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Fluvial systems provide natural resources (e.g., fish and clean water) as well as
cultural and ecological services (e.g., transportation, energy, irrigation, recreation
and waste assimilation) basic to human societies (Naiman et al., 2002). At the start
of the century, large dams contributed to 20% of the world’s electricity supply
and irrigation agriculture produced 40% of the world’s food (Gleick, 1998). This
usage of fluvial natural resources has translated in the lost of more than 40% of
their biodiversity, which will largely compromise their natural functioning (Naiman
et al., 2007). In fact, water shortage and losses of freshwater ecosystem services
may reach to 40% of the world’s population by 2050, given the actual predictions
under Global Climatic Change scenarios (Millenium Ecosystem Assessment, 2005).
Sustaining or restoring the natural functioning of water-dependent ecosystems is
crucial for human being welfare and, in the face of continuing growth of human population
and water demands, constitutes a delicate task for water managers, planners, developers
and decision makers (Postel and Ritcher, 2003). Moreover, fluvial systems are the
arena for much conflict of interests. In Europe, for example, hydropower is being
fostered as renewable energy generation (Ringel, 2006), while many river habitats
and species are protected in the Nature 2000 Network under the Habitats Directive
(CE/1992/43). Moreover, the Water Framework Directive (WFD; CE/2000/60) requires
to all European members that all water bodies must reach at least their best ecological
potential by 2015 (Achleitner et al., 2005), while the Floods Directive (CE/2007/60)
obliges to have a flood risk management plan by 2015 to all state members. Thus,
in the coming years it is expected an increase in the number of river restoration
and river engineering works in order to account for all directives, although the
outcomes of such fluvial engineering works are difficult to predict and they might
follow undesirable directions damaging permanently the fluvial habitat or fluvial
processess that generate and maintain it. Fluvial habitat maintainance is central
to reach the environmental objectives of both HD and WFD.
The HD requires to determine the conservation status of habitats included in Annex
I of the Directive and of those habitats in which species included in Annex II of
the Directive dwell. This entails that, first, river habitats must be defined, second,
their characteristics must be assessed and, finally, contrasted to some reference
condition. On the other hand, the WFD demands the determination of river hydromorphological
quality, which implies to determine hydrologic, hydraulic and morphological river
conditions. Therefore, both HD and WFD need to take into account physical river
habitat characteristics in order to establish “habitat conservation status” and
“hydromorphological quality”. The determination of the most relevant physical habitat
characteristics and of the fluvial processes that generate and maintain them are,
thus, crucial for the future development of river habitat assessment tools.
In an ecological sense, habitat is understood as an area that contains the necessary
resources and conditions for a population to persist (Ricklefs & Miller, 2000),
in fact, a habitat is “unique” for each organism. For example, a riffle section
might be a heterogeneous habitat for a mayfly grazing on diatoms but a homogeneous
habitat for a trout feeding on invertebrates. Therefore, “habitat” definition requires
the existence of a biotic element and must be done in terms of those characteristics
that are relevant to the organism concerned (Begon et. al, 1996). Nevertheless,
“Habitats” can be classified in terms that apply to all organisms or a subset of
them. For example, lotic and lentic habitats are relevant to filamentous algae,
macrophytes, invertebrates and fishes.
River habitats have been defined as the local physical, chemical and biological
features that provide an environment for instream biota (Jowett, 1997). River physical
habitat can be understood as the result of the interaction between discharge regime
and the structural and hydraulic components of the river channel, which are organised
as a dynamic mosaic (Maddock, 1999). While the chemical and biological aspects of
river habitats count with a well established evaluation methodology, physical habitat
assessment is far less developed (Maddock, 1999). Moreover, defining and characterising
river habitats is somewhat difficult, as rivers are highly complex structured ecosystems,
which integrate processes occurring at different spatial and temporal scales.
Despite the complexity of assessing river habitat, a wide array of methodologies
have been developed everywhere, covering from the basin scale (e.g., Rosgen, 1996)
to the microhabitat (e.g., PHABSIM: Bovee, 1996). However, there is a lack of standardisation
on physical habitat assessment protocols, and different characteristics are being
assessed depending on the scale and objective of the study. Important questions
that deserve further investigation to improve river habitat assessment are:
• The possibility of reflecting physical habitat seasonal variation in relation
to discharge
• The need of river habitat assessment across a range of spatial scales
• The establishment of physical habitat reference conditions for different river
types
• The need to link population dynamics, biological community composition and structure
and/or ecosystem processes to relevant river habitat attributes
References
Begon, M., J. L. Harper, and C. R. Townsend. 1996. Ecology: Individuals, Populations
and Communities. Blacwell Science, Oxford.
Bovee, K.D. 1996. Perspectives on Two-Dimensional River Habitat Models: the PHABSIM
Experience. Quebec, Canada: INRS-EAU. B149- p.
Jowett, I. 1997. Instream flow methods: a comparison of approaches. Regulated Rivers:
Research & Management 13:115-127.
Maddock, I. 1999. The importance of physical habitat assessment for evaluating river
health. Freshwater Biology 41:373-391.
Oliveira, S. V., and R. M. V. Cortes. 2005. A biologically relevant habitat condition
index for streams in northern Portugal. Aquatic Conservation: Marine and Freshwater
Ecosystems 15:189-210.
Ricklefs, R. E., and G. L. Miller. 2000. Ecology, 4th edition. W.H. Freeman and
Company, New York.
Rosgen, D. L. 1996. Applied river morphology. Wildland Hydrology, Pagosa springs,
CO.
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