Neuston, also called pleuston, are organisms that live at the surface of a body of water, such as an ocean, estuary, lake, river, wetland or pond. Neuston can live on top of the water surface or submersed just below the water surface. In addition, microorganisms can exist in the surface microlayer that forms between the top- and the under-side of the water surface. Neuston has been defined as "organisms living at the air/water interface of freshwater, estuarine, and marine habitats or referring to the biota on or directly below the water's surface layer." Neustons can be informally separated into two groups: the phytoneuston, which are autotrophs floating at the water surface including cyanobacteria, filamentous algae and free-floating aquatic plant (e.g. mosquito fern, duckweed and water lettuce); and the zooneuston, which are floating heterotrophs such as protists (e.g. ciliates) and metazoans (aquatic animals). The word "neuston" represents the nominalized neuter of the Greek neustos meaning "swimming"; compare also "plankton". This term first appears in the biological literature in 1917. The alternative term pleuston comes from the Greek plein, meaning "to sail or float". The first known use of this word was in 1909, before the first known use of neuston. In the past various authors have attempted distinctions between neuston and pleuston, but these distinctions have not been widely adopted. As of 2021, the two terms are usually used somewhat interchangeably, and neuston is used more often than pleuston.
Overview
The neuston of the surface layer is one of the lesser known aquatic ecological groups. The term was first used in 1917 by Naumann to describe species associated with the surface layer of freshwater habitats. Later in 1971, Zaitsev identified neuston composition in marine waters. These populations would include microscopic species, plus various plant and animal taxa, such as phytoplankton and zooplankton, living in this region. In 2002, Gladyshev further characterised the major physical and chemical dynamics of the surface layer influencing the composition and relationships with various neustonic populations" The neustonic community structure is conditioned by sunlight and an array of endogenous (organic matter, respiratory, photosynthetic, decompositional processes) and exogenous (atmospheric deposition, inorganic matter, winds, wave action, precipitation, UV radiation, oceanic currents, surface temperature) variables and processes affecting nutrient inputs and recycling. Furthermore, the neuston provides a food source to the zooplankton migrating from deeper layers to the surface, as well as to seabirds roaming over the oceans. For these reasons, the neustonic community is believed to play a critical role on the structure and function of marine food webs. Yet, research on neuston communities to date focused predominantly on geographically limited regions of the ocean or coastal areas. Consequently, neuston complexity is still poorly understood as studies on the community structure and the taxonomical composition of organisms inhabiting this ecological niche remain few, and global scale analyses are yet lacking.
Types
There are different ways neuston can be categorised. Kennish divides them by their physical position into two groups:
epineuston: organisms living on the water's surface hyponeuston: organisms within a region of specified depth directly below the surface layer To this can be added the organisms living in the microlayer at the interface between air and water:
microlayer neuston: organisms (microorganisms) living in the surface microlayer sandwiched between the upper and under surface. Rapoport and Sánchez further split epineuston into two categories:
transitory, accidental or rain epineuston, which may survive on the water after being washed there by flooding but prefer land permanent, or epineuston proper: adapted and specialized to life on the water, preferring the surface of the water over soil Marshall and Burchardt divide neuston into three ecological categories:
euneuston: organisms with maximum abundance in the vicinity of the surface on which they reside day and night facultative neuston: organisms concentrating at the surface only during certain hours of the day, usually during darkness pseudoneuston: organisms with maximum concentrations at deeper layers but reaching the surface layer at least during certain hours.
Freshwater neuston Freshwater neuston, organisms living at lake or pond surfaces or slow moving parts of rivers and streams, include beetles (see whirligig beetle), protozoans, bacteria and spiders (see fishing spider and diving bell spider). Springtails in the genera Podura and Sminthurides are almost exclusively neustonic, while Hypogastrura species often aggregate on pond surfaces. Water striders such as Gerris are common examples of insects that support their weight on water's surface tension.
Floods
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