The marine biogeochemistry component used in UKESM1 is the Model of Ecosystem Dynamics, nutrient Utilization, Sequestration and Acidification (MEDUSA-2; henceforth MEDUSA; Yool et al., 2013a).

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).

Figure 1. A schematic diagram of the biogeochemical components and interactions currently included in the MEDUSA model. Boxes represent model components, such as nutrients or plankton types, while arrows represent flows of material between these, for instance where one plankton type eats another. The second from bottom row of boxes are simple chemicals dissolved in the seawater, such as nutrients and carbon. These are consumed by the two sizes classes of marine algae (non-diatoms and diatoms) in the row above through photosynthetic production. In turn, the algae are consumed by the two size classes of animals, the larger of which also consumes the smaller animals. Finally, death and waste processes of both algae and animals produce particles of detritus – marine snow – that sink into the ocean and consumed by implicit bacteria back to dissolved chemicals. Because it is sinking, some of the detritus reaches the seafloor, and it is channelled there into the bottom row of boxes that represent benthic reservoirs that are gradually returned back into solution. To simplify the diagram, dissolved oxygen and its connections to the other components are omitted here.

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.

Within UKESM1, MEDUSA plays a role in the exchange of CO2 between the ocean and the atmosphere, as well as in the production of volatile organic compounds (VOCs), such as dimethyl sulphide (DMS), that affect cloud formation and radiation balance in the atmosphere. In turn, iron-rich dust produced in dry regions of the Earth is transported via UKESM1’s atmosphere to the ocean where it regulates the productivity of MEDUSA’s plankton ecosystem.

Figure 2. An intercomparison of observational (left) and simulated (right) integrated marine primary production for June–July–August (top; northern summer) and December–January–February (bottom; northern winter). This is the production of organic material by marine algae as they grow and divide, and it consumes dissolved inorganic carbon in seawater. Consequently, it affects the transfer of CO2 between the ocean and atmosphere. The observational field shown here is an average of the three satellite-derived estimates based on remotely sensed surface chlorophyll (the main pigment in marine algae). Production in g C m−2 d−1.