The physical and chemical characteristics of the world ocean are not uniform. Temperature, salinity, light, density, nutrients, and oxygen can vary by location, depth, and time. Despite being highly variable, there are broad patterns in physical and chemical characteristics. Water stratification patterns by depth can be observed by looking at vertical profiles of these characteristics. The upper layer of the ocean is well lit, warm, and highly oxygenated with low salinity, density and nutrients. There is also a thick bottom layer with relatively high salinity, density, and nutrients and low temperature and salinity. Oxygen is low at mid depths and then increases at higher depths.

Physical and chemical factors vary with depth for a number of reasons. For example, vertical profiles in the ocean vary with the seasons depending on latitude (SF Fig. 2.5). The area on earth indicated by each of the locations in SF Fig. 2.5 are shown in SF Fig. 2.6.