Zooplankton are also found in freshwater systems, and play very much the same role in the food web as described by Zarah
. The functional classification
of different types of zooplankton is different in lakes and rivers. Instead of being grouped into holo- and meroplankton, limnologists
tend to refer to zooplankton as being either pelagic
. Also, fish larvae are not normally grouped with other zooplankton in freshwater systems; they are considered independently.
One of the principle differences between marine and freshwater zooplankton is their mean size. In lakes and rivers, it is rare to find zooplankton species larger than 5 millimeters in total length, while this is common in marine systems. However, in the freshwater environment we commonly encounter densities several orders of magnitude greater than in marine systems. They generally consume phytoplankton, but can consume other zooplankton and bacteria. In freshwater systems, the larger zooplankton species can swim against all but the strongest currents, and also perform diel vertical migration.
Most freshwater zooplankton species are crustaceans (the notable exceptions being the rotifers and some rare species of freshwater jellyfish). Some common freshwater zooplankton taxonomic groups are:
- Daphnia -- These are some of the largest zooplankters, and are commonly called water fleas
- Copepods -- The three types are cyclopoids (predatory), calanoids (herbivorous) and harpactacoids (benthic)
- Rotifers -- there are a phenomenal number of species of these small zooplankton (generally < 250 μm)
- Sididae -- These are large benthic crustaceans
- Mysis -- A freshwater shrimp species