JINDR ˇICH D URAS 1, 2 and J OSEFH EJZLAR 2, 3 1 Water Quality Laboratory, Vltava River Authorities, Denisovo nábrˇezˇí 14, 304 20 Plze ň, Czech Republic 2 Faculty of Biological Sciences USB, Branisˇovská 31, 370 05 ˇ Ceské Budeˇjovice, Czech Republic 3 Hydrobiological Institute AS CR, Na sádkách 7, 370 05 ˇ Ceské Budeˇjovice, Czech Republic The Effect of Outflow Depth on Phosphorus Retention in a Small Hypertrophic Temperate Reservoir with Short Hydraulic Residence Time key words: reservoir, phosphorus, sedimentation, water-sediment exchange, phytoplankton Abstract Stratification and phosphorus fluxes (input, output, sedimentation, and release from sedim were studied in ˇ Ceské údolí Reservoir (49°43 ′ N, 13°21’E; V – 2.65 × 10 6 m 3 ; A – 1.04 × 10 6 m 2 ; z max – 5.5 m; surface altitude – 314 m a.s.l.) during two summer stratification periods which differed outlet operation and in hydraulic residence time (1997: surface outlets and 14 days; 1998: bottom lets and 23 days). Use of bottom outlets resulted in weaker thermal stratification, a less pronounce gen deficit in the hypolimnion, and significantly lower P retention (17%) in comparison with surfac discharge (42%). Factors apparently contributing to lower retention of P during the use of bottom lets were: (i) faster flushing of the hypolimnion which intensified longitudinal transport of particles more intense P release from sediments due to a lower concentration of nitrate in the water column to more frequent contact of sediments in the shallows with water of pH > 9. During both periods, p plankton production was limited only by light, not by nutrients. Phytoplankton biomass was able in both years despite some differences in vertical distribution. 1. Introduction Eutrophication has attracted the attention of limnologists for several decades. An imp tant field of investigation is the behaviour of reservoirs in relation to the nutrient budge retention in reservoirs, expressed as a proportion of incoming P load, has recently been marised by S TRAS ˇ KRABA et al. (1995), S TRAS ˇ KRABA (1999a) and K ENNEDY (1999). The reten- tion of P depends above all on hydraulic residence time (HRT), P loading and inflow P c centration. In general, retention of P increases with increase of HRT (especially when H is lower than 100 days) and inflow P concentration. Moreover, HRT coupled with reserv operation controls thermal stratification and thus many other water quality param well (R EYNOLDS , 1992; S TRAS ˇ KRABA , 1999b). All the authors mentioned above have noted that great care is needed when applying the relationships found to shallow and highly fl ing reservoirs. The behaviour of these reservoirs could be very individual and more data needed before general conclusions can be drawn. The aim of this study was to compare P retention in a small, rapidly flushing tempera reservoir during stratification periods in two years differing in outflow operation (discha from surface or bottom). Factors influencing internal phosphorus fluxes – specifically se sedimentation, water/sediments exchange and sediment resuspension – were evalu understand differences in P retention between both observation periods. Internat. Rev. Hydrobiol. 86 2001 6 585 – 601 © WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 1434-2944/01/0611-0585 $ 17.50+.50/0