Phytoplankton living in the mixed layer of a river, lake or ocean are exposed to fluctuating irradiance. These include fast fluctuations from surface waves, moderate changes from daily weather cycles and mixing in rivers, lakes and oceans as well as long-scale effects such as seasons. Phytoplankton entrained in a water column that is well mixed move between the surface and bottom waters in a time scale ranging from seconds in shallow, well-mixed rivers (Harding et al., 1987) to minutes and hours in deeper rivers, estuaries, lakes and the ocean (Reynolds, 1987; Mallin and Paerl, 1992). This vertical movement within the water column is associated with constant variation in intensity of light available to phytoplankton for photosynthesis in these time scales, such as high light intensities at the water surface and low light intensities outside the euphotic zone. The available literature on phytoplankton responses to fluctuations in light fields has primarily focused on physiological processes such as photosynthesis, respiration and variations in chlorophyll a while fewer have examined growth rates [see (Litchman, 2000) for a summary]. Field methods utilized often include movement of bottles through the water column (Marra, 1978) or varying the amount of light from a source (Mallin and Paerl, 1992). Laboratory-based studies have usually used rotating incubations or fluctuated light from a source (Flameling and Kromkamp, 1997). Most studies have concentrated on marine and estuarine waters (Marra, 1978; Gallegos and Platt, 1982; Mallin and Paerl, 1992), while some have focused on freshwater plankton (Gibson and Fitzsimons, 1992; Litchman, 2000).