It is presently unclear to what extent coastal and oceanic chromophoric dissolved organic matter (CDOM) is remnant of diluted inputs from the terrestrial biosphere, is the product of in situ biological processes, or is a derivative of both sources. The development of a persistent phytoplankton thin layer (2--4 m thick) in East Sound, Washington, provided an opportunity to study the link between CDOM formation and in situ phytoplankton production. Spectral CDOM absorption ag(λ), spectral particulate absorption ap(λ), and hydrographic parameters were simultaneously recorded in high-resolution vertical profiles. Significant fine-scale variability was observed in ag(412). The majority of this fine structure covaried with salinity, consistent with mixing between an oceanic end member water type (low CDOM, high salinity) and a riverine one (high CDOM, low salinity). During the development of the thin layer, deviations from this ag(412)-salinity relationship were observed. To isolate only the changes in ag(412) resulting from in situ processes, the initial state, represented by the ag(412)-salinity relationship, was subtracted from the measured ag(412). The remaining in situ change or residual ag(412) fraction was linearly correlated with oxygen supersaturation and ap(440). Above the phytoplankton layer centered at 3--6 m the ag residual was negative, suggesting removal by photodegradation. A positive ag residual within the thin layer is the first direct evidence for rapid in situ CDOM production associated with phytoplankton primary production. Although the in situ fraction was 10% or less of the total CDOM absorption in East Sound during this period, in the open ocean this fraction may comprise nearly all of the absorption by dissolved materials. ¿ 2001 American Geophysical Union