USING HYPOLIMNETIC DISSOLVED-OXYGEN DEPLETION RATES FOR MONITORING LAKES

Volumetic hypolimnetic oxygen depletion, caused primarily by the decomposition of organic material in the hypolimnion (VHDR(org)), is considered to be an important indicator of the trophic state of lakes and is used to detect change of trophic state. The observed volumetric hypolimnetic 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 rate at which the observed hypolimnetic dissolved oxygen depletes are reviewed. In many instances, if the limnology of the system is understood, the perturbations to the hypolimnetic respiration rate, VHDR(org), caused by these phenomena can be ignored and VHDR(obs) used for lake management. Guidance is provided as to when the effects of these phenomena can be ignored and when they cannot be. Should corrections be necessary, methods for calculating the effects of physical perturbations are outlined. However, most of the effects caused by biological perturbations from changes in algal species cannot be eliminated, except for those caused by hypolimnetic production.