The effect of a sewage plume on the biology, optical characteristics, and particle size distributions of coastal waters was evaluated around the Sand Island, Hawaii, outfall diffuser. In situ physical and biooptical data and Niskin bottle samples were collected during a 1-week cruise from September 25 to October 1, 1994. One or two layers affected by sewage could be distinguished in the water column: recently discharged (new) sewage plume waters and old plume waters. In conditions characterized by high Froude number the distribution of chlorophyll fluorescence in new plume waters was the same as for ambient waters, while for low Froude number, chlorophyll fluorescence increased within the plume, demonstrating the importance of physical forcing on effluent and phytoplankton interactions. New plume waters were associated with at least 2.7-fold increases in particle load, high concentrations of particles larger than 70 μm, increases in ammonium, phosphate, and silicate, and high levels of heterotrophic bacteria and Prochlorococcus compared to surrounding waters. Both new and old plume layers, but not phytoplankton layers, showed distinct increases in fluorescence for the excitation/emission (Ex/Em) wavelength pair Ex/Em=228/340 in nm, interpreted as particulate tryptophan-like fluorescence. Such fluorescence may be useful as a new in situ real-time indicator of waters affected by effluent discharges. ¿ 1997 American Geophysical Union