Maximal sustainable sinking velocity of phytoplankton

Abstract

Most phytoplankton species have a tendency to sink. Phytoplankton require light for photosynthesis, however. Therefore, phytoplankton species that sink too fast will not be able to sus- tain a viable population in the euphotic zone. This points to the existence of a maximal sustainable phytoplankton sinking velocity. Using a reaction-advection-diffusion model of phytoplankton growth in a stratified water column, we derive that this maximal sinking velocity is inversely proportional to the turbidity of the water column. In other words, clear waters can sustain species with high sinking rates, whereas turbid waters can sustain species with low sinking rates only. We show that this prediction is both qualitatively and quantitatively supported by empirical data. An intriguing impli- cation is that export production of sinking phytoplankton might be sensitive to the turbidity of the water column.