Teka Kom. Ochr. Kszt. Środ. Przyr. – OL PAN, 2009, 6, 200–205 PHYTOPLANKTON – ELEMENT IN ECOLOGICAL STATUS ASSESSMENT FOR LAKES OF THE WEL RIVER CATCHMENT AREA Agnieszka Napiórkowska-Krzebietke * , Agnieszka Pasztaleniec ** , Andrzej Hutorowicz * * Inland Fisheries Institute, Oczapowskiego str. 10, 10–719 Olsztyn, akrzebietke@infish.com.pl ** Institute of Environmental Protection, Kolektorska str. 4, 01–692 Warsaw Summary. Determination of the role of phytoplankton in the ecological status assessment was taken up for ten lakes of the Wel river catchment area. Spring (April) and summer (July) 2009 results of the total biomass and phytoplankton composition were compared. Based on the func- tional groups structure, the Q index was calculated for three chosen lakes. The greatest phyto- plankton biomass (above 20 mg dm -3 ) in spring was recorded in four lakes situated on the Wel river. In July the planctonic algae biomass increased downstream and maximum value (52 mg dm -3 ) was observed in the farthest situated lake in Wel river catchment area. In summer Q index indi- cated a poor ecological status for Lake Rumian, a moderate state for Lake Lidzbarskie and a good status for Lake Kiełpińskie. Key words: phytoplankton, lake, ecological status assessment, the Wel river catchment INTRODUCTION Phytoplankton plays a very significant role in lake ecosystems, because of its position at the base of aquatic food-chain, and provides relatively unique information about the environmental condition of the ecosystems [McCormick and Cairns 1994]. Algae biomass and assemblages as important indicators are widely used for routine monitoring or the ecological status assessment [Padisák et al. 2008, Nõges et al. 2009]. The purpose of this study was to determine the role of phytoplankton in the ecological status assessment for lakes of the Wel river catchment area. These are preliminary results being the effect of a study conducted in the catchment area of the lowland river Wel (Central Poland, 791 km 2 ) within the framework of the