The characterization and field testing of a compact in situ optical instrument using fluorescence, absorbance, and scattering to identify and quantify contaminants and natural substances in water bodies are described. The instrument, based on a technology named LEDIF (LED-Induced Fluorescence) developed by Ng et al. (2012a) was configured for in situ long-term continuous monitoring at a fixed location. The optical performance and sensing capabilities of the instrument were assessed by calibrating against laboratory-prepared and commercial standards, and compared with several commercial sensors and published values. The effects on sensor response of temperature, pressure, and particle interference due to turbidity and total suspended solids were assessed and reported. Instrumental baseline shift over time, instrument detection limits, and the capability of LEDIF to determine characteristics such as quantum yield and intramolecular deactivation of fluorescence response, were also assessed. Experimental results match well with results from commercial instruments and published data. To demonstrate field deployment, the LEDIF instrument was installed at Chinese Garden of Singapore for longterm continuous monitoring of chlorophyll a (Chl a) and turbidity at a management-selected location. The analytes were chosen based on management's needs in recording temporal trends of algal biomass and water cloudiness to develop effective controls of inflow and outflow water quality. A monitoring record showing the temporal response of algal biomass to ambient conditions and turbidity variations is presented and discussed.