A Lagrangian model of phytoplankton photosynthetic response in the upper mixed layer

A previously developed Lagrangian model considered the distributionand light exposure of individual phytoplankton cells in a weaklystratified upper ocean layer mixed by depth-dependent turbulence.This model is coupled with a phytoplankton photoresponse model,previously applied in a Eulerian context, that includes a temporallydecaying dependence on inhibitory light exposure. The combinedmodel output provides the trajectory, light experience and photoresponseof each individual, or the photoresponse central tendency andvariability of the population that occurs at a given depth ata given time. The time dependence of the photoresponse is themain variable considered and determines the shape of the verticalprofile of primary production. Shorter response times yielda strong photoinhibition event that is nearly synchronous withinhibiting incident radiation. Longer response times are characterizedby weaker photoinhibition events that outlive the occurrenceof the inhibiting incident radiation for an interval relatedto the imposed time constant. Under the depth-dependent turbulentmixing regime imposed, photoinhibition is detectable even atwind speeds up to 10 m s^–1. Modeling and technologicalapproaches that compare the bulk characteristics of populationsto the statistical characteristics of the individuals formingthose populations are both under active development. A fruitfulfeedback relationship is likely in the near future.