Plankton

Plankton. The organisms that make up the freshwater plankton are all those microscopic creatures, including photosynthesizers,  living free in the open water, whose careers depend more on the currents of water movement than on any swimming ability of their own. Planktonic animals may be able to rise and fall in the water column, depending on whether they are attracted to sunlight or flee it, but their powers are so slight that they are at the mercy of water movements. Wind and wave, upwelling currents and downdrafts, are equally in control. Plankton comes from the same Greek stem as planet: both are "wanderers".
 
Algae, organic debris or solutes, and bacteria: these form the three common bases of the planktonic food web.
 
Freshwater plankton are most abundant in still waters that are rich in algae or organic matter that supports bacteria. In fast-moving streams, planktonic organisms tend to be swept away. In a swift upland stream, there may be many insect larvae, but there are few planktonic creatures. Freshwater plankton more characteristically develops in the slower-moving waters of lakes and swamps, and of course our aquaria provide comparably hospitable lentic habitats. No matter how powerful your filtration or how strong the outflow current from your filters, aquarium systems are "lentic" rather than "lotic," characteristic of still waters rather than flowing ones.
 
A. Thienemann gave a talk (archived on-line) on "Tropical freshwater plankton" that was based on the German expedition of 1928-29 investigating lakes on Java, Sumatra and Bali, at a time when virtually nothing was known of tropical freshwater plankton in southeast Asia; the report finally saw print in 1952! The scientists were surprised to find, in contrast with other, strictly  terrestrial flora and fauna, so many familiar genera of algae and members of familiar families of cladocerans and rotifers.