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).