Towards end-to-end (E2E) modelling in a consistent NPZD-F modelling framework (ECOSMO E2E_v1.0): application to the North Sea and Baltic Sea

Abstract

Abstract. Coupled physical–biological models usually resolve only parts of the trophic\nfood chain; hence, they run the risk of neglecting relevant ecosystem\nprocesses. Additionally, this imposes a closure term problem at the\nrespective “ends” of the trophic levels considered. In this study, we aim to\nunderstand how the implementation of higher trophic levels in a\nnutrient–phytoplankton–zooplankton–detritus (NPZD) model\naffects the simulated response of the ecosystem using a consistent\nNPZD–fish modelling approach (ECOSMO E2E) in the combined North Sea–Baltic Sea system.\nUtilising this approach, we addressed the above-mentioned\nclosure term problem in lower trophic ecosystem modelling at a very low\ncomputational cost; thus, we provide an efficient method that requires very\nlittle data to obtain spatially and temporally dynamic zooplankton mortality. On the basis of the ECOSMO II coupled ecosystem model we implemented one\nfunctional group that represented fish and one group that represented macrobenthos\nin the 3-D model formulation. Both groups were linked to the lower trophic\nlevels and to each other via predator–prey relationships, which allowed for the\ninvestigation of both bottom-up processes and top-down mechanisms in the\ntrophic chain of the North Sea–Baltic Sea ecosystem. Model results for a\n10-year-long simulation period (1980–1989) were analysed and discussed with\nrespect to the observed patterns. To understand the impact of the newly\nimplemented functional groups for the simulated ecosystem response, we\ncompared the performance of the ECOSMO E2E to that of a respective truncated\nNPZD model (ECOSMO II) applied to the same time period. Additionally, we\nperformed scenario tests to analyse the new role of the zooplankton mortality\nclosure term in the truncated NPZD and the fish mortality term in the\nend-to-end model, which summarises the pressure imposed on the system by\nfisheries and mortality imposed by apex predators. We found that the model-simulated macrobenthos and fish spatial and seasonal\npatterns agree well with current system understanding. Considering a dynamic\nfish component in the ecosystem model resulted in slightly improved model\nperformance with respect to the representation of spatial and temporal variations\nin nutrients, changes in modelled plankton seasonality, and nutrient profiles.\nModel sensitivity scenarios showed that changes in the zooplankton mortality\nparameter are transferred up and down the trophic chain with little\nattenuation of the signal, whereas major changes in fish mortality and fish\nbiomass cascade down the food chain.