wider role this may play in marine biogeochemistry. Studies have shown that mixotrophs are much more important for the marine ecology than previously assumed, and comprise more than half of all microscopic plankton. Their presence act as a buffer that prevents the collapse of ecosystems during times with little to no light. Size groups Plankton are also often described in terms of size. Usually the following divisions are used: {| |width="120"| Group |width="100"| Size range (ESD) |width="350"| Examples |- | Megaplankton ||> 20 cm || metazoans; e.g. jellyfish; ctenophores; salps and pyrosomes (pelagic Tunicata); Cephalopoda; Amphipoda |- | Macroplankton || 2→20 cm || metazoans; e.g. Pteropoda; Chaetognaths; Euphausiacea (krill); Medusae; ctenophores; salps, doliolids and pyrosomes (pelagic Tunicata); Cephalopoda; Janthina and Recluzia (two genera of gastropods); Amphipoda |- | Mesoplankton || 0.2→20 mm || metazoans; e.g. copepods; Medusae; Cladocera; Ostracoda; Chaetognaths; Pteropoda; Tunicata |- | Microplankton || 20→200 µm || large eukaryotic protists; most phytoplankton; Protozoa Foraminifera; tintinnids; other ciliates; Rotifera; juvenile metazoans – Crustacea (copepod nauplii) |- | Nanoplankton || 2→20 µm || small eukaryotic protists; small diatoms; small flagellates; Pyrrophyta; Chrysophyta; Chlorophyta; Xanthophyta |- | Picoplankton || 0.2→2 µm || small eukaryotic protists; bacteria; Chrysophyta |- | Femtoplankton || < 0.2 µm || marine viruses |- |} However, some of these terms may be used with very different boundaries, especially on the larger end. The existence and importance of nano- and even smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity. The microplankton and smaller groups are microorganisms and operate at low Reynolds numbers, where the viscosity of water is more important than its mass or inertia. Habitat groups Marine plankton Marine plankton includes marine bacteria and archaea, algae, protozoa and drifting or floating animals that inhabit the saltwater of oceans and the brackish waters of estuaries. Freshwater plankton Freshwater plankton are similar to marine plankton, but are found inland in the freshwaters of lakes and rivers. Aeroplankton Aeroplankton are tiny lifeforms that float and drift in the air, carried by the current of the wind; they are the atmospheric analogue to oceanic plankton. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of their tiny size. Scientists can collect them for study in traps and sweep nets