Modelling particle transformations and the downward organic carbon flux in the NE Atlantic Ocean

Previous attempts to model aspects of the biological pump have either focused on predicting Particulate Organic Carbon (POC) flux to depth using pelagic biological data, or have investigated aspects of particle dynamics such as aggregation. Combining these approaches offers a powerful tool both to develop a mechanistic understanding of the factors controlling POC flux, and to predict this flux. In this study, prior to running the particle transformation model, a biotic particle assemblage—representative of NE Atlantic surface waters in post-bloom conditions—was prescribed using regional foodweb data in conjunction with a published foodweb model. A comparison of the composition of this assemblage with available planktonic abundance data was favourable. These particles were then divided into those large enough to settle out of the mixed layer within a specified period (particle nuclei), and those too small to sink out, the latter form a residual particle field (RPF). In the particle transformation model runs, three generic particle nuclei, a diatom, copepod exuvia and a faecal pellet were considered, and each was modified concurrently by bacterial solubilisation and particle aggregation (with RPF only) over a 3000 m water column. Other features of the model included a particle residence time in surface waters, particle contact rates driven by upper ocean physics, and vertical gradients in particle solubilisation rate. A comparison of predicted and observed particle properties was favourable, however lighter particles were generated in the simulations and reasons for this apparent discrepancy are discussed. Sensitivity analyses indicated particle geometry, solubilisation rate, contact rate and residence time all played major inter-related roles in determining downward particle trajectories. The model provides a mechanistic understanding of why particles of similar size have different sinking rates, and points to the important role of residence time and the properties of the RPF. Downward POC flux (0–3000 m) was also predicted by this model and was comparable to that recorded by deep-moored sediment traps over summer in the NE Atlantic. This approach therefore permits assessment of the main factors controlling downward POC flux, and the ability to predict reliably this flux.