Aquatic Organisms 9

Aquatic organisms “Is ditch water dull? Naturalists with microscopes have told me that it teems with quiet fun.” 9 — G.K. Chesterton healthy stream is a highly diversified the quality of available water. Fish occupy an ecosystem. Its complex food chain ranges important position in the aquatic food chain and A from microscopic diatoms and algae to obtain their food supply from several sources. large fish, birds and mammals. The diversity of The amount of food available in a stream is species, particularly aquatic organisms, and their determined by the physical and biological condi- numbers are important to any stream study for tions of the area. When producers are plentiful, two reasons: consumers also flourish. Diatoms coating a rock • as indicators of water quality in the stream feed primary consumers such as mayflies. They, and in turn, feed higher-order consumers like stoneflies and fish. • as parts of various food chains, including Overhanging Vocabulary fish. vegetation supplies a benthic A wide variety of organisms inhabit water. variety of terrestrial hyporheic The size and diversity of a population depend on insects to the menu. plankton Oregon Department of Fish & Wildlife Aquatic organisms • 307 Many aquatic insects use streamside vegetation underground and under-river ecosystem. These during emergence and adult stages of their life organisms contribute to the health and productiv- cycle. ity of the river by supporting the aquatic food Some aquatic insects leave their positions chain that extends to and beyond the water’s among boulders and gravel in riffles and are surface. carried downstream short distances before reat- taching to the stream bottom. When insects are moving in a water column, as drift or during The diversity of species, emergence, they are most vulnerable to being eaten. particularly aquatic organisms Benthic (bottom dwelling) organisms are found on stones or in mud or vegetation. Because and their numbers, are a streambed serves as a place for attachment, most organisms in a fast-moving stream will be important to any stream. benthic. Organisms in fast water have many special- ized methods for obtaining food. To gather food Evidence suggests that hyporheic (from in a water column, they grasp it quickly or filter Greek for “below” and “flow”) exchange is it from the water while remaining stationary. significant in large streams, like the Santiam Others gather food on the bottom. River of Oregon’s Willamette Valley. Some Plankton can be producers or consumers and scientists feel the stability of many streams may float or swim freely throughout a stream. Few depend largely on these hyporheic zones, which organisms can live in rapid sections of streams exchange water and materials with the river without being swept downstream by the current. channel. The hyporheic zone may extend 15 feet Consequently, plankton are abundant in slower to 30 feet below the river bottom and two miles waters of large streams and rivers. to either side of the river. Stream ecologists have found a complex The knowledge of hyporheic zones, and the community of small animals living in the ground organisms found there, challenges traditional water below the stream channel and sometimes views of how rivers work. It may have an effect for miles on each side. Many types of small blind on river system assessments. It could also mean shrimp, primitive worms, bacteria, algae, and that measures to protect streams from pollution various kinds of immature insects live most, if or alteration may need expansion to include not all, of their lives in a maze of channels in this wider areas along the watercourses. 308 • The Stream Scene: Watersheds, Wildlife and People Oregon Department of Fish & Wildlife Food processing “And in the water winding weeds move round.” 9.1 —Wallace Stevens n autumn, forest floors are piled high with Functional feeding groups leaves. But in spring, the Earth’s load is light- What or who is responsible for all this aerobic ened; the leafy carpet has worn thin and I decomposition? Leaf litter can be broken down seems to disappear with the melting snow. Where and decomposed slowly by abrasion and micro- have the leaves gone? Those that stay where they bial action, but streams also harbor invertebrates fall are decomposed, for the most part, by soil invertebrates and microbes. But many of the “disappearing leaves” are carried down hill slopes into small, heavily canopied forest streams. What is important is Most leaves and other organic materials blown by the wind, washed from the surrounding not so much what, landscape, or fallen directly from overhanging but how the animals eat. limbs into watercourses do not get very far. They are trapped by rocks, logs and branches close to where they entered the water. They become part of the food or energy base of the stream. that help decompose leaves and other organic Some of this material settles out in pools and materials under a variety of conditions. A rich, backwaters. Leaves that get buried will decom- diverse population of aquatic insects is keyed to pose anaerobically. Because anaerobic processes the varied quality of this food base. are much slower than aerobic ones, buried leaves Although most of us have seen our share of remain intact longer. These leaves can be recog- crayfish and snails, other aquatic invertebrates, a nized by their black color. Eventually the buried bit smaller and often a bit quicker, can easily leaves are re-exposed, and decomposition contin- elude us. The aquatic invertebrates we are inter- ues aerobically, much as if they had never been ested in here are inconspicuous aquatic insect buried. larvae and nymphs (immature forms). It is hard to distinguish one species from another at this Vocabulary aerobic gatherers This section is adapted from “Turning Over a anaerobic predators Wet Leaf,” by Rosanna Mattingly, and used with permission from The Science Teacher, collectors scrapers September 1985. filters shredders Oregon Department of Fish & Wildlife Food processing • 309 immature stage, and the nymphs’ names are and fruits—by biting into them or by cutting or based, in general, on their adult characteristics. boring through them. These insects are called So, rather than identify these animals individu- shredders. Shredders generally reduce whole ally, we can group them according to the mode of leaves to masses of small particles, but they often feeding for which each animal is adapted. What leave the midrib and veins intact. Thus, they is important is not so much what, but how the “skeletonize” the leaves. Many shredders prefer animals eat, hence the distinct functional feeding leaves that have been partially decomposed by groups. microbes; with microbial decomposition, leaves become tender and digestible. Shredders In the Pacific Northwest, litter from many Some aquatic invertebrates feed on leaves or soft-leaved shrubs is quickly colonized by mi- other organic material—such as wood, needles crobes. This microbe conditioning makes leaves Figure 11. Food Processing in Streams Direction of energy flow or "eaten by" SUN Contributes to FPOM Food source Feeding group Coarse Particulate Fine Particulate Organic Matter Organic Matter (CPOM) (FPOM) leaves, needles, cones, fecal pellets, plant and twigs fragments Algae (mostly green algae and Bacteria and Bacteria and diatoms) other microbes other microbes Shredders Collectors Scrapers Examples: Examples: Examples: Organic case caddis Net-spinning caddis Minneral case caddis RIPARIAN PLANTS RIPARIAN Craneflies Midge larvae Snails Dull color stoneflies Blackfly larvae Mayflies Most often found: Mayflies Most often found: Leaf packs Most often found: Rocks Water-logged wood On rocks and in mud Open-canopied areas Headwater streams Lower stream reaches Mid-stream reaches Predators Examples: Mottled stoneflies, beetle larvae, dragonfly larvae, free-living caddis, fish Most often found: Throughout stream Adapted from: Ken Cummins, “From Headwater Streams to Rivers,” American Biology Teacher, May 1977, p. 307. 310 • The Stream Scene: Watersheds, Wildlife and People Oregon Department of Fish & Wildlife into palatable, nourishing invertebrate meals filterers and gatherers feed, at least for short before most other leaves are ready. Though they periods, on particles of little or no nutritional are somewhat slower to decompose than herba- value. Apparently, some appear to pay no atten- ceous leaves, alder leaves are also a favorite. tion to what they are eating. Other types of leaves must remain in a stream Filter feeders include blackfly larvae— longer before they become soft enough for the elongate animals that are bulbous near the bot- animals to eat, so shredders end up with a “time- tom end where they attach themselves to stream release” menu. substrates. Blackfly larvae have fans with which they strain particles from the water column. These fans are coated with a sticky substance that catches small particles that would otherwise pass through their fans. Collectors are often more Freshwater clams feed in a similar manner by abundant than shredders in passing food over mucous-covered gills that filter out small food particles. Some free-living low-gradient streams. caddisflies spin nets of various mesh sizes and thereby selectively collect particles of certain By chewing on leaves, shredders expose leaf surfaces and edges to further attack by microbes. In streams, organic materials Shredders also biochemically alter organic sub- are produced, received, strates as the material passes through their diges- tive tracts. So, shredders excrete material usually stored and decomposed. composed of particles that are smaller and of a different quality than what they ate. Many stonefly and caddisfly larvae are shredders. Caddisflies are especially intriguing sizes. Mayfly nymphs and beetle and fly larvae because many use the same leaf bits and other are particularly abundant gathering collectors. organic fragments they eat to construct the cases Collectors are often more abundant than in which they live. shredders in low-gradient streams where fine particles are not washed away so rapidly.

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