Do Temporal Diversity Dynamics Reconcile Viral and Grazer Based Theories for the Propocalypse?

Understanding the relative importance of top-down vs. bottom up controls for setting plankton populations is an open challenge in marine ecology. Recent work has focused on the sharp spatial decline in Prochlorococcus biomass when moving northward in the North Pacific transition zone. This work has argued against bottom up controls like temperature and for top down mechanisms like apparent competition or viruses setting the location of the collapse. However, as temperature and light modify the underlying rates in the system we would expect them to play some role in the temporal dynamics. Here, we seek to unify bottom up and top down controls to make predictions about the seasonal progression of the ‘Propocalypse’ and the associated predators of Prochlorococcus. Observations suggest that the Propocalypse occurs further poleward in the summer and about 10 degrees further south in the winter. Here, we use models to confirm that ecological interactions allow for the existence of the transition, and that the meridional movement is determined by growth rate changes due to light and temperature in the spring, and by mixed layer depth changes during the fall and early winter. We go on to seek a diversity-mortality relation for Prochlorococcus connecting viral mortality to the seasonal motion of the Propocalypse.