Biotic Factors.

http://en.wikipedia.org/wiki/Lotic_system_ecology

Bacteria

Bacteria are present in large numbers in lotic waters. Free-living forms are associated with decomposing organic material, biofilm on the surfaces of rocks and vegetation, in between particles that compose the substrate, and suspended in the water column. Other forms are also associated with the guts of lotic organisms as parasites or in commensal relationships.Bacteria play a large role in energy recycling.