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Organisms: Plankton (Protista and Chromista)

Many small and large organisms make their homes in aquatic ecosystems, including bacteria, fungi (molds), plants, animals (fish, birds, reptiles, amphibians, insects, crustaceans, worms and more), protists (parameciums, euglena, green algae) and chromista (diatoms and some algae). 

Understanding the relationship of an aquatic food web helps organize the importance of some tiny organisms (plankton and others) to the survival of the ecosystem. There is a strong interdependence between organisms and then with their abiotic factors such as sunlight and water. 

We will look mainly at plankton as a group because these directly influence an organism's ability to survive. Many small macroinvertebrates (insects and crustaceans) and plankton (protists and algae) make up 80% of many food webs (as primary consumers and producers).

Some categories are not as straightforward as you might expect since algae and seaweed can belong to multiple kingdoms (plants, protists, bacteria and chromista).

A basic outline would look like this:

(single celled)
(multi-celled, except some green algae)
(single and multi-celled)
(many single-celled or colonies of singles)
 paramecium     scuds, clams, mayflies
 majority of green algae (mostly freshwater)
 water molds
 diatoms and dinoflagellate
     kelp and seaweeds
 ciliates    brown and red algae (marine)

Plankton can include organisms that often resemble bacteria, but they are not. 

Macroinvertebrates include many larval stages of insects who spent the beginning part of their life in the water and later turn into insects that can leave the aquatic life, but still stay near water. Some like scuds remain aquatic dwellers.