aquatic=All living things interact with other living things and with their surrounding environment. A community of living things functioning together with all of the non-living physical factors of the surrounding environment is called an ecosystem. 

All organisms require energy and nutrients to live, grow, and reproduce. Plants get energy from the sun, but animals get energy from the food they consume.  Organisms living in an ecosystem depend on each other to get the energy and nutrients they need to live. Human actions that affect the natural world can change entire ecosystems because of the links between living organisms and their environment. 

Food webs are simplified diagrams that can be used to illustrate the relationships among organisms in an ecosystem. The adjacent diagram shows a hypothetical aquatic food web. The arrows show how nutrients and energy are transferred from one organism to another in feeding relationships. The transfer of energy from organism to organism in a sequence is known as a food chain and the position on a food chain is known as the tropic level of that organism. A detailed food web would include many more organisms than are shown here, including decomposers (such as bacteria) that break down dead organic matter, recycling nutrients within the ecosystem.