Phytoplankton Size Structure

Abstract

The size of phytoplankton cells ranges widely over at least 9 orders of magnitude, from a cell volume around 0.1μm3 (equivalent to a diameter of c. 0.5μm) for the smallest cyanobacteria to more than 108μm3 (a diameter of 1000μm) for the largest diatoms. Cell size is a master functional trait that affects many aspects of phytoplankton physiology and ecology over multiple levels of organization, including individuals, populations and communities. Phytoplankton size structure, the partitioning of biomass among species of different cell sizes, is a fundamental property of pelagic ecosystems that largely determines their food-web organization and biogeochemical functioning. Plankton communities where small phytoplankton dominate are characterized by complex food webs, a predominance of recycling processes, and little export of biogenic carbon towards the ocean's interior. In contrast, phytoplankton assemblages with larger mean cell size are associated with shorter food chains and enhanced downward export fluxes. Small cell size confers a competitive advantage in terms of resource acquisition, which explains the dominance of small cells in environments where nutrient and/or light are in short supply. Large phytoplankton, compared to their smaller counterparts, are less strictly controlled by grazing, but they suffer stronger losses through sinking. Contrary to predictions by Kleiber's rule, the relationship between phytoplankton cell size and both biomass-specific metabolic rate and maximum growth rate is unimodal: cells of intermediate cell size (50–500μm3) sustain the fastest pace of metabolism and growth. Accordingly, phytoplankton blooms in the ocean are dominated by species of intermediate cell size. The biogeography and temporal variability of phytoplankton size structure are largely independent of a direct effect by temperature and driven by changes in resource supply.