Complexity in Biogeochemical Models: Consequences for the Biological Carbon Pump

Abstract. Ocean biogeochemical models underpin projections of future marine ecosystem change, including anticipated shifts in the biological carbon pump (BCP) and broader biogeochemical cycles. However, their outputs remain highly sensitive to model complexity and parameterisation choices. Here, we evaluate five configurations of the Pelagic Interaction Scheme for Carbon and Ecosystem Studies (PISCES) to quantify intramodel variability in net primary productivity (NPP), carbon export (C_exp), and export efficiency (e-ratio) over the 21^st century under the high emissions RCP8.5 scenario. The tested PISCES configurations differed from the standard model through distinct modifications to phytoplankton growth processes, but are forced by identical physical variables, representing an ensemble opportunity. All configurations resolve NPP and C_exp within the range of remote-sensing variability. The more complex Quota-based configurations produce 15–21 (10–18) Pg C yr^-1 more NPP than the simpler Monod-quota models in the reference (future) period, but this increase, driven by elevated small phytoplankton biomass, does not enhance C_exp, yielding lower e-ratios (0.14–0.17) than in the Monod-quota configurations (~0.25). The introduction of a picophytoplankton functional type (PFT) emerges as one of the most influential parameterisation choices. It drives opposing future NPP responses between 30–60º N/S, an increase in the Monod-quota configurations versus a decline in the Quota-based ones, as well as contrasting latitudinal trends in C_exp within the same region. Other parameterisations, such as a low-iron scheme, an added diazotroph PFT, and explicit manganese cycling, exert more modest, regionally confined effects under high emissions scenarios, influencing NPP and C_exp primarily at biome scales rather than driving large-scale divergence in model behaviour.