Diatom aggregate formation and fluxes: a modeling analysis under different size-resolution schemes and with empirically determined aggregation kernels

A hierarchical analysis is presented for evaluating the accuracy of different formulations to simulate diatom aggregate formation and diagnose its associated carbon fluxes. We find that, for diagnostic purposes, a two-class arrangement of sizes (small particles and aggregates) is an adequate compromise between the accuracy of estimated fluxes and the level of resolution implemented for particle sizes. Ignoring the existence of aggregates severely underestimates fluxes, whereas increasing the size resolution (up to 7 size classes) yields a small improvement in accuracy. We demonstrate that this two-size-class arrangement is also the minimum resolution for accurately modeling the formation of aggregates. This arrangement requires only one aggregation kernel, which we estimate from experimental data without imposing any assumption for the interaction between particles. Although the kernel is robust to changes in the nature of the particles, the simulations obtained with this approach are very sensitive to the initial concentration of aggregates implemented to run the model. This sensitivity to initial conditions does not appear in a model with higher size resolution whose kernel tensor for aggregation has also been derived from experimental mesocosm data through an inverse modeling procedure. Although the level of accuracy achieved in a simulation with this kernel tensor seems promising, its robustness requires further tests in conditions other than a mesocosm experimental design.