Rhithral

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The rhithral ( old Gr . Ῥεῖϑρον, Bach ) is a technical term in limnology and hydrology for the Bach habitat. It is divided into epi , meta and hyporhithral (upper, middle and lower stream section). The Krenal (source region) connects towards the source and the Potamal (river region) towards the mouth .

According to fishing law, rivers are divided into so-called fish regions according to the key fish species living there . The upper and lower trout region corresponds to the epi- and metarhithral, the grayling region to the hyporhithral .

Ecological conditions

The main characteristic of the rhithral as a habitat is the current . Streams tend to have a faster current than rivers or spring rivulets. Although there is a wide range of possibilities from slow-flowing lowland brooks to fast-flowing mountain brooks, only species with special adaptations to the current can live in the Rhithral. A community of fast flowing ( lotic ) and slow flowing ( lenitic ) zones can be distinguished. Organisms of the lotic zone in particular usually have special adaptations, e.g. B. Adhesion or holding devices, flow-adapted physique, etc.

Depending on the flow, different substrate types are formed in the rhithral . The transport force of flowing water increases with the flow velocity. A bed made of gravel and stones (flow speeds greater than 60 cm / s) is therefore typical for the fast-flowing mountain streams. In slow-flowing sections and behind flow obstacles, the smaller grains of sand can be deposited and form sandbanks. A sandy bottom is generally typical in slow-flowing lowland brooks. The rhithral is characterized by a high proportion of hard substrate-dwelling species ("lithal"), while soft substrate dwellers, i.e. colonists of mud or sand, are more common in rivers. In lowland brooks, almost all typical brook species are hard substrate colonists; In addition to gravel occurring here, they give way to v. a. looking for deadwood.

Due to the mostly narrow water level of the brooks, they are usually fully shaded by the trees on the banks . In the natural landscape, streams were always accompanied by an alluvial forest . In agricultural landscapes, the stream is often accompanied by a narrow gallery forest of alders and willows. Fully sunny streams do not correspond to the natural state and show a different community. The shading of the streams has serious consequences for living conditions. This way, streams usually stay cool all year round. The summer water temperature, which increases with increasing distance from the source, reaches around 10 ° C in the epi- and metarhithral and up to 15 ° C in the hyporhithral. In exposed streams or (here: naturally) in rivers, the water temperature rises to over 25 ° C. Due to the cool water, the streams have a consistently high oxygen content. In streams that are not organically polluted, it is always close to saturation and usually above 8 mg / l. Lower oxygen contents are also typical towards the source (in the krenal) because the groundwater is usually free of oxygen.

Another consequence of the shading is that there are hardly any taller aquatic plants in streams. Although some specialized plant species such as spring moss can occur in some types of streams, typical mountain streams in particular are completely free of aquatic plants . Single-cell algae occur as a coating on the stones (especially diatoms ). Red algae that live in fresh water , e.g. B. of the genera Lemanea and Hildenbrandia form coatings on stones. Phytoplankton, on the other hand, are usually meaningless. The main source of food for the stream fauna is therefore primarily materials that come into the stream from outside. The most important source of food is often the fallen leaves that fall from the bank trees into the stream. Typical food types for the Rhithral are therefore chopper (they break down the dead leaves) and pasture-goers (they graze the organic coating on the stone surfaces). Less common are z. B. filter feeders and detritus feeders.

The living conditions in the Rhithral are therefore quite extreme for some factors, e.g. B. flow, low temperatures, low productivity. Other factors, however, are conducive to colonization: high oxygen content, relatively constant living conditions. As a consequence, the rhithral shows a very characteristic and independent, species-rich community in some animal groups. Plants (because of the shade) and vertebrates (because of the lack of food), on the other hand, are represented rather poorly.

Cohabitation

In invertebrates, especially in some insect orders, the rhithral community is so rich in species and also so rich in specialized and characteristic species that only a small selection of typical forms can be named below. The most species-rich insect orders are the caddis flies (Trichoptera), some families of the two-winged flies (Diptera), the mayflies (Ephemeroptera) and the stone flies (Plecoptera). Other characteristic representatives are the dragonflies (Odonata) and the beetles (Coleoptera). If one looks at the biomass, there are also some species-poor groups, which can be very rich in individuals and can even surpass the much more species-rich insect larvae in terms of mass. In the first place this concerns the amphipods from the family Gammaridae. Large vortex worms (Turbellaria), the planarians, are also important. Less common than in rivers (but also with some characteristic species involved) are z. B. mussels and snails, annelids and leeches. The invertebrate community of the river bed (visible to the naked eye) is summarized as macrozoobenthos . Species with an emphasis on life in the free water body play no role at all in streams due to the current. Even the fish species mostly live on the bottom.

fishes

Compared to the Potamal, the Rhithral is rather poorer in fish species. The number of fish species therefore increases significantly with the size of the running water from the Epirhithral to the Hyporhithral.

  • Brown trout characteristic of the stream middle and upper courses (epi- and metarhithral), it occurs with the same frequency as far as the hyporhithral, ​​here accompanied by other species.
  • Grayling . Typical for stream underflows (hyporhithral)

These species serve to characterize the waters in so-called fish regions . Typical fish species of the Rhithral are also

Shellfish

Snails

Vortex worms

  • Triangular-head planaria ( Dugesia gonocephala )

Leeches

  • Large snail leech ( Glossiphonia complanata ), more often only in polluted streams

Crustaceans

Flea crabs (Gammaridae) are the most individual and biomass-rich rhithral colonists in many stream types. Since they can only tolerate moderate currents, they step back in very fast flowing mountain streams without being completely absent. Flea shrimp are absent from naturally acidic or acidic streams.

  • Brook flea shrimp ( Gammarus fossarum ). More in low mountain streams
  • Common flea shrimp ( Gammarus pulex ). More in lowland streams. more pollution tolerant.
  • Stone cancer . Southern Germany, in the low mountain range. Rare today due to the cancer plague

Mayflies

Dragonflies

Stoneflies

  • Protonemura . Species-rich genus, very sensitive to pollution. In unpolluted mountain streams regularly with three to four species
  • Nemoura . Species-rich genus with a pollution-tolerant species ( Nemoura cinerea )
  • Isoperla . One of the three common species in any unpolluted mountain stream
  • Perla marginata

Bed bugs

Mud flies

  • Sialis lutaria , only in slow flowing streams

Beetle

  • Hook beetle . Species-rich family, in almost every unpolluted stream with one or mostly several species. Very common: Elmis maugetii , Elmis aenea , Limnius perrisi , Limnius volckmari , Esolus parallelepipedus . Species of the genera Riolus and Oulimnius typical of lime streams with lime sinter precipitation . Imagines plastic breathers .
  • Hydraena ( long-button water beetle ). Species-rich genus. In mountain streams mostly with several species
  • Swimming beetles (Dytiscidae). Typical species e.g. B. Deronectes platynotus , Oreodytes sanmarkii , Agabus guttatus

Caddis flies

Due to the abundance of species of caddis flies in the Rhithral, ​​only selected ones can be named even from the most typical representatives.

  • Rhyacophila . Regularly three or four species in clean mountain streams, and one or two in lowland streams. robber
  • Glossosoma conformis
  • Hydropsyche . Regularly three or more species in streams. Most common species in low mountain streams: H. siltalai .
  • Drusus biguttatus . These and other species of the genus are typical in Epirhithral.
  • Anomalopterygella chauvinia , typical Hyporhithral species.
  • Chaetopteryx villosa , often found in all brook types, often the most common caddisfly species.
  • Potamophylax sp., V. a. P. cingulatus and P. luctuosus .
  • Silo . One of the three common species in any unpolluted stream.
  • Lepidostoma basale . Typical type of hyporhithral. Should prefer deadwood as a living space.
  • Sericostoma personatum . Preferably in sandbanks in the upper streams
  • Odontocerum albicorne . Not missing in any unpolluted mountain stream, but it is very sensitive to pollution.

Two-winged

From this very species-rich order, only a few families are regularly represented in the rhithral of unpolluted streams, but they are among the most frequent colonists. An excessively high proportion of Diptera in the community indicates brook degradation due to pollution or water development.

  • Mosquitoes . The family is widespread in all types of water with a barely manageable abundance of species. Many of the (difficult to identify) species are rhithral specialists. In the case of water pollution, there are sometimes mass occurrences of fewer species.
  • Black flies . Widespread in all rivers. Typical Rhithral species are the species of the genus Prosimulium and the subgenus Nevermannia of the very large genus Simulium, e.g. E.g .: Prosimulium latimucro , Prosimulium hirtipes , Simulium costatum
  • Wilts , especially in slow flowing streams with numerous species.

Birds

Typical of the Rhithral is mainly the dippers .

Community of the brook banks

Edge zones influenced by the running water are smaller in the Rhithral than in the Potamal, but are also part of the typical habitat here. The Bachaue is naturally characterized by alluvial forest . The alder-ash-brook floodplain forest (vegetation unit: Association Carici remotae-Fraxinetum in the plant-sociological system) is particularly typical for Bachauen Although the floodplain is connected to the stream and there are significant interactions, it is normally (as a terrestrial habitat) not counted as part of the actual rhithral itself. The actual bank area is narrow and is essentially formed by the zone of the seasonally fluctuating water level. In the case of fast-flowing streams with high transport power, more or less extensive gravel or sand banks can fall dry at low tide. These are much smaller than in the Potamal, but have an independent community in which predators dominate. Different species of ground beetle are common , especially those of the genus Bembidion . The shore bug Saldula c-album and the wolf spider Pirata knorri are also typical . A typical invertebrate colonist of the damp sandbanks is still the larva of the reticulated winged Osmylus fulvicephalus ( European brook-like ). The characteristic vertebrate species on the banks is the water shrew ( Neomys fodiens ).

Herbaceous bank plants and reed beds are more common on lowland streams. In the plant-sociological system, the reeds on streams are set up as independent vegetation units (societies of the Glycerio-Sparganion association). Typical types are, for example flood final swath ( Glyceria fluitans ), Blue Water speedwell ( Veronica anagallis-aquatica ), brooklime ( Veronica beccabunga ), Epilobium hirsutum ( Epilobium hirsutum ) and Bachberle ( Berula erecta ). Stream reeds are found preferentially on lowland streams. Butterbur ( Petasites hybridus and Petasites albus ), rarely also the ostrich fern ( Mattheucia struthiopteris ) and the winter horsetail ( Equisetum hyemale ) sometimes form typical populations on low mountain streams .

Brook typology

The life community of the rhithral is different depending on the characteristics of the individual body of water (factors flow velocity , substrate type , course development / curvature, shading, pH value , saprobic , etc.). Basic types are the mountain stream with a very steep gradient, shooting current and rocky bottom, the mountain stream with a gravel or block bottom and branched or slightly meandering channel and the lowland stream with slow current, substrate made of sand or organic material and mostly meandering course. The basic types are further differentiated into numerous individual types. Mountain streams are typically more species-rich than low-mountain streams, which in turn are more species-rich than lowland streams. This also applies to the typical and distinctive species: while mountain streams have numerous characteristic species, low mountain streams are mainly characterized by the lack of these specialists, while very few new species are added. Lowland streams have an even more impoverished fauna and virtually no independent species. In contrast to the other types of brooks, lowland brooks are characterized by the occurrence of unspecialized, ubiquistic freshwater species that are absent from the specialized cenosis of mountain brooks. There are also special cases: streams that are characterized by special factors and therefore do not fit into the general typology, e.g. B. Bog brooks (called as a type "organic brook"), naturally dry-falling brooks, brooks influenced by brackish water and a few others. Details on the river typology can be found in the article River type .

The community of the Rhithral is different depending on the type of stream. In addition, there is a broad base of rhithral species that are equally widespread in virtually all stream types.

Stream pollution and stream degradation

Streams as ecosystems are naturally closely related to their catchment area . In a landscape that is populated and dominated by humans, the resulting changes naturally affect the structure and community of the Rhithral. For decades, the most important influencing factor has been pollution with easily organically degradable substances, which has been caused by domestic sewage and faeces since the introduction of the alluvial sewer system in the 19th century. The influences are measured with the saprobic system , in which the community of the water serves as a measuring instrument for the pollution. Therefore, the effects of this factor on the community of the Rhithral are very well known.

Although this stress factor has by no means become insignificant, the conditions have noticeably improved thanks to the construction of better sewage treatment plants since a low point at the beginning of the 1970s. Diffuse pollution is important, especially from agriculture, from nutrients, pesticides or eroded topsoil. In addition, the effect of the morphological degradation has proven to be significant. This summarizes the consequences of river expansion and straightening. The natural community of the Rhithral is therefore essentially only found in streams that have a predominantly forested catchment area. Since the pollution increases further downstream, the Rhithral is comparatively much better preserved than the Potamal. Especially in the low mountain ranges there are still numerous very little or no impaired streams.

literature

  • The importance of rivers in the ecosystem. In: Ulrich Dumont: Handbook of transverse structures. Water management. Published by Harald Friedrich . Ministry for the Environment and Nature Conservation, Agriculture and Consumer Protection, Düsseldorf 2005, ISBN 3-9810063-2-1 , Chapter 2 (PDF; 3.16 MB).
  • Josef Blab: Basics of biotope protection for animals. (= Series of publications for landscape management and nature conservation. 24). 2nd Edition. extended revision. Kilda, Bonn et al. 1986.
  • Peter Haase: Zoo zoenoses, chemistry and structure of regional brook types in the Lower Saxon and North Hessian highlands. (= Ecology and environmental protection. 18). Dissertation. University of Kassel. Comprehensive University of Kassel - Department of Ecology and Nature Conservation, Witzenhausen 1999.
  • Leonie Jedicke, Eckhard Jedicke: color atlas landscapes and biotopes of Germany. Ulmer, Stuttgart 1992, ISBN 3-8001-3320-2 .
  • Reference waters of the river types in North Rhine-Westphalia. Part 1: Tobias Timm and others: Small to medium-sized rivers. (= LUA leaflet. 16). State Environment Agency North Rhine-Westphalia, Essen 1999. ( online ; PDF; 23.81 MB).
  • Jürgen Schwoerbel : Introduction to Limnology. (= UTB for science. Uni pocket books. 31). 6th, revised edition. Fischer, Stuttgart et al. 1987, ISBN 3-437-20395-9 .
  • Dietrich Uhlmann, Wolfgang Horn: Hydrobiology of inland waters. A floor plan for engineers and scientists. (= UTB. 2206). Ulmer, Stuttgart 2001, ISBN 3-8001-2757-1 .

Individual evidence

  1. ↑ floraweb.de
  2. ↑ floraweb.de