Streams and rivers have a longitudinal zonation or profile (Fig. 4.12) instead of horizontal in lakes and ponds. lilies and Botosaneanu (1963) have recognized two major subdivisions of a river course: the steep and torrential upper course (“rithron “) and the flat, slow-flowing lower course (“potamon “).
However, a true rhithron having water temperature below 20°C, may be often absent in the tropical rivers. By contrast, temperate rivers are characterised by relatively lengthy rhithron zones.
Rhithron:
A rhithron is characterized by the presence of:
(i) Steep, narrow and shallow riffles or rapids, and
(ii) Flatter, wider and deeper reaches, termed pools. Intermediate areas of moderate current often found in larger streams and rivers are termed runs. Riffles have high, turbulent flow, coarse bottom of boulders, rocks or pebbles and limited attached vegetation. Pools have lower flow, bottom of somewhat finer material and some rooted vegetation. Low temperatures and high values of dissolved oxygen are important characteristics of the rhithron zone.
Communities of Rhithron:
Biotic communities of the rhithron zone consist of plankton, periphyton, “aufwuchs”, nekton and a variety of benthos or bottom-dwelling forms. During periods of spate there is no phyto – or zooplankton, although during low water transient blooms may occur as flow drops and pools become isolated.
In rhithron zone of relatively undisturbed stream, the main primary producers are the attached algae and diatoms (e.g. filamentous green algae like Ulothrix and Cladophora, and diatoms like Nizschia and Gomphonema). Those organisms that attach firmly to a substrate (submerged rocks, plants, debris) but do not penetrate it (in contrast to plants rooted in the bottom or certain parasites) are collectively called “aufwuchs “(German word proposed by Ruttner, 1953).
They comprise all attached organisms except the macrophytes in contrast to the English equivalent “periphyton” (meaning “on plants”) which includes the plants and animals adhering to parts of rooted aquatic plants. The diatoms and small algae are sometimes referred to as microphytes. Besides, in sunny areas productive streams may have luxuriant growth of algae like Cladophora which is a mesophyte (intermediate between microphytes and higher plants or macrophytes).
The zooplankton of the rhithron zone mainly consists of ciliate protozoans (e.g. Vorticella), rotifers (e.g. Asplancha, Brachionus, Keratella, Philodina), and crustaceans (e.g., Daphnia, Bosmina, Cyclops, Diaptomus, Gammarus pulex). There are also many insect species adapted to the on the stream bottom where they shelter among or cling to the rocks.
Commonly encountered insects in streams are:
(a) The mayflies Ephemeroptera (e.g., Baetis rhodani, Rhithrogena, Ecdyonurus, Ephemerella, Caenis, Ameletus, Epeorus).
(b) The stoneflies Plecoptera (e.g.,Nemoura, Protonemura, Perlodes, Diura, Arcynopteryx, Perla, Isoperla, Leuctra, Brachypterd).
(c) The caddisflies Trichoptera (e.g. Hydropsyche, Rhyacophila, Philopotamus, Drusus, Limnephilus, Agraylea). _
(d) The two-winged flies Diptera (e.g., Simulium (blackfly) Chironomus riparius, (bloodworms), Antocha, Liponeura).
(e) The dragonflies Odonata (e.g.,Libellula).
Some worms (e.g., flatworm Dugesia, roundworm Dolichodorus, annelid worm Tubifex), leeches (Haemopis, Glossiphonia, Erpobdella), molluscus (e.g. Bithynia, Dreissena) and crustacea (e.g., Gammarus) may also occur among the benthic forms.
The nekton of the rhithron zone consists of water bugs Hemiptera (e.g., Notonecta, Micronecta, Gerris, Corixa), beetles Coleoptera (e.g., Dytiscus) and fish. The resident fish species in rhithron zones are entirely rheophilic and belong to two main groups. Firstly, there are those species, which live among rocks of the bottom and are found mainly in the riffles. These are of small size and are adapted to grip or cling to the substrate (e.g., Chiloglanis, Astroblephus, Glyptothorax, Pseudecheneis). Other species such as Mastacembelus have pointed snout and eel-like shape that enable them to get into the holes in the rocky bottom.
The second group of fish is those species, which are adapted to swim fast as to resist the current, and some of them even move against it (e.g., Barbus, Salmo, Barilius, Schizothorax, Tor). Because of the severity of the habitat, the diversity of resident species tends to be low.
Adaptations of Rhithron Community:
Most animals living in the rhithron zones of streams are easily recognizable from their adaptations for living in swift current (Fig. 4.13).
Some important adaptations are as follows:
(i) Streamlined bodies:
Almost all stream animals possess more or less egg-shaped body which offers minimum resistance to water flowing over it. Best examples of hillstream fish with streamlined body are trout (Salmo), Barilius and Schizothorax species.
(ii) Flattened Bodies:
Many stream animals possess flattened bodies, which enable them to find shelter under stones. Some Ephemeroptera like Epeorus, Rhithrogena, and fishes such as Noemacheilus (Cobitidae) and Pseudecheneis (Sisoridae) are the best examples.
(iii) Hooks and Suckers:
These enable riffle animals to grip smooth surfaces. For example, Bibicocephala and Liponeura have a row of ventral suckers.
(iv) Adhesive Apparatus:
Some hill stream fishes, e.g., Glyptothorax, Garra, and Pseudecheneis possess a well developed adhesive apparatus on the ventral surface.
(v) Sticky Undersurfaces:
Some snails and flatworms are able to adhere to the substrate by their sticky undersurfaces.
(vi) Positive Rheotaxis:
Stream animals orient themselves upstream and, if capable of swimming movements, move against the current (rhieo, current; taxis, arrangement). This is an inherent behaviour pattern and is just as important an adaptation as the morphological features given above.
(vii) Positive Thigmotaxis:
Many stream animals have an inherent behaviour pattern to cling close to a surface or to keep the body in close contact with the substratum (thigmo, touch or contact). Almost all the larvae of mayflies and stoneflies and many fishes exhibit this type of behaviour.
(viii) Permanent Attachment:
Some organisms of the lotic environment remain permanently attached to a firm substrate such as a stone, log, or leaf mass. Green algae Cladophora, encrusting diatoms such as Nitzschia, Gomphonema, Melosira and some aquatic mosses (Fontinalis) are good examples. Such organisms are known as “aufwuchs”.
Potamon:
The potamon zone of a river is the flat, slow-flowing lower course. Zonation within the potamon is both longitudinal and lateral (Fig. 4.12). Longitudinally, there is a repetition of differing habitats associated with the meanders of the channel. Laterally, there is the distinction between the main channel and its floodplain. The floodplain is normally an area of relatively flat land flanking the main channel.
The channel at each band is deeper by the outer, concave bank where the current is fastest, whereas the inner, convex bank consists of a sandy or muddy point bar. The plain itself contains many types of water body, some of which retain water throughout the inter-flood period. Because of deposition of silt, such features show a succession from open lagoon, through vegetation- lined pools and heavily vegetated swamps to dry land.
In short, the potamon zone is environmentally more complex and differs from the rhithron zone in the following features:
(i) Geomorphology of the channel
(ii) Physico- chemical qualities of the water (Hi) Nature of substratum
(iv) Biota; including floating, emergent and submerged vegetation
(v) High bacterial density
Communities of Potamon:
The above mentioned physico-chemical, geologic and biotic features have a great bearing on the communities of the potamon zone. The community consists of the populations of the following groups:
(i) Plankton:
The occurrence of plankton is closely related to the flow conditions. During the floods planktonic organisms may be present but are rare, whereas during the dry seasons algal blooms may develop within the lentic waters of the plain and also in the main channel. In short rivers these are generally confined to backwaters. In longer rivers the time taken for individual masses of water to travel downstream is sufficient to allow the development of plankton. In rivers whose flow has been slowed by other hydraulic work, plankton also develops to a greater degree. However, the contribution of the free plankton to the primary production is slight.
(a) Algae and Diatoms:
Algal species may be categorized into three groups on the basis of their sensitivity (Bilgrami and Datta Munshi, 1985). The first group comprises such algae which exhibit maximum pollution tolerance and can be used as pollution indicators. They include Oscillatoria limosa, Microcystis aeruginosa, chlorella vulgaris, Stigeoclonium tenue, Ankistrodesmus falcatus, Scenedesmus quadricauda, Synedra ulna, Navicula viridula, Euglena viridis, Phacus caudatus and P. viridis and are usually confined to the sites of urban and industrial discharge.
The second group includes forms recorded from clean as well as moderately polluted zones. Important of them were Oscillatoria princeps, Phormidium uncinatum, Anabaena circularis, Closterium acerosum, Pediastrum duplex, Scenedesmus dimorphus, Gomphonema parvulum, Cymbella turgida, Navicula cuspidata and Synedra acus. The third group includes highly sensitive clean water forms such as Merismopedia glauca, Oscillatoria subbrevis, Phormidium calcicola, Pediastrum simplex, Melosira ambiguans, and Gomphonmea species.
(b) Zooplankton:
Zooplankton of the potamon zone consists of members of protozoa, rotifera, cladocera and copepoda. Forms like Filinia longiseta, Brachionus angularis, B .calyciflorus, B. forficula, B. quadridentata, Keratella tropica and K.cochlearis may be present in clean as well as moderately polluted waters. But forms like Brachionus rubens, Platiyas polyacanthus and Rotatoria rotatoria are characteristic of polluted parts of the potamon zone.
(ii) Nekton:
Some insects, beetles, and fish dominate the nekton of potamon zone, although forms which are not true nekton such as Kachuga kachuga, Trionyx gangeticus, Lissemys punctata, Gavialis gangeticus, Platanista gangetica have been also reported from the Ganga and other rivers.
Two main groups represent fish communities of the potamon zone. Firstly, there are fish species adapted to resisting low dissolved oxygen concentration. Their adaptations may be in the form of auxiliary respiratory organs for using atmospheric oxygen as in Channa, Clarias, Erythrinus and Notopterus, or may be physiological as with Carassius, or even behavioural as with many cyprinodonts. The same species often have a capacity to tolerate high temperatures. The other group of fish includes the species which usually live in the main channel and are fast swimmers such as Mystus (Aorichthys) species, Labeo species, Catla catla and Cirrhinus mrigala.
(iii) Benthos:
The benthos of the potamon is relatively poor as unstable mud bottoms, heavy siltation and seasonal desiccation do not favour well settled bottom living organisms. The benthic forms may include some molluscs (e.g. Lymnaea, Bithynia, Corbiula), chironomids, ephemerptera, plecoptera, trichoptera and worms and leeches.
(iv) Macrophytes:
The presence of macrophytes is a characteristic feature of the potamon zone. Most aquatic macrophytes are angiosperms (flowering plants), but aquatic ferns, mosses, liverworts, and even the large algae of the Charophyceae may be present.
The free-floating macrophytes such as Salvinia (water fern), Pistia (water lettuce), Eichhornia crassipes (water cabbage) may be quite common. In temperate and subtropical parts much smaller free-floating plants such as the duckweed Lemna may be present in backwaters. Rooted macrophytes may have all or part of their vegetative and sexually reproductive parts above the water or may be completely submerged.
Some examples of rooted macrophytes of streams are Nymphaea (waterlily), Potamogeton (pond weed), Ceratophyllum (coontail), Myriophyllum (milfoil) and the water crowfoot (Ranunculus). The eel grass (Zostera) and widgeon grass (Ruppia) are the common rooted macrophytes in estuaries.