Killing the predator: impacts of top predator mortality on global-ocean ecosystem structure

Abstract. Recent metanalyses suggest that microzooplankton biomass density scales linearly with phytoplankton biomass density, suggesting a simple, general rule may underpin trophic structure in the global ocean. Here, we use a set of highly simplified food-web models, solved within a global general circulation model, to examine the core drivers of linear predator-prey scaling. We examine a parallel food-chain model which assumes microzooplankton grazers feed on distinct size-classes of phytoplankton, and contrast this with a diamond food-web model allowing shared microzooplankton predation on a range of phytoplankton size classes. Within these two contrasting model structures, we also evaluate the impact of fixed vs. density-dependent microzooplankton mortality. We find that the observed relationship between microzooplankton predators and prey can be reproduced with density-dependent mortality on the top predator, regardless of choices made about plankton food-web structure. Our findings point to the importance of parameterizing mortality of the top predator for models to recapitulate trophic structure in the global ocean.