Hydrobiologia 506–509: 759–765, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 759 Trophic state of a lowland reservoir during 10 years after restoration Ryszard Goldyn, Tomasz Joniak, Katarzyna Kowalczewska-Madura & Anna Kozak Department of Water Protection, A. Mickiewicz University, Drzymaly 24, 60-613 Poznañ, Poland E-mail: rgold@amu.edu.pl Key words: trophic state, reservoir ageing, chlorophyll-a, total phosphorus, Secchi depth Abstract The restored Maltañski Reservoir was studied from its filling with water in 1990 till 2000. Total phosphorus, chlorophyll-a, and Secchi depth, as well as the Carlson’s trophic state index (TSI) values based on those three parameters showed characteristic patterns of changes among seasons and years. Within each year, the lowest trophic state was usually observed in winter and the highest in summer. Because of the high loads of phosphorus received by the reservoir, this element did not limit primary production. TSI values calculated on the basis of total phosphorus were always markedly higher than calculated on chlorophyll-a and Secchi depth (similar to each other). The trophic upsurge phase lasted only a few months after the filling of the reservoir in 1990. Similar symptoms were observed after its refilling in the spring of 1993. The trophic depression phase lasted until the end of 1995. After that time a significant correlation between phosphorus concentration in the reservoir and in river waters flowing into the reservoir was observed. The successive phases of reservoir ageing, determined on the basis of phosphorus concentration, were not accompanied by changes in chlorophyll-a content. The influence of the top- down mechanism (biomanipulation effect) resulted in relatively low values of chlorophyll-a after the filling of the reservoir with water in 1990 and in 1993. As early as 1992 chlorophyll-a values reached a very high level and stayed at that level until the end of the study in 2000 (except for the short decline in 1993). Introduction The process of ageing of dam reservoirs has been stud- ied extensively, especially in the former Soviet Union, Czechoslovakia and the United States (for reviews, see Ostrofsky, 1978; Straškraba et al., 1993). Most of the studies concerned large reservoirs formed by flood- ing vast areas of meadows, fields, thickets, deforested land (sometimes even uncleared forest patches), or abandoned human settlements. As a result of leaching and decomposition of organic matter, large amounts of nutrients were usually released from the flooded areas into the water. This resulted in a strong increase in the trophic state of the reservoir after its flooding. The next phase concerns the gradual decrease of nutri- ent concentration and lasts usually several years. This decrease in trophic state is followed by slow eutroph- ication process, associated with the influence of the catchment (Straškraba et al., 1993). In this study the process of ageing of the small and shallow Maltañski Reservoir, created on a pol- luted river was analysed. Before its filling with water, the bottom was reshaped artificially. The vegetation and the fertile layer of the soil were removed to min- imize their influence on the quality of stored water. The objective of this paper was to analyse the age- ing phases in this lowland reservoir influenced by the high external loads of nutrients and to determine if the special preparation of the basin before flooding had a long-term effect on the trophic state of such reservoirs. Study area and methods The Maltañski Reservoir is located in the City of Poznañ, in the lowlands of mid-western Poland. It is relatively small and shallow (area 64 ha, volume 2 × 10 6 m 3 , mean depth 3.1 m, max. depth 5 m). Its mean water residence time is 34 days, while the mean discharge of the Cybina River, which flows through the reservoir, is 0.67 m 3 s −1 .