Cladocera: A suborder of CRUSTACEA, order Diplostraca, comprising the water fleas. They are benthic filter feeders that consume PHYTOPLANKTON. The body is laterally compressed and enclosed in a bivalved carapace, from which the head extends.Daphnia: A diverse genus of minute freshwater CRUSTACEA, of the suborder CLADOCERA. They are a major food source for both young and adult freshwater fish.Zooplankton: Minute free-floating animal organisms which live in practically all natural waters.Rotifera: A class of minute animals of the phylum Aschelminthes.Copepoda: A huge subclass of mostly marine CRUSTACEA, containing over 14,000 species. The 10 orders comprise both planktonic and benthic organisms, and include both free-living and parasitic forms. Planktonic copepods form the principle link between PHYTOPLANKTON and the higher trophic levels of the marine food chains.Fresh Water: Water containing no significant amounts of salts, such as water from RIVERS and LAKES.Population Density: Number of individuals in a population relative to space.Population Dynamics: The pattern of any process, or the interrelationship of phenomena, which affects growth or change within a population.