Glossary Description
Age class distributionThe age class distribution of the different fish species represents an important biological quality element to judge the population structure according to the FIA. It can be classified as
  1. all age classes represented, natural age class distribution, juvenile fish dominating
  2. all age classes represented, but juvenile fish clearly underrepresented, and adult fish overrepresented
  3. lack of some age classes, disturbed age class distribution e.g. only juveniles or only adults, subadults lacking
  4. strongly disturbed age class distribution, only low density of fish, single fish of different sizes
  5. No fish
For more information see Haunschmid et al. (2010).
Allen curveThe 'Allen' curve can be seen as a generalization of natural length frequency distributions of fish populations, starting with high numbers of larvae and juveniles, and ending up with only single very large individuals. It can be described as a negative expontential function of the number of individuals per length class.
AlgaeAlgae are a very large and diverse group of simple, typically autotrophic organisms that, besides fish, benthic invertebrates and aquatic macrophytes can be used to judge the ecological status according to the EU-WFD.
Aquatic macrophytesA macrophyte is an aquatic plant that grows in or near water and is either emergent, submergent, or floating. In lakes macrophytes are a central element of nutrient cycling, provide cover for fish and substrate for aquatic invertebrates, produce oxygen, and act as food for some fish and wildlife. Macrophytes also are used to judge the ecological status of water bodies according to the EU-WFD.
Biological element (EU-WFD definition)A collective term for a particular characteristic group of animals or plants present in an aquatic ecosystem (for example phytoplankton; benthic invertebrates; phytobenthos; macrophytes; macroalgae; angiosperms; fish).
Biological indicator (EU-WFD definition)An indicator is parameter that can be monitored to estimate the value of a biological quality element. Indicators may include the presence or absence of a particularly sensitive species (Environmental Agency: Water for life and livelihoods - River Basin Management Plan glossary).
Biological quality element (EU-WFD definition)A characteristic or property of a biological element that is specifically listed in Annex V of the Water Framework Directive for the definition of the ecological status of a water body (for example composition of invertebrates; abundance of angiosperms; age structure of fish) (Environmental Agency: Water for life and livelihoods - River Basin Management Plan glossary). The Austrian guidelines for the assessment of the biologocial quality elements according to the EU-WFD can be found here
BiomassThe amount of living matter in a given habitat, expressed either as the weight of organisms per unit area or as the volume of organisms per unit volume of habitat. Biomass (below 8-40 kg/ha, depending on the geomorphological setting, acts as KO criterion in the FIA) (Haunschmid et al., 2010).
Causal relationA causal relation is a relationship between an event (the cause) and a second event (the effect), where the second event is understood as a consequence of the first. Commonly causality is also understood as the relationship between a set of factors (causes) and a phenomenon (the effect).
ChannelizationChannelization can be described as a process that includes ‘all processes of river engineering for the purposes of flood control, drainage improvement, maintenance of navigation, reduction of bank erosion or relocation for highway construction’. Channelization historically was extensively applied to increase arable land by reducing flooding and draining land by river straightening and bank stabilization, often constructed as uniform rip-rap structure (Brookes, 1988).
Connectivity interruptionWeirs and dams built by humans in rivers represent artificial physical obstacles to e.g. migrating fish. Rivers become fragmented and habitat availability for different life stages of fish is reduced, leading to reduced genetic exchange between sub-populations and potential local extinction of species.
DensityThe number of individuals per unit area.
DischargeThe discharge is the volume of water transported by a river through a specific cross section in a certain amount of time. The unit used to express discharge is m³/s.
Dominant speciesMost abundant species inhabiting a specific river section.
Ecological status (EU-WFD definition)Ecological status applies to surface water bodies and is based on the following quality elements: biological quality, general chemical and physico-chemical quality, water quality with respect to specific pollutants (synthetic and non synthetic), and hydromorphological quality. There are five classes of ecological status (high, good, moderate, poor or bad). Ecological status and chemical status together define the overall surface water status of a water body. (Environmental Agency: Water for life and livelihoods - River Basin Management Plan glossary).
EU Water Framework Directive (EU-WFD)The Water Framework Directive (Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy) is a European Union directive which commits European Union member states to achieve good qualitative and quantitative status of all water bodies (including groundwater and marine waters up to one nautical mile from shore) by 2015. The ecological status of surface waters are assessed according to biological quality (fish, benthic invertebrates, algae and aquatic macrophytes) and the chemical status according to environmental quality standards for specific pollutants. The WFD explicitly enforces a catchment based management approach.
FishThe term fish is used to refer to any aquatic vertebrate that has a skin covered with scales, two sets of paired fins, some unpaired fins, and a set of gills. During evolution, fish have adapted to specific environmental conditions in rivers, including physical habitats available in different river types, seasonal discharge, temperature and flood regimes etc. That`s why the deviation from the natural expected fish community (reference fish community) can be used to judge human alterations of river ecosystems. Fish, as benthic invertebrates, algae and aquatic macrophytes are a central quality element to judge the ecological status of water bodies according to the EU-WFD.
Fish communityA fish community is an assemblage of two or more fish populations of different species occupying the same geographical area, e.g. a specific river section.
Fish Index Austria (FIA)The Austrian tool to estimate the ecological status with regard to the EU-WFD based on fish (Haunschmid et al., 2006). The FIA analyzes the deviation of the actual fish assemblage from historically and actually pre-defined
reference fish assemblages according to river types using nine metrics: percentage of dominant species, percentage of subdominant species, percentage of rare species, presence of reproductive guilds, presence
of flow preference guilds, FRI, biomass, population structure of dominant species and subdominant species. The result is a decimal number between 1 (high), 2 (good), 3 (moderate), 4 (poor), and 5 (bad). Biomass is defined as an additional knock-out criterion and leads to a status class of four or five if below 50 or below 25 kg/ha respectively. The same applies to significant deviation of the fish region index regarding reference values. Deviations larger than 0.6, 0.9 or 1.2 lead to total status classifications of 3, 4 or 5 respectively.
Fish Region Index (FRI)The FRI is based on the probability of occurrence of fish species in a given river region. From Epirhithral to Metapotamal for each individual fish species a FRI between 3 to 7 (from up to downstream) has been defined (Haunschmid et al., 2010). The FRI of a specific river section is then calculated by the density-weigthed FRI's of each individual species characterizing the species associations of specific river sections. The FRI is used to describe changes in a fish community compared to reference longitudinal fish species compositions. Reference values for different fish regions are 3.8 for epirhithral, 4-4.5 for metarhithral, 4.9-5.2 for hyporhithral, 5.7-6.3 for epipotamal, 6.4 for metapotamal (Haunschmid et al., 2010).
Fish regions"The distribution of organisms, resources, and biological processes change along rivers and depend on large-scale processes (e.g. climatic, hydrological, geomorphological) as well as local ones (e.g. biotic). The first attempt to categorize such discontinuities is the Stream Zonation Concept (Illies & Botosaneanu 1963), which defined a series of distinct communities along rivers, separated by major faunal transition zones (e.g. the rhithral–potamal transition)" (Maiolini and Bruno, 2007). These stream zonations can also be called river regions (Epirhithral=upper trout region, Metarhithral=lower trout region, Hyporhithral=grayling region, Epipotamal=barbel region, Metapotamal=bream region, Hypopotamal=brackish water region, flounder region, Schmutz et al., 2000b). For the FIA these generic fish regions have been further specified to better meet differences in species composition and productivity (Haunschmid et al., 2010).
Flow regimeDepending on the climate region, its situation in the river network and the geomorphological setting, a river section can be characterized by a specific annual flow regime with seasonally chaning river discharges.
GuildA guild (or ecological guild) is a group of species exploiting the same resources. E.g. migratory guilds exploit the same range of open continua in rivers. Zauner and Eberstaller (1999) classified the Austrian fish fauna into guilds according to ecological requirements. (habitat structure, water velocity, and spawning requirements). In th FIA, presence and absence of reproductive guilds and flow preference guilds are used as biological quality elements.
Habitat A habitat is the physical environment that is inhabited by a particular species or group of species.
Habitat qualityHabitats can have different suitability for different species, which is often expressed as habitat quality.
HydropeakingHydropeaking is a specific hydrological alteration characterized by strong daily oscillations of discharge; the change between minimum and maximum discharge occurs extremely quickly, typically in 15’-30’. Hydropeaking is created by hydropower plants, which concentrate the electricity production during the time, when the energy price is at the highest (usually during specific intervals during the day), and store water in those hours where the price is low (usually at night). The impact on the aquatic biota, unable to adapt to such quick and repeated variations, is usually dramatic. Hydropeaking impacts are mainly (but not only) related to the stream reach downstream of the release. Hydropeaking affects:
o Biological elements (all biological elements of the river bed and of riparian areas)
o Hydromorphological elements (flow and morphology alteration)
o Water quality (changes of temperature and chemical characteristics and increase of suspended solids)
Hydromorphological pressuresHydromorphological pressures like connectivity interruptions, impoundment, water abstraction, hydropeaking and impaired river morphology represent the major human impacts on Austrian rivers according to the EU-WFD assessment (BMLFUW, 2005). Many river sections are subjected to multiple pressures, complicating the assessment of the contribution of each individual pressure type to the degradation of the ecological status.
IHGInstitut für Hydrobiologie und Gewässermanagement der Universität für Bodenkultur BOKU Wien (Institute of Hydrobiology and Aquatic Ecosystem Management at University of Natural Resources and Life Sciences BOKU, Vienna).
ImpoundmentAn impoundment is the river section above a dam with decreased flow velocities, an increase of water depth, increased sedimentation and lack of sediment in the river stretch below the dam. Due to the accumulation of fine sediments in an impoundment, from time to time sediment flushing is required to maintain the water storage capacity. The flushing of large amounts of fine sediments within a short time period is known to have significant negative effects on aquatic life below dams.
IndifferentAn indifferent species has no specific requirements with regard to environmental factors like flow velocity etc.
Macrozoobenthos (MZB)Macrozoobenthos (MZB) is another term for benthic invertebrates, inhabiting the sediment at the river bottom The presence and abundance of macroinvertebrates which are very sensitive to
pollution can be used as an indicator for the ecological status of a river.
Natural riverA river is a body of inland water flowing for the most part on the surface of the land but which may flow underground for part of its course. Upland rivers are generally fast flowing and lowland rivers are generally slow flowing and meandering. Natural rivers are part of the hydrological cycle, with water in a river generally received from precipitation through a catchment from surface runoff and other sources such as groundwater, springs, and the release of stored water in natural ice and snow (e.g. from glaciers). Without human intervention, depending on the climate region, its situation in the river network and the geomorphological setting, rivers naturally develop different morphological forms being dynamically maintained over time ('dynamic equilibrium'). This allows specific fish communities to adopt to these conditions and build up sustainable populations.
Number of speciesDepending on the climate region, its situation in the river network and the gemorphological setting, a river section can be inhabited by a different number of species being adapted to the natural river dynamics being characteristic for the specific river type.
PressureHuman activities exert 'pressures' on the environment, as a result of production or consumption processes, which can be divided into three main types:
  1. excessive use of environmental resources
  2. changes in land use
  3. emissions (of chemicals, waste, radiation, noise) to air, water and soil
(Kristensen, 2004).
Rare speciesSpecies only rarely occurring in a river.
Rehabilitation‘Rehabilitation’ (in contrast to ‘restoration’) is understood as an action to move significantly towards a previous natural condition, but without the implication of perfection of fully restoring the original situation.
RestorationThe term ‘restoration’ (in contrast to ‘rehabilitation’) is usually used to describe a complete return to an original state of the environment with the implication of returning to a perfect natural state.
Rheophilous speciesA rheophilous species prefers higher flow velocities, and usually reacts sensitive to a reduction of flow velocity in impoundments or by water abstraction.
River sectionA section of a river within a specific fish region.
River type Depending on the climate region, its situation in the river network and the geomorphological setting, rivers naturally develop different morphological forms (straight, meandering, braiding etc.) with specific hydrological and hydraulic characteristicts, so called natural river types.
River type specific speciesSpecies adapted to the specific environmental conditions in a specific river type. In the FIA species are divided into dominating species, typical accompanying (subdominant) species and rare species, and the deviation from a river type specific fish community is judged (Haunschmid et al., 2010).
River type specific species compositionDifferent river types are naturally inhabited by river type specific biological communities.
Sediment flushingDue to the accumulation of fine sediments in impoundments upstream of weirs and dams so called ‘sediment flushing’ is needed to maintain the water storage capacity. During these events high amounts of fine sediments are flushed downstream with potential severe effects on the fish community below the weirs and dams.
SpeciesA species is usually defined as a group of organisms able to interbreed and reproduce fertile offspring.
Subdominant (typical accompanying) speciesSpecies occurring at medium abundance in a specific river section.
Typical speciesRefers to characteristic dominant river type specific species of a specific river section.
Water abstractionAt dams in rivers water might be abstracted for hydropower production or irrigation. The river section below the dam receives a significantly reduced amount of water, leading to the reduction of flow velocities, river width, water depths and consequently to a loss of available habitat for river type specific biota in this river section.

    More relevant definitions related to application of the EU-WFD in Austria can be found QZV Ökologie OG, BGBl II 99/2010.