Plankton trophic interactions and their spatial variation in the Northern Humboldt Upwelling System

Marine ecosystems are traditionally thought to be bottom-up controlled. However, there is evidence that top-down interactions can occur in certain ecosystems at specific times. Understanding trophic controls is necessary for a better representation of the marine ecosystems, a step towards a better prediction of future changes in the ecosystem.

Eastern Boundary Upwelling Ecosystems are characterised by a high primary productivity sustaining abundant stocks of small pelagic fish, which feed directly on plankton and support larger predators. Among all of them, the Northern Humboldt Upwelling System (NHUS) produces about ten times more fish catches than the others and hosts the largest single species fishery on the planet. The reasons for this exceptionally large fish productivity have been subject of debate for several years. However, less attention has been paid to the effect that fish production could have on the plankton community.

We use a coupled physical-biogeochemical model to investigate the dynamics of the plankton community in the NHUS with a focus on the top-down controls. Our model results show a spatial succession of plankton groups. Large phytoplankton blooms near the coast, where cold nutrient-rich deep water is upwelled, and as water is advected offshore, mesozooplankton, which is its main grazer, develops. We also found a trophic cascade induced by fish predation, implicitly modelled by changing the zooplankton mortality rate in the model, affecting all plankton groups until small phytoplankton. This dynamic behaviour might in turn affect the distribution of pelagic fish.

Investigating the mutual dependencies of different plankton groups and fish through the upper closure term (mortality rate) in this highly productive and spatially variable area serves as the first step in the development of an end-to-end modelling system, where plankton is the source of food for higher trophic levels.