From Biogeochemistry to Fisheries and Back Again : Modelling the Upwelling Ecosystem off Peru
Marine ecosystems are subjected to increasing top–down and bottom–up anthropogenic pressures. Top–down pressures affect higher trophic levels (HTL) such as fish, for instance, by harvesting them. Bottom–up pressures affect the ecosystem by changing the environment, which in turn affects lower trophic levels (LTL), such as plankton. The northern Humboldt Current System (NHCS) is the most productive eastern boundary upwelling system in terms of fish catches. In this project, I used physical-biogeochemical and HTL models to understand how the NHCS is affected by top–down and bottom–up drivers. Over four studies, I explored two key questions: How does variability in the biogeochemistry affect fish? And how does fishing pressure affect fish and how does variability in fish affect the biogeochemistry? The first study looks at the impact that fish variability may have on the plankton community. Zooplankton mortality in the physical–biogeochemical model CROCO-BioEBUS was modified to implicitly simulate the change in the biomass of fish which would prey on zooplankton. Large zooplankton was the main driver of the community response. For the second study, the impacts of LTL variability on the HTL of the system were studied by coupling CROCO-BioEBUS with the multispecies HTL model OSMOSE. Interannual variability had an impact on fish biomasses but this was small compared to the high variability seen in observations. Then, I explored the top–down impact of fishing on the Peruvian anchovy and Peruvian hake. I observed a higher resilience of anchovy to increased fishing pressure than hake. Finally, mesopelagic fish in OSMOSE were compared with another model. It was concluded that the life-cycle and trophic interactions in OSMOSE affect the response of simulated mesopelagic fish to changes in LTL. The thesis concludes with a reflection on possible next steps for improving the representation of the NHCS ecosystem.