WATER QUALITY: Technical Information--Briefing paper on aquatic biology: Drift Organisms in Streams by Keith V. Slack
                                                March 14, 1978
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 78.04
Subject: WATER QUALITY: Technical Information--Briefing paper on
         aquatic biology: Drift Organisms in Streams by Keith V.
         Slack
The attached description of "Drift organisms in streams" by Keith
V. Slack (WRD, Menlo Park, California) represents a continuation
of the series of briefing papers on biological quality initiated
by the Quality of Water Branch in August, 1974. Please circulate
the briefing paper as widely as possible in all district,
laboratory, and project offices.
                            R. J. Pickering
                            Chief, Quality of Water Branch
Attachment          .
WRD Distribution: A,B,S,FO-L,PO
                DRIFT ORGANISMS IN STREAMS
                  By Keith V. Slack
An earlier Briefing Paper 1/ discussed stream biology and the
various means by which stream invertebrates are adapted to
maintaining their position in flowing waters. It is to be
expected, however, that an occasional organism will lose its hold
and be carried downstream by the current. In fact, organisms of
all sizes are transported by stream waters. Algae, bacteria,
viruses, and the detritus of larger plants and animals are very
common in the organic load of streams. The term "organic drift"
sometimes is used for this material. However, the term "drift" in
stream ecology usually means the downstream transport of benthic
invertebrates (measured by the number or biomass of invertebrates
passing a sampling point in a unit time).
If drift consisted only of the rare, clumsy organism that missed
its footing momentarily and became entrained in the flow, it would
be of little ecological consequence. Studies have shown, however,
that large numbers of benthic invertebrates are regularly found in
the drift. During the 24 hours of 18 April, 1946, an estimated 64
million invertebrates, weighing 200 kg, passed under Boonville
Bridge on the Missouri River (Berner, 1951). A study of a swift
trout stream in England concluded that during a year, 19.6 g of
organisms drifted over each square meter of bottom which supported
a biomass of 1.5 to 3.6 g at any one time (Horton, 1961). These
quantities are so large that aquatic biologists have questioned
the productive capacity of the streams to withstand such high
rates of attrition.
Several types of drift can be distinguished (Waters, 1972).
"Catastrophic" drift results from the physical disturbance of the
benthic fauna, such as bottom scouring caused by floods, but may
result also from drought, high temperature, anchor ice,
insecticides or other toxicants, and organic pollution. The effect
of catastrophic drift on benthic invertebrate populations can be
severe. "Behavioral" drift results from behavior patterns
characteristic of certain species. It occurs at night or at other
consistent periods of time. Although behavioral drift may occur in
large quantities, it does not appear to deplete upstream areas.
"Constant" drift is the continuous downstream flow of
representatives of all species in low numbers. It has the least
effect on benthic invertebrate populations.
1/ "Stream Biology" by K. V. Slack, Quality of Water Branch
Technical Memorandum No. 75.04 (September 24, 1974).
Diel periodicity
Although several factors have been observed to affect the kinds
and amounts of organisms drifting, the rate of drift usually
varies between day and night. The discovery, in about 1960, of
day-night differences in drift stimulated research on stream
invertebrate drift. Studies in many parts of the world have shown
that drift usually increases sharply at about the time of full
darkness (fig. 1 ). Increased but variable drift continues
throughout the night, then ends with a sharp reduction in numbers
at dawn. However, a few species, mostly caddisflies, are day-
active, showing higher rates of drift during the daytime.
It is postulated that night-active drift is the result of feeding,
and that the nocturnal foraging behavior of stream invertebrates
has evolved because darkness provides them the greatest protection
against predators. Thus, the organisms hide during daylight under
rocks or in interstitial spaces, but move to upper surfaces during
dark to forage.  Herbivores seek algae growing on the tops of
rocks, and their predators follow. The result is increased
exposure to the current and greater chances of dislodgement of
herbivores and predators causing both to exhibit the observed
periodicity in drift.
Day-active periodicities may be the result of a direct metabolism-
activity relationship to water temperature. As growth occurs,
crowding, and subsequent loss of living space may result in
increased activity, dislodgement, and difficulty of reattachment.
This may explain the higher drift which occurs at times of most
rapid growth.
Prepupation or pre-emergence activity in some species may result
in a drift periodicity. Light appears to act in an "on-off"
fashion, triggering increased activity by many benthic species
when it decreases to a threshold intensity of about 1 to 5 lux
(0.1 to 0.5 ft-candles) of total energy measured at the water
surface. Other environmental factors that affect the amount of
drift but not its periodicity include current speed, and, for some
species, water temperature.  In addition, riffles produce more
drift than do pools.
At any one time, the density of drifting organisms is fairly
constant in the cross-section.  Each organism probably moves a
relatively short distance downstream during its entainment before
it can reattach or is eaten by a fish or other predator.  The
total downstream displacement by drift probably occurs in a
saltatory fashion in which the drifting organisms frequently are
returned to the substrate by turbulence and are replaced by
others.  Drifting, therefore, is a temporary event in the life of
many members of the bottom fauna.  However, many organisms fall
prey to predators while in the drift, or are carried out of areas
which are suitable habitats and ultimately die.
Drift organisms
.The most abundant taxa in the drift are amphipods, the insect
Orders Ephemeroptera (mayflies), Plecoptera (stoneflies), and
Trichoptera (caddisflies), and the Family Simuliidae (black flies)
of the Order Diptera (Waters, 1969). In all of these groups, there
are species exhibiting no apparent behavioral drift even though
abundant on the stream bottom. Other groups have been reported in
the drift in fewer numbers. Almost all amphipods, mayflies,
stoneflies, and black flies are night-active, whereas caddisflies
have both night- and day-active species. Notably absent in drift
periodicities are most burrowing forms, large strong swimming
predators, molluscs, stone-cased caddisflies, and dipterans other
than black flies. Chironomidae (midge) larvae apparently show
little or no drift periodicity. The entire mayfly genus, Baetis,
exhibits definite periodicity. Baetis, some species of Gammarus
(amphipods), and some black flies are the main groups exhibiting
extremely high rates of drift (Waters, 1969). In drift downstream
from impoundments, lake benthos and zooplankton may dominate, and
may show diel periodicity.
Relation of drift to production and life cycles
Early observations of drift magnitude led to speculation as to its
role in population dynamics and the processes by which upstream
areas adapted to such an apparent high rate of attrition. One of
the earliest discussions (Denham, 1938) suggested that over-
crowding and competition contributed to drift. A second suggestion
(Muller, 1954) has come to be known as Muller's "colonization
cycle". It postulated an upstream flight of adults for egg laying
with a concentration of eggs and young larvae in the upper
reaches. As small larvae grew, they were forced to seek new space,
and the consequence was downstream drift, which also resulted in
colonization of all suitable habitats throughout the stream.
Emergence was followed by an upstream return of the adults to
complete the cycle. Thus, drift kept population densities reduced
to the carrying capacity of the environment and provided a
mechanism for colonization.
No evidence is available to show that behavioral drift reduces
population densities below carrying capacity. I# it does not,
drift merely represents production that exceeds the carrying
capacity of the stream.  Neither upstream compensation nor the
excess production hypothesis is universally applicable.  It is
certain that variation exists and that a variety of mechanisms are
involved in the ecology of stream invertebrate drift (Waters,
1972).
Methods of drift sampling
Sampling methods for drift analysis are described in U.S.
Geological Survey Techniques of Water Resources Inv., Book 5,
Chap. A4, (Greeson and others, 1977), in Weber (1973), in Elliott
(1970), and in American Public Health Assoc. and others (1976).
Results are expressed as drift rate, which is the amount of
biomass or number of invertebrates passing a sampling point in
unit time, or as drift density, which is the amount of biomass or
number of invertebrates per unit volume of water.
Application of drift data
Invertebrate drift is a fascinating example of a natural
biological cycle which is of interest in water resources
investigations for several reasons. One consequence of drift is
that unpopulated or depopulated areas are rapidly recolonized by
animals from upstream. The areas affected by drift in this way
range from artificial substrate samplers to entire reaches of
river channel. Measurement of drift provides a basis for comparing
streams (Diamond, 1967), is a means of sampling benthic
invertebrates in reconnaissance studies (Slack and others, 1976)
It also is an indicator of water quality (Larimore, 1974).
Qualitative or quantitative changes in drift can indicate the
effect of insecticides or other toxicants on stream biota
(Coutant, 1964). The study of invertebrate drift is an important
method for investigating insect life histories (Anderson, 1967)
and secondary production (Waters, 1962, 1966). Some species of
fish, notably the brown trout, feed largely on drifting organisms
(Hynes, 1970). This is the basis of angling with a wet artificial
fly.
Much remains to be learned about invertebrate drift and its
relation to the stream environment. We can expect that knowledge
developed by research on drift will contribute increasingly to our
understanding of the function and succession of stream
communities. This will enhance our ability to use ecological
approaches in water resource monitoring and appraisal.
References
American Public Health Association and others, 1976, Standard
methods for the examination of water and wastewater (14th ed.):
New York, Am. Public Health Assoc., 1193 p.
Anderson, N. H., 1967, Biology and downstream drift of some Oregon
Trichoptera:  Canadian Entomologist, v. 99, p. 507-521.
Berner, L. M., 1951, Limnology of the lower Missouri River:
Ecology, v . 32, p . 1-12 .
Coutant, C. C., 1964, Insecticide sevin: effect of aerial spraying
on drift of stream insects: Science, v. 146, p. 420-421.
Denham, S. C., 1938, A limnological investigation of the West Fork
and Common Branch of White River: Invest. Indiana Lakes and
Streams, v. 1, p. 17-71.
Diamond, J. B., 1967, Evidence that drift of stream benthos is
density related: Ecology, v. 48, p. 855-857c
Elliott, J. M., 1970, Methods of sampling ivertebrate drift in
running water:  Annales de Limnologie, v. 6, p. 133-159 .
Greeson, P. E., Ehlke, T. A., Irwin, G. A., Lium, B. W., and
Slack, K. V., eds., 1977, Methods for collection and analysis of
aquatic biological and microbiological samples: U.S. Geol. Survey
Techniques Water Resources Inv., book 5, chap. A4, 332 p.
Horton, P. A., 1961, The bionomics o# brown trout in a Dartmoor
stream: Jour. Anim. Ecol., v. 30, r. 311-338.
Hynes, H. B. N., 1970, The ecology of running waters: Toronto,
Univ. of Toronto Press, 555 p.
Larimore, R. W., 1974, Stream drift as an indication of water
quality: Trans Am.. Fisheries Soc., v. 103, p. 507-517.
Muller, Karl, 1954, Investigations on the organic drift in North
Swedish streams: Rept. Inst. Freshwater Research Drottningholm,
v. 35, p. 133-148.
Slack, K. V., Nauman, J. W., and Tilley, L. J., 1976, Evaluation
of three collecting methods for a reconnaissance of stream benthic
invertebrates: Jour. Research U.S. Geol. Survey, v. 4, p. 491-495.
Weber, C. I., ed., l973, Biological field and laboratory methods
for measuring the quality of surface waters and effluents: U.S.
Environmental Protection Agency, Environmental Monitoring Series,
EPA-670/4-73-001.
Waters, T. F., 1962, A method to estimate the production rate of a
stream bottom invertebrate: Trans. Am. Fisheries Soc., v. 91, p.
243-250.
______1966, Production rate, population density, and drift of a
stream invertebrate: Ecology, v. 47, p. 595-604.
______1969, Invertebrate drift-ecology and significance to stream
fishes, in Northcote, T. G., ed., Symposium on salmon and trout in
streams: H. R. MacMillan Lectures in Fisheries, Vancouver, Univ.
British Columbia, p. 121-134.
______1972, The drift of stream insects: Annual Rev. Entomol., v.
17, p. 253~272