We discuss the effect of embedding a simple plankton model in the Hamburg model of the oceanic carbon cycle (HAMOCC3) [Maier-Reimer, 1993]. The plankton model consists of five components: phytoplankton, zooplankton, detritus, dissolved organic carbon, and nutrients. Interactions between compartments are described by one global set of parameters. Despite its simplicity the plankton model reproduces regional differences in seasonal oceanic pCO2 and improves the biogeochemical tracer distributions at the depth of the oxygen minimum in the Pacific Ocean. The predicted seasonal turnover of organic material is consistent with recent atmospheric O2 measurements in the remote areas of the Southern Ocean. ¿ American Geophysical Union 1996