Résumés
Résumé
La dynamique écologique des cours d'eau est fortement liée à leur régime hydrologique qui en façonne la structure morphologique à l'occasion de crues caractéristiques mais qui en rythme également l'habitat physique quotidien. L'hydrologie joue aussi un rôle majeur dans la modulation saisonnière des caractéristiques thermiques et chimiques, autres paramètres abiotiques fondamentaux.
Deux exemples concrets pris dans un cours d'eau alpin, la Séveraisse, démontrent l'intérêt d'une étroite connexion entre l'hydrologie et l'hydroécologie.
La quantification de l'habitat des poissons repose sur la méthode des microhabitats dont les principes ont été définis par l'US Fish and Wildlife Service (Instream Flow Incremental Melhodology, USA, Fort Collins, Colorado). Cette méthodologie combine une description détaillée de stations représentatives du cours d'eau par mesure puis modélisation hydraulique des paramètres de l'habitat tels que la hauteur d'eau, la vitesse du courant et le substrat avec des modèles de préférence de localisation des poissons à différents stades vitaux. Des valeurs d'habitat potentiel par stade vital pour les différentes espèces étudiées (ici la truite Fano, Salmo frutta fario) sont calculées pour différents débits.
Des chroniques hydrologiques de qualité sont par conséquent nécessaires pour analyser la dynamique de l'habitat et en reconstituer l'historique.
Les faibles débits lors des étiages estivaux correspondent souvent à un habitat limitant pour l'adulte de truite.
D'autres épisodes hydrauliques critiques peuvent aussi exister. Ce fut le cas d'une crue sévère de 7D m3/s sur la Séveraisse dont l'effet biologique a pu être évalué : iode diminution de la densité et de la biomasse de truite, structure de taille désorganisée.
Ces deux exemples soulignent l'importance des données hydrologiques en hydroécologie. Les évènements exceptionnels, les épisodes structurants, les étiages, le rythme saisonnier des débits définissent la variabilité spatiale et temporelle de l'environnement aquatique, qu'il faut ensuite relier à la dynamique biologique des hydrosystèmes.
Mots-clés:
- Hydrologie,
- géomorphologie fluviale,
- modélisation,
- habitats aquatiques,
- poissons
Abstract
Hydrobiology studies deal more often with pollution problems in running waters than with those concerning stream regulation or deterioration of physical integrity of streams and rivers.
It is more and more obvious that running water ecology is strongly related to hydrological characteristics which structure aquatic habitats.
This latter challenge requieres a better understanding of the relationships between physical and biological parameters in particular in natural sites.
We have summarized basic knowledge on the role of hydrological characteristics in aquatic systems :
- Hydrology structures the morphology of the river bed and banks : bankfull discharge is often considered as responsible for hydraulic geometry of river channels in association with sediment load. The channel forming flows correspond to the flood discharges of frequency 1.5 to 2.0 years in most stream types.
- Hydrology punctuates the dynamic of habitat available for aquatic biota. Physical habitat is discribed as a combination of spatial attributes : depth, current velocity, which are directly dependent on instantaneous discharge, and substrate and cover, resulting from the morphology discribed above. For example fish need different habitat for each physiological function to achieve a whole life cycle : reproduction and incubation of embryos, nutrition, test, and hiding. For the biologists, it is essential to develop knowledge on the autoecological requirements of species, in particular fishes in original ecological conditions.
- Hydrology influences the thermal regime determining the oxygen concentration and several associated physico-chemical processes. Severe abiotic conditions could exist in periods of prolonged low flows.
In a second part, we demonstrate the interest of a close collaboration between hydrological and hydroecological sciences with two concrete examples in an alpine stream, Severaisse (Hautes-Alpes).
We use a quantitative methodology derived from the Instream Flow Incremental Methodoly (IFIM) of the Fish and Wildife Service (USA, Fort-Collins).
One of the hypothesis of this methodology is that physical habitat plays a major rate in structuring fish populations, certain habitats acting as « bottlenecks ».
In Severaisse, a representative study reach was chosen in a braided part of the stream, including each identified geomorphotogical unit.
The physical characteristics are described on perpendicular transects (2 or 3 transects per unit). On representing homogeneous area, on each transect different measurements of depth, current velocity and substrate type are taken at irregular internals, with more points in heterogeneous portions. These points divide the represented area in cells. Topographic data complement the description and are necessary for hydraulic simulation at different discharges.
The physical characteristics of the reaches are illustrated on maps, thus it is possible to follow the evolution of one parameter or a combination of them with changes in discharge (500 ls-1, 1 m3 s-1, 2 m3 s-1, 10 m3 s-1).
Fish habitat requirements are obtained from preference curves for each physical parameter and life stage. The studied species is the brown trout (Salmo truffa Fario, L., 1759).
The available habitat is then calculated cell by cell by converting physical data into suitability criteria. The resulting Wheighted Usable Area (WUA) for brown trout is then computed versus discharge.
Adult WUA increases quickly between 0.5 and 1.0 m3 s-1 and is quite linear up to 10 m3 s-1 and a rapid decrease up to 4 m3 s-1.
In Severaisse the low discharge is in winter with a monthly average stream flow of 2 m3 s-1 (period 1959-1985). This value corresponds to the maximum WUA (120 m2) for adult and to a medium WUA (300 m2 for a maximum of 450 m2 at 0.5 m3 s-1) for alevins and juveniles. The lower values for adult are often encountered.
The summer average discharges, when fish are more physiologically active and in a period of active growth, give also good WUA (10 m3 s-1, 100 m2 for adult). When the hydrological events are close to the mean average interannual pattern, brown trout population tends to an equilibrium with a number of adults defined by the minimum WUA. This type of population structure has been observed in September 1986.
This first example shows the importance of hydrological data which can be translated into habitat values for fish in two ways : real date by chronics or statistical approach (variability between years).
The second example consist in a biological assessment of trout population before and after a sever flood (70 m3 s-1) in the same stream. The number of fish was depleted and the site structure severely impacted with no more fish greather than 240 mm (total lenght) and a drastic loss in 1 + year class.
The flood resulted also in a morphological modification with displacement of the channel. During the flood, it was not possible to measure current velocities but it could be assumed that they were to high for fish to maintain. Fish might have drifted downstream with no recolonisation after 9 months.
These two examples have emphasized the importance of hydrology in aquatic ecology. Flood regime pattern can be translated in potential habitat values for fish or other biota.
This new habitat methodology can also improve the quality of impact studies, in particular chose dealing with stream regulation.
Therefore stream flow variability and predictability are essential to define temporal and spatial patterns of lotic environments.
Keywords:
- Hydrology,
- fluvial geomorphology,
- modelling,
- aquatic habitats,
- fishes