A Modeling Approach to Determine How Prey Size Choice Reveals an Emergent Keystone Predator Effect in Planktonic Communities

Across the world’s oceans, herbivorous protists are often the dominant consumers of primary producers, but the behavioral and taxonomic diversity of herbivorous protists make them difficult to incorporate faithfully in models. Here, we developed a size-based mechanistic model of grazing dynamics explicitly parameterized for these organisms. We examined three consumer size classes--5, 50 and 200-μm. We contrasted predictions from a simple closed nutrient-phytoplankton-zooplankton model with those from a one-dimensional environmental model. In both frameworks, consumer:prey size ratios did not follow the canonical 10:1 ratio. The smallest herbivorous protists consumed prey up to 4-μm in diameter, 50-μm consumers up to 256 μm, and 200-μm consumers up to 512 μm. With all three consumer sizes present, the smallest consumers grazed the smallest phytoplankton while the largest consumers had unchanged grazing dynamics. Similar patterns emerged in the one-dimensional biogeochemical model. In both frameworks, grazing on smaller phytoplankton freed up resources for larger phytoplankton. Phytoplankton sizes ranged from 1 to 8 μm when the only consumers were 5 μm, but from 1 to 512 μm when 200-μm consumers existed. These patterns reflect the keystone predator effect in which a predator consumes the most competitive prey, allowing less competitive prey to exist. Consumer-specific subsistence prey concentrations highlight the competitive advantage of the largest consumer and how parameter alterations could favor other sizes. This study underscores the sensitivity of modeled ecosystem structure to the details of grazing dynamics, which can significantly impact biogeochemical cycles and our understanding of the base of the food web.