Why are plankton populations generally absent from streams?

In the context of inland water ecosystems, plankton populations are generally considered absent from streams because these environments are characterized by flowing water, or lotic conditions, which differ significantly from the standing water found in lakes and ponds.12

While plankton—organisms that drift because they are too small or weak to swim against currents—form a fundamental part of the biological community in lakes, they do not establish similar populations in streams. Instead, the ecology of running waters is defined by the downstream transport of materials and the fact that much of the energy in a stream is derived allochthonously, meaning it originates from outside the stream itself. Ecological processes in these systems are often described by concepts such as the river continuum, which accounts for how communities change along the length of a river system, and nutrient spiraling, which tracks the cycling of nutrients as they are carried downstream.13