We investigated the optical properties of surface water in areas of the West Florida Shelf influenced by riverine discharge and by the occurrence of a phytoplankton plume. Results of absorption and fluorescence spectroscopy analyses and determination of dissolved organic carbon (DOC) concentration showed that the injection of riverine colored dissolved organic matter (CDOM) strongly affected the optical properties and DOC concentrations over the shelf. Riverborne nutrients contributed to an increase in primary productivity. However, during the study period, the increase in primary productivity did not result in the production of significant amounts of CDOM.Fluorescence spectroscopy results showed that optical properties of riverine CDOM were lost close to the mouth of the rivers. A simple mathematical model describing mixing between riverine and marine end‐members demonstrated that most of the observed changes in optical properties of CDOM along salinity gradients can be explained by mixing. Laboratory mixing experiments between riverine water and seawater indicated that flocculation of organic matter during estuarine mixing did not affect the optical properties of CDOM. Remote‐sensing studies of the Gulf of Mexico using historical data from the Coastal Zone Color Scanner show a recurrent, seasonal chlorophyll bloom over the West Florida