Water covers over 75% of the globe and is essential to all life. This unit focuses on how physical, chemical, and biological components of marine and freshwater ecosystems function and interact. Ecological parallels and contrasts between marine and freshwater habitats are explored along gradients (fresh to salty, land to water). The unit begins by comparing the physical and ecological features of freshwater and marine habitats, the types of plants and animals in these habitats and the roles they play in fundamental ecological processes such as organic matter cycling. These ecological principles are linked to practical skills of sampling water quality and biota from a range of aquatic ecosystems. The unit concludes by reviewing how ecological principles can help manage the impacts of human activities on our oceans, estuaries, rivers and wetlands.