A random walk stimulation model was developed to explore the effects of variations in light regimes due to vertical mixing on primary productivity. Cells were allowed to light-shade adapt on some time scale by altering chl: carbon ratios in response to variations in light regimes. Photosynthetic response was adjusted according to variations in chl:carbon ratios by either varying the initial slopes of photosynthesis-irradiance curves, or varying photosynthetic capacities. The model suggests that despite physiological adaptation to light, vertical mixing may have little effect on the integrated water column primary productivity. It is suggested that if photoinhibition does not have a pronounced effect, the average distribution of primary production in a water column is not related to variations in light regimes arising from turbulent diffusion processes.
the maximum intensity at the sea surface. Local atmospheric conditions, clouds, and aerosols modulate the light reaching the sea surface. Atmospheric variations are likely to occur at either low frequencies, 3 to 10-d periods between storms, or at very high frequencies, the travel time for clouds to cross the sun. Low frequency variations are too long to influence diel productivity and the passing of clouds, while perhaps important on some days, is too intermittent to be a general characteristic of light variations. The light received by a cell is perhaps best predicted from the surface intensity and depth of the cell. In a turbulent ocean, cells are displaced vertically about a daily mean depth and these displacements also create variations in light. Marra (1978 b) suggested that vertical turbulence may increase water column productivity and inferred that the effect is due to variations in light experienced by the cells. In this paper we examine the role turbulence plays in creating variations in light, and the effect of these variations on the daily productivity of the water column.