Vltava cascade as teaching grounds for reservoir limnology

Abstract

The empirical and modelling limnological studies of the reservoirs of the Vltava Cascade spanning over 30 years have demonstrated the major differences between stratified throughflowing reservoirs and lakes. The most significant limnological differences are caused by the generally deep outflows due to the extreme position of the maximum depths, and high flow rates causing flow-driven gradients. Limnological conditions in dam reservoirs are very sensitive to the theoretical retention time of water in the reservoir (RT), in particular in the range RT < 200 days. The primary effect of the high inflow rates is the weakening of stratification, due mainly to increasing bottom temperatures leading to deeper mixing, which results in lower surface temperatures. The hydrophysical changes subsequently affect the fate of chemical constituents and the primary production; the effects are propagated to higher levels and fed back to both chemical and physical water quality. A reservoir being second or successive in a cascade differs limnologically in that it is affected by the dominantly hypolimnetic releases of the upperlying reservoir. The differences in hydrophysics, hydrochemistry and hydrobiology have been specified and the findings are, in general, supported by modelling studies.