Complexity is an intrinsic property of natural systems. In the oceanic system, it is linked to many interactions with the atmosphere, geosphere and biosphere with which it exchanges energy and matter. Complexity of the ocean system has, at different spatial and temporal scales, hydrodynamic mechanisms of these exchanges and dynamics of elements and compounds, they are involved in biogeochemical cycles or used as tracers. By its pedagogical approach, it defines the terms, methods, techniques and analytical tools used. Then, it analyzes the consequences of climate change, future projections, human impact and the concept introduced with planktonic pelagic ecosystem component.