Population dynamics of sinking phytoplankton in stratified waters

We analyze the predictions of a reaction-advection-diffusion model to pinpoint the necessary conditions for bloom development of sinking phytoplanktonspecies in stratified waters. This reveals that there are two parameter windows that can sustain sinking phytoplankton, a turbulence window and athermocline-depth window. The two windows are delimited by fourcritical parameters: a minimal and maximal turbulence, and aminimal and maximal thermocline depth. Simple analytical expressions for these four critical parameters are presented, and their relation to critical parameters previously introduced by Riley, Sverdrup, and others are explained.Both parameter windows disappear if the phytoplankton sinkingvelocity becomes too high. This indicates the existence of a maximal phytoplankton sinking velocity that can be sustained. We derive that this maximal sinking velocity is inversely proportional to the background turbidityof the water column. In other words, clear waters can sustain species with high sinking rates, whereas turbid waters can sustain species with lowsinking rates only. We demonstrate that this prediction is both qualitatively and quantitatively supported by empirical data. An intriguingimplication is that export production of sinking phytoplankton can be very sensitive to the turbidity of the water column. The theory presentedhere offers a new framework for a better understanding of the population dynamics of sinking phytoplankton species in stratified waters.