Zooplankton simulated reality

Beneath the ocean’s surface lives a huge diversity of animals smaller than a grain of rice. This is the world of the copepods, the most abundant animals on Earth. Every night, billions of copepods rise to the sea surface to feed. This daily migration is the largest animal movement on our planet.

Copepods graze on microscopic plants called phytoplankton. At high latitudes, copepods vacate surface waters when phytoplankton density decreases in autumn and enter an energy-conserving hibernation at great depths. In spring the copepods emerge from hibernation and ascend to the surface to complete their life cycle. This behaviour is termed the seasonal vertical migration, in contrast to their daily (diel) vertical migration. Vertical migrations are sensitive to environmental conditions, such as light, temperature, salinity, food and predation—and these environmental conditions are shifting rapidly due to climate change. The extent to which copepod migrations can adapt to rapid environmental shifts is unknown. In 2024 the “Migratory Crossroads” project kicked off to bridge this knowledge gap, focusing on the most dominant copepod species in the Norwegian Sea, Calanus finmarchicus.

Tracking vertical behaviours of individual copepods in their natural habitat is a challenging task. In Migratory Crossroads, we have therefore built the copepod simulation model, PASCal (Pan-Arctic behavioural and life-history Simulator for Calanus). This model aims to produce a near-realistic representation of the diel and seasonal vertical migratory behaviours of Calanus finmarchicus, covering their entire life cycle from eggs to adults. In PASCal, virtual individuals are designed as entities with a body size, developmental stage, sex and energy reserve (lipids). Vertical behaviours are implemented as a function of internal (body size, development stage, energy reserves) and external stimuli (light, temperature, salinity, food and predation). To make our simulations as realistic as possible, we drew on literature to understand how copepods respond to internal and external stimuli and how fast they can swim.

To further enhance the realism of simulated copepod behaviours, we have been collecting field data on the vertical distributions of C. finmarchicus in the Norwegian Sea in collaboration with the CliN-BluFeed project (see Further reading). Data have been collected using a combination of traditional and innovative methods including nets, autonomous vehicles and remote sensing. Field data allows us to compare model-driven and real-world behaviours of C. finmarchicus. In PASCal, we take our simulations to the next level by upscaling from a simple unidimensional water-column model to a full three-dimensional representation of the Norwegian Sea. This will allow us to track copepod movements across space under past, present and potential future environmental conditions. Accurate three-dimensional digital representations of C. finmarchicus populations, based on realistic simulations of their life cycles can provide valuable insights into efficient harvesting and sustainable management of Calanus fisheries in the Norwegian Sea.