Natural waters are vulnerable ecosystems facing diverse anthropogenic constituents, natural substances and organisms that may have an adverse effect on their fragile equilibrium and ultimately on human health. To preserve aquatic ecosystems, it is thus crucial to understand chemical and biological processes that take place within them as well as to control relevant types of hazards and quality parameters at an appropriate temporal and spatial scale. This knowledge is required to enable efficient risk evaluation and management. Nowadays, to face these challenges, the trend goes towards sensor observation networks enabling the simultaneous determination of multiple parameters at high spatial and temporal scales. The scope of this doctoral thesis was to contribute to improve the ability of these observation networks by developing and field validating innovative sensing systems targeting the autonomous, in situ, high-resolution monitoring of a range of trace metals in their potentially bioavailable forms for the phytoplankton.