Quoted from: https://ukesm.ac.uk/ukesm-component-models/ocean-biogeochemistry/ocean-biogeochemistry-ukesm1/# 

This model is an ‘‘intermediate complexity’’ model of the plankton ecosystem founded on the oceanic nitrogen cycle. Although highly simplified, MEDUSA is designed to include sufficient complexity for it to address major feedbacks between ocean biogeochemical cycles and anthropogenic drivers such as climate change (CC) and ocean acidification (OA).

In addition to nitrogen, MEDUSA includes the elemental cycles of carbon, oxygen, silicon, and iron, and links these together in a dual size-class nutrient-phytoplankton-zooplankton-detritus (NPZD) plankton ecosystem model.

In terms of nutrients, MEDUSA includes nitrogen, silicon, and iron nutrients that are required (together with sunlight) for the growth and carbon fixation of autotrophic phytoplankton.

MEDUSA’s phytoplankton are represented by “small” nanophytoplankton (typically photosynthetic prokaryotes) and “large” microphytoplankton, both of which require nitrogen and iron nutrients. The latter are assumed synonymous with siliceous diatoms, an important eukaryotic algal group, and additionally require silicic acid for their growth. Both phytoplankton groups are modelled with a dynamic chlorophyll quota to allow them to photoacclimate across a range of surface, submarine and seasonal light conditions.

In turn, MEDUSA’s phytoplankton are consumed by two size classes of heterotrophic grazers, microzooplankton and mesozooplankton. The former are assumed to be faster growing single-celled protists, such as flagellates, that consume “small” phytoplankton, while the latter are assumed to be multicellular metazoans, such as copepods, that consume both phytoplankton size classes as well as the microzooplankton.

Mortality and other loss processes of the modelled plankton components produce particles of non-living detrital material that sink into the ocean interior. As with the other components, these are divided into “small”, slow-sinking particles and “large”, fast-sinking particles. The former are represented explicitly in MEDUSA, while the latter are associated with ballasting biominerals and modelled implicitly.