Examples of the diel variability in chlorophyll and beam attenuation are presented using data from moored in situ fluorometers and transmissometers, and from profiles of the beam attenuation coefficient and fluorescence. The data are discussed in terms of the three primary processes thought to influence the diel variability in chlorophyll: (1) fluorescence yield per unit chlorophyll a, (2) chlorophyll a changes per cell (or, carbon), and (3) changes in phytoplankton carbon (growth). A simple, provisional, model is presented which incorporates these three biological processes, under the assumption that the corresponding diel variability in particle attenuation represents the change in phytoplankton carbon. The analysis produces qualitative agreement with the in situ data. The model suggests that while under high-light conditions fluorescence declines, chlorophyll can still increase during the day. Under low-light conditions, the diel variability of chlorophyll and fluorescence are in phase. The model is limited by (1) physiological understanding of the processes involved in producing fluorescence and chlorophyll changes over diel time scales; (2) biomass-independent variations in particle attenuation; and (3) not including, for the present, physical forcing.