The effect of outflow depth on phosphorus retention in a small, hypertrophic temperate reservoir with short hydraulic residence time

Stratification and phosphorus fluxes (input. output, sedimentation. and rel ease from sediments) were studied in Ceske udoli Reservoir (49 degrees 43'N , 13 degrees 21'E; V - 2.65 x 10(6) m(3); A - 1.04 x 10(6) m(2), z(max) - 5 .5 m; surface altitude - 314 in a.s.l.) during two summer stratification pe riods which differed in outlet operation and in hydraulic residence time (1 997: surface outlets and 14 days 1998: bottom outlets and 23 days). Use of bottom outlets resulted in weaker thermal stratification, a less pronounced oxygen deficit in the hypolimnion. and significantly lower P retention (17 %) in comparison with surface discharge (42%). Factors apparently contribut ing to lower retention of P during the use of bottom outlets were: (i) fast er flushing of the hypolimnion which intensified longitudinal transport of particles, (ii) more intense P release from sediments due to a lower concen tration of nitrate in the water column and to more frequent contact of sedi ments in the shallows with water of pH > 9, During both periods, phytoplank ton production was limited only by light, not by nutrients. Phytoplankton b iomass was comparable in both Years despite some differences in vertical di stribution.