The annual average degree of phosphorus retention (both TP and OP) in reservoirs was found to depend mainly on the theoretical retention time (RT). The phosphorus retention (RP) is shown for some reservoirs in Czech Republic to be well approximated (standard deviation = 10.9) with RP(TP) = 74.7* (1-exp(-0.0255* RT)) (N = 38, 7 ≤ RT ≤ 604 days).
The inclusion of 13 additional reservoirs for which data were assembled from literature resulted in an almost identical relationship: RP(TP) = 76.1* (1-exp(-0.0282* RT)) (N = 51, range of RT nearly unchanged).
The retention of soluble reactive phosphorus RP(OP) studied on data from literature seems to be higher than that of TP.
Statistical comparison with data for RP(TP) of lakes has shown a significantly lower retention in lakes than in reservoirs for comparable RP, resulting in RP(TP) = 66.0* (1-exp(-0.00419* RT)) (N = 59, 5.8 ≤ RT ≤ 260000 days).
The differences between RP(OP) and RP(TP) and differentiation of lakes and reservoirs are discussed and a need for more thorough theoretical analysis stressed. Consequences for watershed management are summarized.