Benthic Fauna of Streams

Abstract

Stream invertebrates are well adapted to the running water environment. The dominant taxa in headwater streams include the immature stages of the insect orders Ephemeroptera (mayflies), Plecoptera (stoneflies), Trichoptera (caddisflies), Megaloptera, Coleoptera (beetles), and certain Diptera. Many other types of invertebrates, attached and planktonic algae, rooted vascular plants, and various vertebrates also are common in streams of higher order. All of these organisms have certain traits that enable them to maintain their position and survive in rapidly flowing waters. Some mayflies (e.g., Heptageniidae) have become dorsoven-trally flattened. Although they inhabit very rapid water, these mayflies live close to the substrate where the water velocity is nearly zero (Fig. 5.3). Many caddisflies (e.g., Limnephilidae) build elaborate cases that not only protect them from predators but also serve as ballast against the current or as attachment points on rocks. Others (Hydropsychidae) build intricate nets on submersed rocks and logs to catch food particles that are being transported downstream. Certain dipteran larvae (e.g., Blepharoceridae) have specialized suckers that are used to attach to the substrate. The common black fly larvae (Simuliidae) spin silk pads and attach themselves to rock surfaces by this means. [See Hynes (1970) for extensive treatment of this subject.]

The wide variety of microhabitats available to stream organisms will become apparent as you investigate stream ecosystems. Rock surfaces, plant surfaces, leaf debris, logs, backwaters, silty or sandy sediments, crevices in gravel, organic debris dams, and other spaces in

the stream all provide special habitats for different organisms. The patchy distribution and abundance of organisms in a given stretch of stream is partially dependent on the availability of these microhabitats. Thus, any sampling of the benthic fauna must take this spatial heterogeneity into account. This patchiness adds both to the fascination as well as to the difficulty of research on streams.

Stream insects play a role in the processing of organic matter [see Cummins (1974) and Cummins et al. (1984)]. Streams in forested regions depend on allochthonous inputs of organic matter for much of the consumer productivity [see Fisher and Likens (1973)]. Many of the insects mentioned above are well adapted to utilize terrestrial organic matter as food. Ingested detrital particles are either assimilated by the organisms or egested and utilized as a food source by other consumers. Bacterial colonization of these organic particles may increase the food value of the detritus by increasing the nitrogen content. Many of the invertebrates themselves become sources of food for other carnivorous invertebrates or vertebrates.

Macroinvertebrates include invertebrate fauna retained by a 500-μm pore sieve or net. Because many of the early life stages important to life histories and production analyses are smaller than this size demarcation, collection methods often employ finer-meshed devices (e.g., 125–250μm). Meiofauna are benthic animals that pass through a 500-μm pore sieve but are retained on a 40-μm pore sieve. Their composition is dominated by rotifers, copepods, ostracods, nematodes, and young stages of chironomid dipterans and oligochaete worms, as well as other animals such as gastrotrichs, tardi-grades, and turbellarians. Because of their small size, the meiofauna has been studied rarely, even though these benthic animal communities dominate in terms of numbers and species diversity, and likely often are more important to ecosystem energetics than are the macroinvertebrates.

Meiofauna, like most macroinvertebrates, occur in all types of standing and running waters, and live on surfaces, including plants, debris, and sediments, as well as within the interstitial spaces of sediments and grains of sand. Because the availability of dissolved oxygen is important to the distribution of meiofauna within the interstitial water of sediments, meiofauna tend to be more abundant within the hyporheic zone of running waters than in organic dominated sediments of lakes.

The types and distributions of benthic macroinvertebrates also have been used widely as indicators quality. The distribution of certain macroinvertebrates and microorganisms can be quite specific because many organisms often have narrow physiological tolerance ranges [e.g., Sladecek, (1973)]. Colonization of many different artificial substrata has been used in comparative analyses of water quality among and within water bodies [e.g., Rosenberg and Resh (1982), Flannagan and Rosenberg (1982), and Lamberti and Resh (1985)].

This exercise briefly introduces some of the sampling equipment and problems of obtaining quantitative estimates of the distribution and abundance of stream invertebrates.