Energy and nutrients, or chemicals, flow through an ecosystem. While energy flows through the ecosystem and cannot be recycled, nutrients cycle within an ecosystem and are reused. Both energy flow and chemical cycling help define the structure and dynamics of the ecosystem.
Primary producers like plants or phytoplankton use solar energy to synthesize sugars through photosynthesis and are the sources of all energy in the ecosystem. Primary producers also need nutrients or chemicals like nitrogen, phosphorus and iron to grow. Nutrients and sugars are available to primary consumers, herbivores that eat the primary producers, and secondary consumers, predators that eat the primary consumers.
The energy flowing through the ecosystem cannot be recycled. Consumers use the sugars, fats and proteins they take from other organisms as a source of energy to grow and maintain their cells. They lose some of this energy as heat. Nutrients are recycled through decomposition. When primary producers or consumers die, fungi and other decomposers obtain energy by breaking down their remains and, in the process, they return key nutrients like nitrogen to the soil so primary producers can use them.
Energy and nutrient availability can constrain an ecosystem's productivity. In the open ocean, for example, light is plentiful at the surface but scarce farther down. Moreover, nutrients like nitrogen and iron are also scarce, so productivity is limited. In regions of the ocean where upwelling brings nutrients to the surface--as, for example, off the coast of Chile in non-El Nino years--productivity increases.