Optimality-based plankton ecosystem model (OPEM) for the UVic-ESCM. OPEM includes optimality-based formulations for phytoplankton, diazotrophs, and zooplankton. Variable C:N:P stoichiometry is implemented for phytoplankton, diazotrophs, and detritus. It is mainly based on the version of Nickelsen et al. (2015) GMD.