- The model ocean contains the same components as the real ocean but it is supposed that the system is at equilibrium
- The real system is not at equilibrium everywhere
• Concentrations of certain elements such as P and N2, Si and OC, are subject of important variations in the surface layer
• Counter argument: Surface layer does represent 5% of the total ocean volume
• Biological activity produces situations far from a state of equilibrium (organic matter preserved in the sediments)