Torn between extremes: the ups and downs of phytoplankton

We present a model to investigate the success and resulting patterns of phytoplankton migration, based on motility which depends exclusively on the internal energy and nutrient state of the cells. The model consists of migrating and non-migrating sub compartments for phytoplankton, and migration is a function of the prescribed threshold values for internal quotas. The different modes of phytoplankton behavior are evaluated in the framework of a coupled physical–biological model that includes wind-driven up- and downwelling. The results show that (1) migration is almost always advantageous with respect to biomass, (2) a wide variety of migration patterns (e.g., subsurface maxima, surface-avoidance behavior) can be reproduced by a relatively simple treatment of motility, (3) multiple deep chlorophyll maxima can be explained as the result of certain threshold values in combination with negligible vertical movement of the water, and (4) descending tongues of high phytoplankton concentration may be caused by migratory behavior and not necessarily by subduction due to frontal convergence. Thus, our model offers explanations for a large variety of observed phytoplankton distributions and migration patterns.The comments of two anonymous reviewers are gratefully acknowledged.