The primary cause of ocean stratification is the variation in density due to differences in temperature and salinity. Warmer, less dense water tends to float above colder, denser water, creating distinct layers or strata in the ocean. This stratification affects nutrient cycling and marine ecosystems, as the layering can impede the vertical mixing of water. Additionally, factors such as climate change and human activities can influence the stratification patterns, impacting marine life and global climate systems.