Abrupt transitions in dynamics of a NPZD model across Southern Ocean fronts

•First spatial realisation of NPZD model with optimal control algorithm.•Model reproduces seasonal processes in open ocean.•Model convergence was better for open ocean than shallower shelf regions.•All variables show marked meridional changes associated with fronts of ACC.

The Southern Ocean is the largest high nutrient, low chlorophyll region in the global ocean and is subject to strong iron limitation. Iron availability is associated with the interaction between the Antarctic Circumpolar Current (ACC) and topographic features such as the Kerguelen Plateau in the Indian Sector of the Southern Ocean. The fronts of the ACC also provide important environmental delineations for Southern Ocean ecosystem structure. Here, we implement a NPZD (nitrogen, phytoplankton, zooplankton, detritus) model together with an optimal control algorithm along three meridional transects across the Antarctic Circumpolar Current. Our study represents the first spatial application of this model, and considers potential environmental drivers of parameter variability and community dynamics.In iron-limited open ocean regions, this model captures seasonal processes well but does not converge in more complex regions, such as the iron fertilized Kerguelen Plateau and sea ice areas. All variables show marked meridional changes, which are likely associated with the fronts of the Antarctic Circumpolar Current. This technique indicates that the different bodies of water associated with the circumpolar current exhibit different ecosystem dynamics, a result that is likely caused by distinct NPZD communities. Overall, our findings contribute to broadscale understanding of the drivers of lower trophic level ecosystem dynamics across physical boundaries in the Indian Sector of the Southern Ocean, and can be used to help guide further modelling efforts.