Aquatic ecosystems are made up of a wide variety of species including bacteria, plants (algae as well as floating and rooted aquatic plants), zooplankton, annelids (worms and leeches), molluscs (mussels and snails), arthropods (crustaceans and insects), fish, birds and mammals, all interacting to form a community. While they can be studied at ecosystem, community, population, species and even at the genetic level, freshwater monitoring is normally done at the community level by examining the organisms present at individual sites and comparing this in terms of diversity and abundance to other sites within a catchment (Table 8.19).