The effect of grazers on the vertical distribution of phytoplankton

Background/Question/Methods   Phytoplankton are known to form heterogeneous vertical distributions in poorly mixed water columns: benthic layers, deep chlorophyll maxima (DCMs), and surface scums. Simple game theoretic models have been successful in explaining how these diverse distributions can result from competition for nutrients and light when phytoplankton can move fast enough to overcome turbulent mixing. However these models do not include zooplankton grazers, which may influence the vertical distribution of phytoplankton. To address this question, we developed a one-dimensional reaction-diffusion-advection model of phytoplankton, nutrients, light, and zooplankton. We analyzed this full model as well as a simplified game theoretic model.

Results/Conclusions   Grazers tend to widen the distribution of phytoplankton, with more efficient grazers having a more pronounced effect. Grazers can widen a DCM until it reaches the surface, the bottom, or both. Spatial redistribution of phytoplankton buffers the top-down effect of grazers until the surface or compensation depth are reached. Therefore, grazers can have significant effects on the vertical distribution of phytoplankton.