Most of our understanding of sources and controls over dissolved organic (DOM) concentration and composition is focused on variability over seasonal or interannual timescales. Spectral properties of absorbance and fluorescence of colored dissolved organic (cDOM) contain intrinsic information about the chemical composition of the DOM as a whole, as well as extrinsic information about the sources and environmental processes. These properties can be measured rapidly in situ, and thus provide new and unique information about high frequency variability - on the order of minutes to hours - in the concentration, composition, and source of DOM. We use continuous in situ measurements of cDOM spectral absorbance and fluorescence to examine changes in DOM at unprecedented time scales. We observe changes that result from diurnal processes in river systems, tidal processes in wetland systems, and hydrologic processes in watersheds. We will present results from these studies in which we observe rapid changes in inferred intrinsic properties such as humic content and reactivity, as well as rapidly changing extrinsic properties such as source and association with metals. These observations provide new insights into biogeochemical processes.Bergamaschi, B.A., B.D. Downing, R.G. Spencer, B.A. Pellerin, and E.S. Boss, 2008. High Frequency Variability in Dissolved Organic Matter Composition and Activity as Determined Using In Situ Optical Measurements. 2008 Ocean Sciences Meeting, Orlando, FL, March 2-7.