Nutrients-Unicellular-Multicellular (NUM) model
Here we provide a short presentation of the NUM model and of the modifications we made related to the temperature norm (see below and the Supplementary Materials for more details). A detailed description of the model and its governing equations can be found in Serra-Pompei et al. (2020). NUM is a semi-chemostat size and trait-based plankton functional type model that accounts for protists and copepod life cycles. The model includes four abiotic factors: light, ambient temperature, mixed layer depth, and nutrient concentration (nitrogen). The plankton populations are homogeneously distributed in the upper mixed layer. Nitrogen enters the system via the mixing rate between the upper and the deep layer and leaves the system via sinking or particulate organic matter, but no vertical migration (daily or seasonal) or other biological activity is resolved in the deep layer. Nitrogen is channeled in the food web via diffusive uptake by protists and predator-prey interactions. Finally, the particulate matter generated by organisms is recycled, returning nutrients to its dissolved form in the surface layer.
Each plankton group in the model represents a population of various individuals with shared traits like size and foraging behavior. The population growth rate depends on the uptake, encounter, ingestion, assimilation, respiration, growth, and reproduction rates. Protists are assumed to be spherical cells, distinguished by their size and temperature norm (below and SM). As potential mixotrophs, protists constantly move within the trait spectrum of autotrophy, mixotrophy, and heterotrophy, depending on the available resources. Their uptake rates follow a Holling type II response, and their biomass concentration is the balance between energy gain and losses from predation and background mortality (losses not caused by predation).
The life cycle of copepods is represented by 8 discrete life stages, 7 juveniles, and one adult. The juvenile stages invest all their energy gain in somatic growth and the adult stage only in reproduction. Like protists, the ingestion rate of copepods follows a Holling type II functional response. All copepods are omnivorous and can prey on protists or other copepods within their grazing kernel. Copepods are distinguished by two traits: the maximum adult body length and foraging mode – active or passive. Active feeders have higher ingestion and growth rates but face increased respiration costs and predation mortality. In contrast, passive feeders have lower rates and predation risks. Copepod losses arise from predation and background mortality, with larger individuals facing additional predation from unspecified larger predators (e.g., fish).