A tributaries join with the main river.

A given river varies considerably over its length, as it changes from a mountain brook to a large river. This is most noticeable in the abiotic factors of the environment, but all features of the ecosystem vary in response.

Characteristic of Lotic Environment:

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Moving waters differ in the three major aspects from lakes and ponds : current is a controlling and limiting factor land-water interchange is great because of the small size and depth of moving water systems as compared with lakes; is almost abundant supply except when there is pollution.

Temperature ex­tremes tend to be greater than in standing water. The most distinctive features of moving water ecosystems are related to their motion, i.e., the rate of flow and the stream velocity.

The rate of flow or discharge refers to the volume water observation point during a specific unit of time; in units such as m.3/sec., ft.3/sec., or acre-feet/sec., (one a foot is equivalent to a volume of water 1 acre in area by 1 foot deep). It tends to increase steadily growing downstream, as tributaries join with the main river.

The velocity of flow is the speed at which the water moves, and is measured in m. /sec., ft. / sec., or mi. /hr. Velocity is variable, but it also tends to increase down­stream with increasing discharge. A factor that may be even more important to the biotic community is the turbulence, or the irregularity of the motion of the wafer particles.

Perfectly even flow in which water particles move parallel to one another, is -ailed laminar flow it is contrasted with turbulent flow in which the movement of water particles is highly irregular. In highly turbulent water the erosive power is great, the sheer for­ces at the water-sediment interface at the bottom of the river are powerful, and the amount of oxygen incorporated into the water is very high.

In streams with laminar flow the erosive power is lower, the sheer forces at the bottom of the stream are lower, and relatively less oxygen is incorporated into the water than turbulent waters.

Fig. 20.7. Diagram showing the differences between turbulent (A) and laminar (B) flow of water in a stream (after Clapham, Jr., 1973).

The moving-water ecosystems can be divided into following two ecosystems depending up on the velocity and rate of flow of water body:

Rapidly flowing water:

Rapidly flowing water (of fast moving torrential streams, etc.) can be defined as the portion of the stream in which the flow is both rapid and turbulent. Everything that is not attached or weighty is swept away by the current; this includes organisms and sediment particles alike. The substrate tends to be rock or gravel, and the fragments are rounded and smoothed by the water.

The habitat itself is an extremely diverse one. Physical parameters such as sheer force and rate of water movement tend to be quiet different on top of a rock fragment, bet­ween rock fragments, or beneath rock fragments, and different species can exploit these differences in microhabitats.

Aquatic plants, in addition, provide microhabitates for some torrential animals. The rarely occurring phytoplankton of rapid or riffle zone of stream includes diatoms, blue-green and green algae (e.g., Cladophora, Ulothrix) and water moss (e.g., Fontainalis) (Fig. 20.8).

1. Plant inhabiting torrential forms:

Some animals of rapidly flowing water streams live among the mosses and flowering plants like Eriocaulon miserum, Hydrous lichen ides, and Duroea walli- chii. Animals inhabiting these plants have a torpedo-like body which enables them to offer minimum resistance to the current.

Some animals such as funnel mouthed tadpoles of Megalophrys and tippled larvae live entangled in the roots of these plants. They possess devices for fixation and have hydrostatic organs.

Some animals such as Cephalopteryx, Helodes, Phalacrocera, Gammarus, etc., live among leaves and stems of aquatic plants. All these animals possess hook-like structures on their body helping them in anchoring on leaves and stems. The larvae of simulids and chironomids, however, live on the exposed surfaces of plants.

2. Rock inhabitant forms:

The animals that live on the tops of exposed rocks have an efficient mechanism for staying in one place; otherwise, they will be swept away. The organisms in this microhabitat are universally flattened. Some, like the fresh-water limpet, Ferrissia, are virtually flat, offering little resistance to the current, are holding themselves in place with a very large powerful foot that extends over almost the entire area of the shell.

Others, such as water penny, the larvae of the riffle beetle, and Baetis lar­vae, are not only almost flat, but each of their legs possesses hoo­ked claws that enable them to hold onto the substrate more firmly. Larvae of Samarium and chironomids cling to rock top by grappling hooks at the posterior end of the body. Caddishly worms contain both of these structures (claws on legs and hooks at posterior end of body).

The mayfly nymphs Iron and Psephenus attach themselves to the rocks by means of functional pads. Some animals, including fresh-water sponges, actually cement themselves to the surface of the rocks. Others, such as caddish flies, build “houses” cut of sedi­ment or wood fragments, which are then cemented firmly to the rocks. Exposed surfaces of rocks also contain few sessile algae and few plants.

A significant amount of organic detritus is washed into rapid-water ecosystems from upstream and adjacent terrestrial areas, however. This is the source of the most of energy in fast water ecosystems, and is much more important as an energy source then primary production within the stream itself. In consequence, the majority of primary consumers of such a microhabitat are detritus-eaters.

3. Inhabitant forms of spaces between rocks:

Numerous diffe­rent kinds of animals live in the spaces between the rocks. Many of them, such as mayfly and stonefly naiads, are flattened and have certain behavioral adaptations to hold them in place. These include tbigmotaxis, by which they cling instinctively to any hard surface such as rock or another insect larva. Kheofaxis, by which the ani­mals orient themselves to face into current and move upstream, is another adaptation of this sort.

In many of these naiads, the combi­nation of the organisms shape and behaviour is such that the current presses the insect tightly against the rock, increasing the friction between the animal and its substrate. Other insect larvae, such us the hellgrammite, are large and covered with spines their size make it somewhat harder for the current to sweep them away, while the spines help in holding the larvae in places between stones.

4. Inhabitants occurring beneath rocks:

Many species such as flatworms, annelid worms, other insect larvae, clams and some species of snails, live beneath the rocks. The current is weaker here and animals are less likely to be carried away. They lack any spe­cial adaptation.

Finally, if the current is sufficiently slow, certain swimming organisms such as fish will be present. The fish of fast-water eco­systems tend to be stenothermal cold-water fish, such as trout. The fishes of fast-water ecosystems have small body size and stream­lined body. Trout and masheer are capable of moving against the current or waterfall by muscular effort.

Majority of torrential fishes, however, live on or among rocks and boulders, over which water flows very swiftly. Small loaches (Noemaclicilus) or loach-like fishes (Ambyceps olyra) are met with at the bottom and limpet-like fishes (e.g., Glyptosternum, Balilora) or fishes with special modifications, Garra, Glyptothorax, Pseudecheneis) occur on the rocks.

The loaches and carps adhere to rocks like the limpets, with the ventral surface, inclusive of paired fins forming one large suckcr, e.g., Gastromyzvn of Borneo, the Indian loach Baletora of the eastern Himalayas and Assam hills (Hora, 1947).

Physico-chemically, the fast-water ecosystems resemble with cold, deep lakes. Water temperatures tend to be quite low, produc­tivity is also quite low, and diversity is high. In the fast-water eco­system, the main control of productivity is the current, which seriously limits the amount and type of autotrophy production that can take place.

Slowly flowing water:

A slowly flowing water eco­system is a very different type of system from the fast streams. Because the flow is both slower and more likely to be laminar, the erosive power of the stream is greatly reduced, and smaller sedi­ment particles (silt) and decaying organic debris, instead of being carried away by the stream, are deposited on the bottom.

In addi­tion to these the slow streams have a higher temperature, conse­quently, plank tonic organisms, especially protozoans, occur in large numbers in this ecosystem the detritus-feeding benthos of stow water ecosystems include those which either live on the bottom, such asisopods (sow bugs), mollusks (Sphaenius, Pisidiunu and Anodonta dominata), and mayfly and damselfly naiads, or which burrow into the sediment, such as tubeworms, naiads of the burrow­ing mayflies, Sialis (alderfly), midge Chironomus, and several other insect larvae, as well as, clams, nematodes, snails and rotifers.

Swimming organisms are also abundant, including not only fishes, such as carps, cat fishes, suckers, stingers, spoonbills, etc., which tend to be different species from those of fast-water areas, but also larger Crustacea such as amphipods (fresh-water shrimps).

Finally, several insects spend most of their time at the surface of the stream. These include forms such as water striders, water boatmen, back- swimmers, and predaceous diving beetles.

Zooplankton is abundant, including a rich assemblage of Protozoa and smaller Crustacea such as Cladocera (water-fleas) and copepods. The richness of wood afforded by slow streams attracts a large number of reptiles (water- snakes, crocodiles, turtles) and amphibians.

Plant life is also abundant in a slow-water ecosystem. It includes rooted vascular plants such as pond weeds and grasses firmly attached aquatic mosses and large multicellular filamentous algae. Motile algae such as diatoms and flagellates may abound in the open in all, the primary productivity of slow-water ecosystem is than that of the rapid-water ecosystem, and the community is relatively less dependent on food materials from outside.

Further, detritus food chain consisting of organisms such as bacteria t: fungi is much better developed in this community as decomposed organic debris that comprises the main food for these organisms accumulates in the mud bottom some slow-moving streams, in fact, the bottom muds contain more organic material than mineral fragments.

In slow-water streams oxygen concentration is main limiting factor. The high level of animal activity, coupled with an active detritus food chain, can withdraw a large amount of oxygen from water of a slowly moving stream.

In addition, the low level of tur­bulence means that less oxygen is incorporated into the water at the surface. Thus the dissolved oxygen content of a slowly moving stream is likely to be much lower than that of a fast-moving stream. This is commonly reflected in the fauna of the stream.

Fishes tole­rant of low oxygen levels such as carps and catfishes are the most common fishes in slow water, while species with high oxygen dem­ands, such as trout are found in fast water.