the Dovinė river catchment. ere is about 7960 kg P y –1 retained in the five largest lakes of the basin. e lowest content was found in Lake Dusia (80 mg m – ²) and the highest in Lake Simnas (980 mg m – ²). Key words: the catchment of the Dovinė river, Žuvintas Biosphere Reserve, phosphorus load, phosphorus retention, eutrofication Julius Taminskas, Rita Linkevičienė, Rasa Šimanauskienė Institute of Geology and Geography, T. Ševčenkos 13, LT-03223 Vilnius, Lithuania E-mail: julius.taminskas@geo.lt, rita.linkeviciene@geo.lt, rasa.simanauskiene@geo.lt INTRODUCTION e influence of land use on riverine systems is a frequent object of research. Nutrient losses from agricultural catch- ments are especially visible. e augmentation of eutroph- ication, which depends on the ratio of phosphorus and nitrogen, is observed in such conditions. Nitrogen levels are normally high in the catchments of rivers and lakes situated in plains. However, the lack of phosphorus due to its loading in pristine conditions reduces euthrophication processes. erefore, the inflow of anthropogenic phos- phorus accelerates the euthrophication of water bodies. e phosphorus and nitrogen ratio changes significantly aſter deforestation or under intensive agricultural activity. For example, nutrient ratios in streams were altered from P to N ratio of 1:120 before deforestation to 1: 33 aſter de- forestation in the United States (Gergel et al., 2002). is is related with the augmentation of phosphorus outflow from eroded lands. However, there are several phosphorus sources that increase euthrophication in river catchments. For example, there are several point sources of phospho- rus (Simnas fishery ponds, settlements situated in the river valley and exploited peatlands), as significant as phospho- rus losses from agricultural land in the Dovinė river catch- ment characterised by melioration channels and a scarce forest cover. ese facts impede the sustainable manage- ment of the catchment. During the intensive and unsustainable catchment’s usage, the amount of mineral phosphorus increased up to 10 times in some places of the Dovinė river cath- ment (Tamošaitis et al., 1986) and influenced the outflow of phosphorus from the catchment to the Baltic Sea, i.e. phosphorus load increased continuously. However, the in- fluence of phosphorus increase was strongest on lake eu- throphication in the catchment. ere are several lakes characterised by different troph- ic conditions in the Dovinė river catchment. Žuvintas is the main lake in the Žuvintas Biosphere Reserve situated in the central part of the Dovinė river catchment. Due to euthrophication, the area of the lake was decreasing by 0.2 ha each year during the period 1934–1960. e wa- ter volume diminished almost by half in the same period (Bieliukas et al., 1962). e decrease of the lake area and water volume continued in the second half of the 20 th cen- tury (Taminskas et al., 2005). e overgrowth with aquatic macrophytes decreases the lake’s area. A high inflow of biogenes, especially phosphorus and organic matter, from the catchment accelerates the dominance of macrophytes. Rapid euthrophication processes are also observed in the Amalvas and Simnas lakes of the Dovinė river catch- ment. e overgrowth of Lake Simnas with macrophytes has been especially rapid over the last three decades. e lowest euthrophication is found in Lake Giluitis. ere are no fundamental changes in the euthropication of Lake Dusia situated in the upper reaches of the catchment. e main goal of the present work was to evaluate the main phosphorus sources in the Dovinė river catchment, its retention in the lakes and outflow from the catchment.