Resolving zooplankton in marine ecosystem models using the functional size spectrum framework

Zooplankton are the linchpin of the marine ecosystem, serving as the main energy pathway from phytoplankton to fish. However, despite their critical role, zooplankton are simplistically represented or ignored altogether in current marine ecosystem models, with the most common assumption being that the zooplankton community does not change across environmental gradients. Recent developments in functional size spectrum modelling, which allow organisms to be resolved based on traits such as body size and feeding behaviour, offer a promising way forward to improving the resolution of the zooplankton. I developed a functional size-spectrum model of the marine ecosystem, which resolves the body size ranges, size-based feeding characteristics and carbon content of nine of the most abundant zooplankton groups (heterotrophic flagellates and ciliates, larvaceans, omnivorous and carnivorous copepods, chaetognaths, euphausiids, salps and jellyfish). The model is run across the global ocean, allowing the zooplankton community to emerge based on their traits and the environment. I identify shifts in zooplankton community composition across global environmental gradients, which agree with empirical observation. I use the model to explore how changes in zooplankton community composition affects ecosystem transfer efficiency from phytoplankton to fish.