Abstract
Ecosystems comprise all the life forms (microbes, plants, animals) and the inorganic context (sediment types, water regimes, water quality, etc.) that they live in. Together, the life forms interact with each other. They influence, and are influenced by, their environment, carrying out ecosystem processes such as the transport of nitrogen and carbon. This chapter introduces the terminology used to describe lake and river ecosystems. It describes the function of freshwater ecosystems and the methods scientists use to quantify these functions, including concepts such as ecosystem productivity, metabolism, food webs, populations, and metapopulations. Furthermore, this chapter describes ecological communities, their diversity and assembly, and the concepts of metacommunities and metaecosystems.