Top-down control of marine phytoplankton diversity in a global ecosystem model

The potential of marine ecosystems to adapt to ongoing environmental change is largely unknown, making prediction of consequences for nutrient and carbon cycles particularly challenging. Realizing that biodiversity might influence the adaptation potential, recent model approaches have identified bottom-up controls on patterns of phytoplankton diversity regulated by nutrient availability and seasonality. Top-down control of biodiversity, however, has not been considered in depth in such models. Here we demonstrate how zooplankton predation with prey-ratio based food preferences can enhance phytoplankton diversity in a ecosystem-circulation model with self-assembling community structure. Simulated diversity increases more than threefold under preferential grazing relative to standard density-dependent predation, and yields better agreement with observed distributions of phytoplankton diversity. The variable grazing pressure creates refuges for less competitive phytoplankton types, which reduces exclusion and improves the representation of seasonal phytoplankton succession during blooms. The type of grazing parameterization also has a significant impact on primary and net community production. Our results demonstrate how a simple parameterization of a zooplankton community response affects simulated phytoplankton community structure, diversity and dynamics, and motivates development of more detailed representations of top-down processes essential for investigating the role of diversity in marine ecosystems.