Unless otherwise noted, information contained in each edition of the Kansas School Naturalist reflects the knowledge of the subject as of the original date of publication.

KSN - Vol 24, No 3 - Freshwater Benthos

Volume 24, Number 3 - February 1978

Freshwater Benthos

by Dr. Carl Prophet

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Published by Emporia State University

Prepared and issued by The Division of Biology

Editor: Robert J. Boles

Editorial Committee: James S. Wilson, Gilbert A. Leisman, Thomas Eddy, Robert F. Clarke, John Ransom

Online format by: Terri Weast

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Dr. Carl Prophet is Professor of Biology at Emporia State University. This issue of the Naturalist is the second of a series on aquatic freshwater invertebrates. The first, Freshwater Zooplankton (Vol. 23, No.4, April 1977) is available upon request. Photographs by the author. Sketches by Robert Boles, Division of Biology, ESU.

Freshwater Benthos

by Dr. Carl Prophet

The benthic organisms of lakes and streams play extremely important roles in the ecological processes operating within aquatic communities. In spite of their importance, the casual observer is often unaware that such organisms exist and that the plants and animals within a lake exhibit a definite organization or structure. The structure referred to here concerns, among other features, the diversity or variety of species living in the lake, seasonal changes of species, the spatial distribution of the plants and animals in the community, and differences between species in how and when they utilize the resources of the community.

Community structure is a biological consequence of the interactions between the species comprising the community and of the populational responses of individual species to the stresses of their physical environment. The greater the relative success of different species in coping with the extant environmental conditions within a community and in developing specializations which lessen competition for the available essential resources, then the greater will be the complexity of the community structure. The characterization of community structure, or some facet thereof, is a primary objective of the aquatic ecologist investigating a given lake or stream.

Community Organization

A small lake or stream is an excellent natural laboratory for observing community structure. Perhaps one of the first features to be noticed is that certain kinds of plants and animals appear to be restricted to a particular place in the community. A closer examination of organisms in the community will, indeed, support this hypothesis. While some species may be found throughout the community, others are found in only a certain region. This uneven distribution of species represents zonation or stratification.

Examples of zonation and stratification in a lake are shown in Figures 1 and 2. What causes these
features, and why are they more evident in some lakes than others? When students are involved, these and similar questions can lead to imaginative discussions and class projects to test hypotheses derived from the discussions.

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Upper: Fig. 1. Depth zonation in lakes

Lower: Fig. 2. Rooted plant distribution in the littoral zone

Figure 1 represents a cross-section of a small lake and shows where certain types of organisms are found.
Ecologists characterize that area along the shore in which rooted plants (macropbytes) grow as the littoral zone. Depending upon the kinds of rooted plants present, this portion of the lake can be subdivided into: a zone of emergent plants, a zone of floating plants, and a zone of submergent plants (Figure 2). It is not always possible to distinguish boundaries between these three zones because representatives of more than one type of plant may occur in the same part of the lake. Plants characteristic of the zone of emergent plants are cattails and sedges, while the water lily is a representative of the zone of floating
plants. Pond weeds, coontail, and water milfoil are found in the zone of submergent plants.

The limnetic zone extends beyond the littoral zone into the open water. The only producers found in this part of the lake are the microscopic phytoplanters, principally algae, but upon occasion the tiny free-floating duckweeds may be present. The depth of the limnetic zone is determined by the depth to which sufficient light penetrates to allow photosynthesis to balance respiration. Below the limnetic zone lies the profundal zone. Little or no photosynthesis occurs here.

Numerous additional terms have been coined for characterizing these regions of a lake. For example, the term euphotic zone is often used to designate that part of the lake in which photosynthetic activity exceeds respiration, and this region encompasses both the littoral and limnetic zones. Either aphotic or dysphotic zone is sometimes used rather than profundal zone. Literally, these terms mean without light or poor in light, respectively.

Although plankton and nekton can be found in most any region of the lake, other types of organisms are, like the macrophytes, more restricted. Planktonic organisms are microscopic plants and animals floating in the water column and are termed phytoplankton and zooplankton, respectively (see Kansas School Naturalist, Spring, 1977, for examples). Nekton are large free-swimming animals capable of navigating independently of water currents. Neuston are organisms floating or swimming on the surface and include the whirligig beetle and water strider. Benthos are organisms living in or on the bottom.


Benthic invertebrates are important organisms in lakes and steams. These interesting animals serve many purposes and exhibit many specializations which enable them to perform their ecological functions.

Although there are hundreds of different species of benthic macroinvertebrates (invertebrates large enough to see without the aid of a microscope) inhabiting our Kansas lakes and streams, most species belong to one of but three phyla. The Arthropoda, especially the class Insecta, contributes the most species, followed by the Mollusca (clams and snails) and the Annelida (segmented worms). Of course, species of several other phyla can be collected, but these three are the most common and abundant.

Many of the benthic macroinvertebrates are important food items for some species of fishes, amphibians, and water fowl. Others are useful in that they graze on plant material (are herbivores), are eaten by another larger animal which, in turn, is preyed upon by other species; and thus, help transform the energy
bound in plant matter into forms that can be utilized by other animals. Another important function served by some benthic macroinvertebrates is aiding the breakdown of organic wastes and detritus so that the minerals and elements contained therein can be reused.

Collection and Preservation of Benthic Invertebrates

Species identification of aquatic insects requires the adult stage. Adults of many species of aquatic insects can easily be collected with the aid of a black light a t dusk. This is the time that nocturnal (night active) and crepuscular (active during dawn and dusk periods) species begin their flying activities. If the black light is projected upon a white sheet hung from a tree branch near a stream or lake, the insects attracted by the light can be picked from the sheet by hand and placed in a jar of preservative.

Either 80% ethyl or isopropyl alcohol can be used to preserve many species of aquatic insect adults and larvae and other benthic invertebrates. It is recommended that the alcohol used by changed during the first few days after preserving to insure good preservations. Another procedure is to fix small specimens, especially caddisflies and other small insects, in Kahle's fluid for two or three weeks before storing in 80% alcohol. This treatment will aid some color retention. Kahle's fluid consists of:

Ethyl alcohol 15
Formalin 6
Glacial acetic acid 1
Distilled water 30

In shallow streams and along lake shores, benthic invertebrates can be collected by picking them from the surfaces of rocks and other submerged objects (see cover picture). A pair of fine-pointed forceps is a must for handling the smaller forms.

Numerous special nets and other sampling devices are available for collecting benthos. Stream bottom sampling nets, such as the Surber type (Figure 3) can be used in shallow streams to collect a quantitative sample. The net is placed on the bottom with the mouth of the net pointed upstream. A known area of stream bottom in front of the net is stirred to dislodge the invertebrates, and the current washes them into the net. The net contents are then transferred to a container for later sorting. Large rocks in the area to be sampled must be removed and the attached organisms are picked from the rock by hand.

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Figure 3. A. Surber-type net for use in shallow streams

Artificial substrate samplers (Figure 3B) can also be used to collect certain types of invertebrates. The samplers are submerged in a stream or lake for a period of several weeks to allow the benthos to colonize the sampler. It is then removed and placed in a container of preservative. The sampler is later dismantled in the laboratory, and the invertebrates sorted for identification.

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Figure 3. B. Dredge for deep lakes

The soft sediments of the deep pools of streams and lakes can be sampled with a dredge (Figure 3C). There are many types of dredges, the type to use will be determined by the texture of the substrate to be sampled. The dredge is cocked and lowered to the bottom. The jaws are triggered and take a quantitative bite of bottom mud. The dredge is raised and the contents dumped into a screened tray for washing away the mud so that the invertebrates can be picked from the tray.

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Figure 3. C. Two types of artificial substrate

Let us now examine a few specific examples of some of the more common types of benthic macroinvertebrates one might find in a Kansas lake or stream.


Insects belonging to the order Tricoptera are commonly called caddisflies. Adult caddisflies are mostly small grey or brown moth-like insects which are frequently overlooked because they are active primarily at night. The adults, of course, are terrestrial, but the larvae are aquatic and can be found in virtually all kinds of freshwater habitats. Caddisfly larvae are one of the most abundant forms of benthos one might expect to find in the rocky riffle zones of many Kansas streams. Some species inhabit the wave-washed shores of lakes and others can be found clinging to sumberged and floating objects.

The Tricoptera undergo complete metamorphosis, usually completing one generation per year. Adults are relatively short-lived; most species have at least five instars, and pupation requires approximately three weeks.

The larval stage varies in length from approximately 2 mm to more than 10 mm in most of our more common forms. Color is various, most often green, light tan, or gray. The head capsule is well-formed with one pair of eyes and a pair of large opposing mandibles. There are three pairs of thoracic walking legs, one pair of each of the three thoracic segments and a pair of termina, clawed anal prolegs which are used to anchor the individual. Tracheal gills, which may be either branched or single, are present on abdominal segments (Figure 4).

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Fig. 4. Caddisfly larvae collected from a small stream

Many caddisfly larvae construct elaborate cases in which the larvae live. In some species, the cases are attached to the substrate; in other species the case is moveable and the larva crawls about dragging the case with it. Although other types of insects also build larval cases, none exhibit as much variety as the Trichoptera. Cases are constructed of silken strands with bits of rock and plant material incorporated on the outer surface, and they vary in design from the spiral snail-like case of Herlicopsyche to the large (50 mm
-/+) cyclindrical and conical cases of some of the Limnephilidae (Figure 5).

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Fig. 5. Examples of caddisfly cases

The larval case not only provides a degree of shelter but is involved in the respiratory process. Undulations of the body forces water through the case and aids in aerating the abdominal gills. Cases and nets of all forms are also involved in feeding, either directly as in the net-spinners, or indirectly in the forms with moveable cases. In the latter forms, the larvqe are able to move into more exposed areas in search of food than might be otherwise possible.

Locate a shallow, flowing reach of a nearby relatively clear stream and carefully view the stream bottom for the seinlike nets that some species construct. In some streams large portions of the bottom are covered with these nets (Figure 6). The larva occupies a small retreat near the end of the net and feeds on small organisms and bits of organic material strained from the water flowing through the net. One can find many different kinds of larvae on the same piece of substrate.

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Fig. 6. A. Seive-like nets of caddlsfly larvae on stream bottom

Identification of species requires the adult stage and experience. There are no popular and inexpensive guides to the identification of the Tricoptera in Kansas. It is possible to recognize some of the more common genera from larvae using the following references. Pennak (1954) is a useful guide for the beginner on most of our freshwater invertebrates.

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B. Caddisfly larvae on surface of rock

Pennak, R . W. 1954. Freshwater Invertebrates of the United States. Ronald Press. N.Y.

Wiggins, G. B. 1977. Larvae of the North American Caddisfly Genera (Trichoptera). Univ. Toronto Press. Toronto. 401 pp.


Mayfly larvae constitute another common and important group of benthic macroinvertebrates in Kansas lakes and streams. Like the caddisfly, mayflies spend the greatest part of their life cycle as larvae in an aquatic habitat. In general, the same techniques used to collect and preserve caddisflies can be used for the Ephemeroptera. However a dredge is required to collect burrowing forms, such as Hexageoia, which inhabit the bottom sediments of lakes and ponds.

Ephemeroptera undergo incomplete metamorpbosis. Most adults live only long enough to reproduce; eggs are deposited in water and hatch into naiads. This immature state has some features that are similar to the adult, but differs in several ways. First, the wings are not yet developed, and lateral gills are present on at least a few abdominal segments. The last abdominal segment usually bears three long hair-like structures (Figure 7). From instar to instar, the naiad takes on a greater similarity to the adult. Emergence time differs from one species to the next, but most species emerge during the spring and summer. Emergence is often almost simultaneous in a given species, the result being the formation of huge swarms of adult mayflies around lakes and bridges. Perhaps you have experienced such an event. The adult is quite harmless, but under such conditions they can be annoying. There have been instances where mayfly swarms have stopped vehicular traffic over bridges of large rivers. Mayfly larvae, and especially emerging and ovipositing adults, are important fish food.

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Fig 7. A. Mayfly naiad

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Fig 7. B. Mayfly immature stage collected from a small stream

Identification requires adults, although generic recognition is possible from immature stages. Naiads can be maintained in the laboratory until emergence; if this is done, it is possible to associate a given type of immature to the adult stage. In addition to Pennak (1954), the following reference can be used to help identify immature mayflies.

Edmonds, C. F. Jr., S. L. Jensen, and L. Berner. 1976. The Mayflies of North and Central America. Univ. of Minnesota Press, Minneapolis. 330 pp.


Stone flies are less abundant than mayflies and caddisflies, but are frequently found when collecting the above insects, especially in a c1eanwater stream. Development is incomplete, and sometimes the inexperienced observer will confuse the stonefly larva with that of a mayfly, or even a damselfly. In general, the stonefly naiad is somewhat flattened dorso-ventrally. Each of the three thoracic segments is covered dorsally with a flat sclerite, The abdomen is elongate and the last segment bears just two long hairlike cerci. There are no abdominal gills (Figure 8).

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Fig. 8. A. Stonefly naiad

Emergence of most species occurs during the spring to fall, but a few species emerge during winter. Adults can be found on bridges, and can be obtained by rearing larvae in the laboratory.

Ricker, W. E. 1952. Systematic Studies in Plecoptera. Indiana Univ. Publ. Science Series. No. 18,200 pp.


Adult dragonflies and damselflies are insects often associated with a lake or marsh. Perhaps the reason for this is the adults are active flyers during daylight and readily seen. The larvae of these insects can be found in most any type of aquatic habitat. They are mainly predators and scavengers and usually less abundant than other forms of benthos. The somewhat truncated body of dragonfly naiads makes recognition easy, but many beginners confuse the damselfly naiad with that of either a mayfly or stonefly. The last abdominal segment of the damselfly naiad bears three fl at anal gills rather than cerci; there are no lateral abdominal gills as in mayflies. The labrum is greatly enlarged and can be extended forward. (Figure 9).

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Fig. 9. A. Larval stages of damselfly, left, and dragonfly, right

Immatures can be collected by picking individuals from the surfaces of submerged objects.

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B. Damselfly larvae from a small stream


Crayfish need no introduction; most people know one when they see it. These crustaceans can be found in virtually all types of aquatic habitats in Kansas and are excellent food for many different vertebrates, including man.

Crayfish are one of our largest freshwater invertebrates (Figure 10). They usually secret themselves under
a submerged object or walk slowly along the bottom in search of food or shelter. If distrubed, they are capable of rapid swimming, using the fanlike uropods on the end of their jointed abdomen. Collection is most easily accomplished using either a seine or a dip net.

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Fig. 10. Crayfish, dorsal view

There are six species of crayfish known from Kansas but only one, Orconectes naias, is widespread. Postmolt males are necessary for identification. The following reference for identifying Kansas crayfish can be obtained from the Museum of Natural History, University of Kansas.

Williams, A. B. and B. Leonard. 1952. The Crayfishes of Kansas. Univ. of Kansas Science Bulletin. Vol. 34, PT. II, No. 15.


The freshwater members of this class of mollusks include the mussels (Family Unionidae) and fingernail clams (Family Sphaeridae). Identifications are based on shell morphology, so it is not necessary to have live specimens. Living clams are most easily collected by wading shallow reaches of a stream and picking up anything that, underwater, fells or looks like a live clam. Species inhabiting deep waters present other problems, and dredges and rakes must be used to collect them. Many unionid shells can be found along stream banks where they were deposited by high water or some animal, such as a raccoon.

Clumps of fingernail clams can be found in and on submerged objects in stream riffles as well as in the soft sediments dredged from a lake or pond.

Examples of common Kansas species of unionid mussels and fingernail clams are shown in Figure 11. The following references can be used to identify species.

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Fig. 11. Comparative size of the freshwater mussel,
Lasmigona, and the small fingernail clams, Family Sphaeridae.

Burch, J. B. 1975. Freshwater Sphaeriacean Clams (Mollusca: Pelecypoda) of North America. Malacological Publications; Hamburg, Mich. 96 pp.

Burch, J. B. 1975. Freshwater Unionacean Clams (Mollusca: Pelecypoda) of North America. Malacological Publications. Hamburg, Mich. 204 pp.

Murray, H D and A. B. Leonard. 1962. Handbook of Unionid Mussels in Kansas. Mus. of Natural History. Univ. of Kansas. 184 pp.

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Fig. 12. A good place to collect benthic samples

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