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 statu