Much of the optical variability in the upper sea is caused by variations in the biogenous microparticles, which include the phytoplankton, the Chroococoid cyanobaccteria, and detritus. It is instructive to consider two sources of variability in these biogenous particles. First, changes in the absorption cross section of individual cells result from responses of phytoplankton to the ambient nutrient and light fields. These responses are relatively rapid (days) and a mathematical description of the response is presented. Second,changes in the concentration of detrital particles and cells are more difficult to predict and occur on time scales of weeks. A crude hypothetical model is presented which attempts to describe the general response of the plantonic community to light and nutrient fields.