USING HYPOLIMNETIC DISSOLVED-OXYGEN DEPLETION RATES FOR MONITORING LAKES

Volumetic hypolimnetic oxygen depletion, caused primarily by the decom position of organic material in the hypolimnion (VHDR(org)), is consid ered to be an important indicator of the trophic state of lakes and is used to detect change of trophic state. The observed volumetric hypol imnetic depletion rate (VHDR(obs)) is usually different from VHDR(org) because of phenomena which can alter the concentrations of oxygen in the hypolimnion. The nine different phenomena which can affect the rat e at which the observed hypolimnetic dissolved oxygen depletes are rev iewed. In many instances, if the limnology of the system is understood , the perturbations to the hypolimnetic respiration rate, VHDR(org), c aused by these phenomena can be ignored and VHDR(obs) used for lake ma nagement. Guidance is provided as to when the effects of these phenome na can be ignored and when they cannot be. Should corrections be neces sary, methods for calculating the effects of physical perturbations ar e outlined. However, most of the effects caused by biological perturba tions from changes in algal species cannot be eliminated, except for t hose caused by hypolimnetic production.