Two types of aquatic ecosystems are distinguished:
- Marine ecosystems. They are characterized by the high salinity of their waters, about 35 g/l, and include the seas and oceans.
- Freshwater ecosystems. The concentration of its salts is less than 1 g/l, and includes rivers, lakes, wetlands, etc.
In aquatic ecosystems, the trophic level of producers, photosynthetic organisms, is made up of algae and phytoplankton, always in the surface area, where sunlight reaches them. The second trophic level, that of the primary consumers, will be zooplankton, made up of simple unicellular and multicellular organisms, which will feed on the first level. Subsequently, secondary and tertiary consumers intervene in the food chain.
The flow of energy begins in the phytoplankton, which captures solar energy, and moves to different levels, in the deepest areas of the sea. Decomposing organic matter settles to the bottom of the sea and takes a long time to re-enter the biological cycle.