Challenging one-way fish modelling: two-way coupling reveals critical marine ecosystem dynamics

• Two-way coupling was implemented between a lower and higher trophic level model. • The impact of two-way coupling was investigated on the North West European Shelf. • Mesozooplankton biomass was most sensitive to two-way coupling, declining by 17% • Inclusion of top-down processes decreased carbon cycling on the shelf. • Using two-way coupled models may change understanding of marine ecosystem dynamics. Anthropogenic stressors such as climate change and fishing are altering marine ecosystem dynamics. To understand and predict how marine foodwebs are changing, both top-down and bottom-up trophic dynamics need to be included in our model systems. A common simplification employed is a one-way coupled modelling approach, where results of lower trophic level (LTL) (plankton) models are used to drive fish models, but feedbacks due to fish predation are not applied to the LTL model. Thus, fish do not perceive the resulting reduction in prey biomass, limiting the predictive capacity of the models. Here we have implemented innovative two-way coupling between an LTL model, ERSEM, and the community size spectrum fish model configuration of MIZER, to produce a complete model chain from photons to fisheries. The model is used to investigate the impact of two-way coupling on the North West European Shelf ecosystem. Within the two-way coupled model, clearance rates of fish and mortality rates of mesozooplankton had to be reduced, highlighting limitations of parameterisations within one-way coupled models. Mesozooplankton was the most sensitive group to two-way coupling with a 17% decrease in biomass across the shelf in addition to changes in phenology. Furthermore, two-way coupling decreased carbon cycling across the shelf driven by the top-down pressures on mesozooplankton reducing their production of organic matter and weakening the microbial loop. This impacted the ocean’s ability to absorb atmospheric carbon and consequently has climate change implications. Two-way coupled fisheries models should therefore be considered in the next generation of earth system models.