Regional ecosystem dynamics in the ACC: simulations with a three-dimensional ocean-plankton model

Within the high nutrient–low chlorophyll regime of the Antarctic Circumpolar Current (ACC), high phytoplankton concentrations are frequently observed in the vicinity of the Antarctic Polar Front (APF). As is typical for frontal systems, hydrography in this region is characterized by meanders and eddies as well as up- and downwelling cells which redistribute nutrients and influence the depth of the euphotic zone. To study the processes leading to the observed phytoplankton distribution, a coupled ocean-plankton model for ecosystem studies in the ACC has been developed. The ocean component is an eddy-resolving version of the s-Coordinate Primitive Equation Model (SPEM). The model has a horizontal resolution of 1/12° and a vertical resolution increased near the surface. The biological model (BIMAP) comprises two biogeochemical cycles—silica and nitrogen—and a prognostic iron compartment to include possible effects of micronutrient limitation. Model results indicate that part of the ecosystemʹs regional variability can be attributed to the effect of vertical and horizontal advection. However, frontal dynamics alone cannot explain the observed enhanced concentrations of phytoplankton biomass near the APF and the minima in the northern and southern ACC. Only when iron limitation is taken into account, the model simulates plankton concentrations in close agreement with observations during the SO-JGOFS cruises. While in the northern ACC phytoplankton growth is limited by silicate, primary production is limited by iron south of the APF. Near the APF, mesoscale iron upwelling enhances primary production, leading to increased phyto- and zooplankton biomass. The meridional structure with two plankton maxima is closely linked to the cross-front overturning circulation. This double-cell circulation with two upwelling branches is caused by the northward sloping large-scale bottom topography.