Modern analytical and remote sensing approaches allow for detailed characterization of the composition, chemistry, and spatial distributions of dissolved organic matter (DOM) in aquatic systems. In collaboration with scientists from the Bigelow Laboratory for Ocean Sciences (Boothbay Harbor, Maine), researchers in the USGS Water Mission Area are employing a wide range of analytical tools, including Moderate Resolution Imaging Spectroradiometer (MODIS) data, to assess the influences of riverine sources on DOM in the Gulf of Maine (GoM). Measurements of dissolved organic carbon (DOC) concentrations, absorption coefficients (at 254 nm, 350 nm, and 412 nm), specific ultraviolet absorbance (SUVA254), spectral slope, fluorescence, DOC fractionation, and isotopic composition are used to determine the amount and nature of DOM from major inflowing rivers, the Gulf of St. Lawrence, and the Gulf of Maine (GoM) itself. Additional chemical analyses are being performed on major fractions of the DOM isolated from a smaller subset of samples from the Penobscot River, Penobscot Bay, GoM waters in the Western Maine Coastal Current (WMCC), a sample from the eastern portion of the GoM (Scotian Shelf waters), and the Pacific Ocean. These samples provide detailed DOM compositional data in support of the more easily collected concentration and optical data obtained from discrete samples, optical data obtained by an in situ glider, and remotely sensed satellite observations. Results from this study are being used to ground-truth and calibrate remote sensing optical data and develop relationships for predicting DOC concentration in the GoM.