Light and nutrient availability affect the size-scaling of growth in phytoplankton.

Communities of marine phytoplankton consist of cells of many different sizes. The size-structure of these communities often varies predictably with environmental conditions in aquatic systems. It has been hypothesized that physiological differences in nutrient and light requirements and acquisition efficiencies contribute to commonly observed correlations between phytoplankton community size structure and resource availability. Using physiological models we assess how light and nutrient availability can alter the relative growth rates of phytoplankton species of different cell sizes. Our models predict a change in the size dependence of growth rate depending on the severity of limitation by light and nutrient availability. Under conditions of growth-saturated resource supply, phytoplankton growth rate (mol C cell(-1)time(-1)) scales with cell volume with a size-scaling exponent of 3/4; light limitation reduces the size-scaling exponent to approximately 2/3, and nutrient limitation decreases the exponent to 1/3 as a consequence of the size-scaling of resource acquisition. Exponents intermediate between 1/3 and 3/4 occur under intermediate availability of light and nutrients and depend on the size-scaling of pigment photoacclimation and the size range examined.