Stoichiometric modulation of zooplankton grazing on ocean organic matter biogeochemistry: Results from idealized food web modeling

Understanding how the ecological stoichiometry of plankton is related to the broader carbon cycle is one of the major questions in ecology and biogeochemistry. However, the role that stoichiometric imbalance between marine phytoplankton and zooplankton play in driving their interaction is not well understood. Here, we use a simple food web model set in idealized, 0- and 1-dimensional model domains in subtropical conditions to study how zooplankton selectivity for a higher food quality prey can shift the plankton community, stoichiometry, and carbon export production. In the model, a single class of zooplankton, whose stoichiometry is close to the Redfield C:N:P ratio, has two prey types: eukaryotic phytoplankton and cyanobacteria. As observed, the cyanobacteria type in the model is a gleaner, has a higher C:N:P ratio, and is thus more nutritionally unbalanced than the eukaryote type. We find that as zooplankton grazing becomes more selective based on food quality, it drives down the eukaryote biomass, allows cyanobacteria to flourish, depletes the ambient nutrient levels, elevates the phytoplankton and organic matter C:N:P ratio, and increases carbon export production. In the 1-dimensional model, these general trends are modulated as the euphotic zone and the mixed layer depths change seasonally. In a novel and important finding, this work indicates that the stoichiometric modulation of grazing can have a direct link to carbon export in the ocean. A more realistic modeling effort, combined with model-data comparison, is needed to confirm this finding.