Phytoplankton growth and light absorption as regulated by light, temperature, and nutrients

Numerous studies of the growth of phytoplankton in the laboratory have demonstrated the dependence of cellular pigment concentration and growth rate upon light intensity, photoperiod, temperature, and nutrient supply. These same environmental parameters vary with season in the polar seas and presumably affect the growth rate and cellular pigment concentration of the phytoplankton crop. Unfortunately, there has not been a complete mathematical description of the interaction of all four environmental parameters. This study presents an approach to describing these interactions. It can reasonably be assumed that the gross specific growth rate, g, is a function of the specific rate of light absorption. The dependent variables in this equation are g, the gross specific growth rate, ?, the maximum carbon-specific photosynthetic rate, and, ?, the ratio of carbon to chlorophyll. The value of all three dependent variables is constrained. The independent variables are E?, the light intensity (assumed constant during the photoperiod), and ?, the photoperiod (as a fraction of 24 hours) that the cells are illuminated, ? is the instantaneous capacity of the dark reactions to assimilate electrons, while the product ap?max E?/? is the instantaneous capacity of the light reactions to supply electrons. If the capacity for photochemistry exceeds the capacity for assimilation, dissipative processes occur, and the quantum yield is low. We have applied this equation to the analysis of the growth and light absorption by Skeletonema costatum cultured under light, temperature, and nutrient limitation. Decreases in nutrient supply and temperature cause decreases in ? and increases in ?; thus both the capacity for electron supply and utilisation decrease. However, decreases in temperature decrease the capacity for electron assimilation more rapidly than the capacity for supply; quantum yield drops. Decreases in nutrient supply cause the capacity for supply and assimilation to drop in parallel; quantum yeield is maintained. Decreases in day length cause decreases in ? and increases in ?. The capacity to assimilate electrons and the capacity to supply electrons increase in parallel; quantum yield is maintained. Decreases in light intensity cause decreases in both ? and the capacity to supply electrons. Although the changes in ? with light intensity arc difficult to assess, the capacity to assimilate electrons appears to be little changed by light limitation. Quantum yields increase with decreasing light levels.