In order for organisims and species to survive they must be able to meet their habitat requirements for food, water, air, space and reproduction. The availability of good habitat determines what organisms will be able to survive in a particular habitat. This web page looks at how aquatic organisms interact with each other and how they obtain what they need from their environment. The study of the interrelationships of organisms with each other and with their environment is the study of ecology.
This web page describes how the physical, chemical and biological characteristics within rivers define different habitats for different organisms. It focuses specifically on how the substrate (stream bottom), dissolved oxygen levels and food sources within different reaches of streams and rivers influence the inhabitants and how macroinvertebrate inhabitants have adapted to get what they need from the environment. Other web pages will deal with macroinvertebrates as bioindicators of water quality. This page also looks at how energy production and consumption balance each other within a healthy ecosystem.
Every natural system must have a source of energy in order to operate. The sun provides the ultimate source of energy in all but a very few unique situations. During photosynthesis green plants use sunlight to change water and carbon dioxide into carbohydrate food. They also release oxygen as a byproduct during this process. Plants that can manufacture food using the energy of the sun are called producers. Producers play an essential role by using sunlight to "produce" chemical energy. In most smaller rivers the major source of energy is the leaves that fall or wash into the river from streamside vegetation. Lesser amounts are from algae and rooted plants that live in the river, though in larger rivers these become more important.
Primary consumers are organisms that eat producers. Secondary consumers eat primary consumers and so on. When plants or animals die, the chemicals that make up their bodies are broken down and released back into the system as nutrients by the decomposers. The major decomposers are bacteria and fungi. Decomposers may even become food themselves when they are attached to a piece of detritus that is eaten.
Organisms that obtain energy from similar sources belong to the same trophic level (feeding level), for example, they might all be secondary consumers. Autotrophs (plants) can make their own food and are the producers. Heterotrophs must obtain their food from something else.
As energy is transfered from trophic level to trohic level some of the energy is "lost". In part that is because organisms do not consume all of the food that is theoretically available nor do they use all of the food they consume - some is eliminated as feces. Another reason energy is "lost" is that organisms also use energy for their own metabolic life processes and that energy is ultimately released as heat and cannot be passed to the next trophic level.
Another way of describing feeding in an ecosystem is to describe animals by what they eat. Animals that eat plants are called herbivores. Animals that eat other animals are called carnivores. Detritivores eat detritus. Omnivores eat everything. Among the carnivores, animals that eat other animals are called predators. Animals that are eaten are called prey. Scavengers eat whatever dead organic material they can find. The role in the ecosystem that an organism plays in relation to other organisms and to the environment is its niche.
Because of their abilty to manufacture food green plants are the base of all food chains. A food chain describes the sequence of energy as it moves along from organism to organism. A food chain in a stream might look like this:
Of course the food chain does not stop there. Most organisms are food for more than one other type of organism within the same system. A food web describes a number of overlapping food chains and is usually a more accurate description of feeding patterns in an ecosystem. Food webs illustrate the interconnectedness of organisms within an ecosystem.
Decomposers are also a part of the food web. All living organisms take up and use nutrients. They can be thought of as living nutrient warehouses. Decomposers break down dead organic materials and release nutrients into the soil or water. These nutrients continue the cycle as producers use them to grow.
Aquatic macroinvertebrates, such as larval insects, crustaceans and molluscs play an important role in aquatic food webs. They can be primary, secondary, or even tertiary consumers. They obtain food by shredding, grazintg, filtering, gathering and preying. One insect can have different food sources and ways of oobtaining food at different stages and in different seasons of its life. Stonefly larvae and a number of the caddis and true fly larvae are shredders. When they feed, they break up leaves and plant parts. Their feeding action changes large pieces into finer particles. Collectors gather or filter out the resulting particles or waste feces from the shredders. Some mayfly nymphs as well as other caddis and true fly larvae are collectors. Predators, such as the dragon fly nymph and dobson fly larvae, eat other aquatic insects. Grazers, such as the water penny beetle, scrape algae off rocks.
Streams and rivers are particularly interesting ecosystems to study because as one travels from the headwaters to their mouths, the physical and chemical characteristics influencing the ecosystems change. These changes, in turn, create changes in the organisms living there. The notion that these changes exist and are somewhat predictable is called the river continuum concept.
There are many factors that influence the physical and chemical characteristics of a stream or river. Physical characteristics such as width, temperature, substrate, water velocity and water volume are influenced by the topography and geologly of the stream. Humans might change these characteristics by building dams, channelizing, affecting runoff in the area or altering the streamside vegetation.
Chemical characteristics such as pH, buffering capacity, dissolved oxygen and nutrient levels are also influenced by the topography and geology of the area. Land use activities such as burning fuels, farming and managing waste can change the chemical characteristics of water bodies. These aspects of water quality are covered in more detail on the page Water Quality Monitoring.
Biological factors are not only influenced by physical and chemical characteristics, but they , in turn, can influence physical and chemical characteristics. Aquatic organisms' life processes such as obtaining nutrients, using oxygen, producing wate and decomposing affect the physical and chemical characteristics of their water habitats.
Terrestrial biota also inluence aquatic ecosystems. Riparian forests (steamside trees) influence many of the physical and chemical characteristics of headwater streams and play a very important role in preserving the health of a waterway:
Let's follow a river along its continuum from the headwaters (first order) to the mouth (up to sixth order). Although stream order is technically defined from orders one to twelve, rivers beyond the sixth order are very difficult to find. Headwater streams have a unique ecology that involves complex interactions among water, land and living organisms.
Every natural system needs a source of energy in order to function. Algae, which are usually a major source of productivity in aquatic environments, play a very small role in headwater steams because the shade from riparian forests limits the amount of sunlight reaching the system. The major source of energy in a headwater stream is actually brought in from outside the system by wind and rain. Riparian forests supply headwater streams with energy. Fallen leaves, twigs and tiny organisms clinging to them are the basis of the food chain in headwater streams. These organic materials fall into the water and start to decompose. Microbes and some aquatic macroinvertebrates use this coarse organic material (detritus) for energy.
Headwater streams are typically rocky, shallow, narrow, cold, clear, fastmoving and high in dissolved oxygen. Consequently, the primary consumers that inhabit these waters must be able to get their food from dead leaves and be able to tolerate shallow, fast moving water. In addition, they usually require high levels of dissolved oxygen. Boulders, rocks and rubble make up much of the steam bottom and create riffles that provide abundant habitat. Macroinvertebrates that inhabit headwater streams are also well adapted for fast flowing water by being strong swimmers and by having streamlined bodies, structures that help them attach to rocks or coverings that protect them.
Shredders and collectors make up the bulk of primary consumers in headwater streams. Naturally, shredders are more prevalent in these streams than along any other part of the continuum because many of the leaves falling into headwaters are still intact. The mouth parts and forelegs of shreeders are adapted for chewing and breaking up these relatively big pieces.
Stonefly nymphs live in headwater streams. These aquatic macroinvertebrates are either predators or detritivores. Scientists believe that stoneflies that eat decaying plant material are really going after the protein rich bacteria and fungi decomposers rather than the decaying leaves themselves. Stoneflies can only survive in water with high levels of oxygen. They breathe through the surface of their skin or through tufts of gills along their sides under their legs. In lower oxygen waters, they increase the flow of oxygen over their bodies by doing "push-ups". Stonefly nymphs are a favorite food for trout. Mayflies, some caddisflies and water pennies are examples of other aquatic invertebrates that need high levels of oxygen and are therefore common in headwater streams.
Hellgrammites, also known as dobsonfly nymphs, are active predators in fast flowing waters. They are one of the larger aquatic invertebrates you might find during a survey. Hellgrammites play an important role in the aquatic food web by eating other aquatic insects. They are also one of the few aquatic insects that might bite a person. Even so, anglers seek them out for their ability to attract fish.
Moving downstream from the headwaters, small streams connect to each other and grow broader and deeper. Through the streches of third to fourth order streams, the primary productivity gradually changes. Through these stretches of the river shade trees have less influence, relatively less coarse material drops in, and fine particles of decomposing leaf litter from upstream appear. As the tree canopy starts to open and allows more sunlight to penetrate to the stream bed, additional sources of primary productivity become available. Periphyton (algae growing on submerged rocks) and rooted aqutic plants eventually join detritus at the base of the food chain.
Third to fourth order streams are somewhat wider, warmer, sunnier and sandier than headwater streams. The stream bottom is likely to comprise a few boulders, more rubble and gravel, and some sand and mud. This variety of substrate provides a variety of habitats. Midreaches have greater biodiversity than either headwater streams or downstream stretches. Midreaches have some of the conditions and qualities of both headwater streams and downstream stretches, therefore they can support a large variety of species. Having a wide variety of species often helps keep a natural system more stable because a catastrophe to one species will not affect as great a proprortion of that system as it would if the system had only a few different species.
The primary consumers in this stretch are adapted for gathering tiny bits of organic material or algae and are dominated by collectors and periphyton grazers. The relative abundance of grazers is greater here than in the headwater streams. Grazers are adapted for adhering to rocks and being able to scrape algae off them. Shredders decrease in abundance as less coarse material is available. Filter feeders are able to survive in stretches where there are enough small organic particles for them to eat.
Caddisflies can be grazers, collectors or predators. They need moderate to high levels of oxygen in order to survive. Caddisfly larvae are probably best known for the cases many build to enclose their soft bodies. They use pebbles, grass, sand or twigs to build tiny homes that protect them from being swept away in flowing water. Some types of caddisflies use their homes as mesh collectors to filter tiny organic particles from the stream flow.
Moving toward the mouth, fifth to sixth order streams form a river - broad, deep, sunny and murky. Streamside vascular plants and planktonic algae, much like those found in lakes and ponds, contribute to primary productivity. Coarse particles of leaf litter are rare and an abundance of fine particles of decomposed organic materials drift in from upstream. These fine particles of decomposing plants, animals, and wastes are the primary sourcs of food. In other words, downstream stretches of river benefit from the inability of organisms in upstream stretches to use all the foodstuffs that are released there.
River bottoms in the lower reaches are likely to have some gravel, but to be dominated by sand, silt and mud that have been deposited. Sometimes lower streches of the river have poor water transparcency. Suspended particles cloud the water and prevent sunlight from reaching the bottom. Sediments may bring excessive phosphorous into the system. The oxygen demand of aerobic decomposition, aquatic animal respiration and chemical oxidation reduce the amount of dissolved oxygen in water in lower reaches. Warmer temperatures and reduced aeration also affect dissolved oxygen levels.
In the lowest reaches of the river, filter feeding macroinvertebrates dominate the scene as primary consumers. Most of these creatures burrow in the sand and silt and use fringed body parts to help them filter food and oxygen from the water. Zooplankton (microscopic floating animals) that cannot withstand the flowing waters of upper reaches are found downstream in quiet, still waters. They feed off phytoplankton throughout the water column. The aquatic marcroinvertebrates that need coarse organic material to eat and cool, clear, highly oxygenated water to breathe cannot survive here. The species that can burrow, filter their food and tolerate lower oxygen conditions survive best in lower reaches.
In addition to the characteristic conditions of lower reaches, there may be land use impacts from upstream and along the river that affect the water qualtiy of this section. Because lower stretches of the river are subject to all that happens in upper stretches and within the whole watershed, they tend to have physical and chemical conditions that create habitats which limit the amount and variety of species that can survive there. Polllution tolerant and hardy types of aquatic invertebrates can live anywhere along the continuum, but are likely to be more prevalent in the lower reaches than their pollution intolerant relatives.
Some macroinvertebrates are adaptable to a wide range of habitat conditions. Midges are interesting aquatic larvae that can survive in a huge variety of habitats from marine waters to arctic bogs. They live everywhere from clear, deep lakes to heavily polluted waters. Midge larvae live in sediments and take in oxygen through their body surfaces. Some of them have a substance in their blood that holds oxygen; the oxygenated blood may make them appear red. This blood substance helps them survive in low oxygen conditions. They can be predators, herbivores or omnivores and are important prey for fish.
Midges also play an important role in sewage treatment ponds. During daylight hours algae in sewage ponds release oxygen that is used by aerobic decomposers. However, the accumulation of dead algae creates problems for the operation of the pond. Midges help to reduce this accumulation by consuming the collected algae.
The river continuum concept describes how abiotic and biotic factors in stream and river ecosystems influence each other. It also illustrates how changes in enegy production and consumption through different portions of a river network help keep the whole system in balance.