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Series of 16 fishponds was monitored as a part of a drainage basin of Hracholusky reservoir, which is subject of a recovery project. All the fishponds were eutrophic – hypertrophic due to wastewaters from villages and little cities (including overflows from sewers) and/or due to too intensive production of “hunters ducks” – the phenomenon that is rarely mentioned.

High eutrophication is the main factor that has recently disabled the Jordan water reservoir for water supply and recreational purposes. Large-scale dredging of sediments was done between years 2011 and 2014. Thereafter a complex monitoring of water quality has been initiated since 2015.

Stratification and phosphorus fluxes (input, output, sedimentation, and release from sediments) were studied in Ceske udoli 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 in outlet operation and in hydraulic residence time (1997: 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 contributing to lower retention of P during the use of bottom outlets were: (i) faster flushing of the hypolimnion which intensified longitudinal transport of particles, (ii) more intense P release from sediments due to a lower concentration of nitrate in the water column and to more frequent contact of sediments in the shallows with water of pH > 9. During both periods, phytoplankton production was limited only by light, not by nutrients. Phytoplankton biomass was comparable in both years despite some differences in vertical distribution.