Scaling of phytoplankton photosynthesis and cell size in the ocean

Abstract

We have determined the scaling relationship between photosynthesis rate and cell size in natural phytoplankton assemblages of contrasting marine environments. We found that phytoplankton photosynthesis in the ocean does not scale as the L‐power of cell size, but scales approximately isometrically with cell size, indicating that a single model cannot predict the metabolism‐size relationship in all photosynthetic organisms. The scaling relationship between cellular chlorophyll a content and cell size is also isometric. Taxonomical changes along the size spectrum may explain the deviation of phytoplankton photosynthesis from the general allometric rule. The size scaling exponent for photosynthesis is significantly higher (1.14) in coastal productive waters than in the oligotrophic open ocean (0.96), which provides a physiological basis to explain the dominance of larger cells in nutrient‐rich environments. The size scaling exponent for phytoplankton abundance is significantly less negative in coastal productive waters (–0.90) than in the oligotrophic open ocean (–1.25). The observed size scaling relationships imply that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in productive ones. By controlling the metabolism‐size scaling relationship, nutrient supply plays a major role in determining community size structure and the energy flow through the pelagic ecosystem.