Invertebrate drift - drift of benthic (benthic) hydrobionts downstream of a river.

The movement of aquatic invertebrates in a stream downstream in the water column, as well as the aggregate of these organisms themselves. Benthic organisms that move in the water column are called rheosyrtons.

In 1972, Waters divided drift into three types: catastrophic, behavioral and passive. Typically aquatic organisms encountered in drift are called autochthonous drift, and organisms entering the water from the air or land are called allochthonous. Catastrophic drift is caused by extreme natural or anthropogenic factors, resulting in a significant reduction in the number of hydrobionts and changes in the community structure. This complex of invertebrates is also called econosyrton. If aquatic organisms actively move into the water column on their own, such drift is called behavioral or active. Usually such a drift occurs at night. In passive or permanent drift, there is an accidental flushing of hydrobionts from the substrate surface. This group is also called eusirton.

Viktor Vsevolodovich Bogatov divided drift into five groups: neutral, negative, positive, catastrophic, and preimaginal. Neutral drift refers to the movement of organisms where the total abundance does not change significantly. A negative drift results in a severe decrease in the abundance of hydrobionts due to flooding or small discharges of pollutants into the water. Positive drift leads to an increase in benthos biomass and occurs during restoration of aquatic ecosystems or settlement of areas flooded during floods. Catastrophic drift occurs during destruction of river ecosystems. Some groups of hydrobionts (amphipods and insect larvae) completely disappear, while others (oligochaetes) increase their numbers. Preimaginal drift is accompanied by mass departure of adults of amphibiotic insects and only mature pupae and larvae participate in it.

The process of downstream drift occurs constantly, but in benthic communities high levels of depletion do not ultimately lead to the disappearance of invertebrates in the upper reaches of rivers. This contradiction has been called the "drift paradox." To explain this phenomenon, Muller proposed the "colonization cycle" hypothesis and Waters proposed the "compensatory production" hypothesis. Muller believed that compensation occurs as a result of upstream migrations of adults, while unmature stages move downstream by drift. Waters believed that compensation for benthos abundance was due to production.

It is reasonable to measure the drift of river benthos by the number or biomass of organisms carried over a certain time through a flow section 1 m wide and high, equal to the depth of the site at the time of the study. For quantitative assessment of hydrobionts drift, taking into account their density on the ground, an indicator of specific drift, and taking into account their production, an indicator of drift intensity.