Automated water quality monitoring of humic lakes by using the optical properties of water

Abstract

Automated water quality monitoring (AWQM) is becoming increasingly common in lakes worldwide. The history of AWQM is relatively short and standard calibration procedures for the measured variables are largely yet to be established. The use of optical AWQM sensors, developed in oceanic environments, raises new questions on the diverse effects which humic compounds may have on the automated optical measurements in inlands waters. The focus of this thesis was to characterize the effects of coloured dissolved organic matter (CDOM) on optical in situ measurements of organic matter (OM) and chlorophyll (Chl) in lakes with varying humic content, and to use AWQM data as a part of traditional monitoring and independently to study current topics in limnology; weather-related episodic events on lake mixing and the effect of hypolimnetic oxygenation (HLO) on the nutrient conditions in a humic lake. The data were collected from AWQM stations on lakes in Finland and Ireland and involved both AWQM and discrete data from open water season. The study on the quality of CDOM revealed that OM fluorometers represented the humic content in water column reliably. However, OM fluorometers did not detect changes in the quality of OM. In situ Chl fluorometer was affected by background OM. The effect was less important when OM remained at constant level. Effects of weather-related episodic events on lake mixing and on Chl-patterns studied with AWQM showed Finnish lakes to face multiple short-lived mixing events during stratified period. The Chl-a content in the lakes varied accordingly. A case study conducted on Jyväsjärvi showed that HLO had a significant effect on the temperature structure of the lake but had little long-term effect on its trophic status