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La définition de l'étang en géographie limnologique

Abstract : A review of the existing definitions about what is a pond leads to a classification which identifies two types: a drainable body of water originally intended for fish farming, or a medium-sized body of water between lake and swamp or between lake and pool. Limnogeography is by nature inclined to the second family of definitions. This is why the guiding thread of this paper is that of spatiotemporal scales. The first part reviews the etymologies of the word "pond" in several languages and concludes that the opposition between artificial and natural origin of the body of water is a criterion that should be overcome. It is proposed to give it a more dynamic meaning, by creating the concept of "limnological chain". In this set of connected and evolving water bodies, pond always forms an intermediary, whatever its origin. Pond is a link in the anthropogenic chain when its impounding takes the place of ancient marshlands. Pond is a link in the natural succession when it comes from an ancient lake due to the process of sedimentary filling-up. The second part replaces the pond on the one hand among continental waters, on the other hand among standing waters. Because of its dimensions, the pond is both a lake and a stream. This hybridization may be expressed in water renewal time, duration of currents, the importance of free convection compared with forced mixing, spatial extension of feedbacks from the water body to its environment (limnoclimate of a region of ponds versus limnoclimate of a single lake). The third part studies spatial scales. The numerical thresholds of the scientific literature (for example a depth of 6 m forming the limit between lake and pond) are mentioned, but it is proposed to prefer them functional thresholds. Depth thresholds separating pond from lake and from marsh are discussed through the concept of limnic bathymetric zonation. Area thresholds separating pond from lake and pool are discussed through the concept of limnic belt zoning. Volume thresholds are discussed through the new concept of "cubing of a water body", defined as the three-dimensional structuring of all limnic phenomena in the form of distinct water masses. In other words, the lower is the relative depth (ratio of depth to the mean diameter), the more the pond cubing tends towards the marsh, the higher it is, the more it tends towards the lake. The geographical identity of a body of water lies in the relationship between (i) its submission to the external environment (especially to its basin), (ii) the autonomy of its internal functioning (structuring) and (iii) its effect (impact) on the environment (especially on the outlet). Among limnosystems, pond is defined by the spatial scales of these interrelationships (Fig. 1). It is suggested to create a “stagnustrine” limnosystem between lacustrine and palustrine limnosystems. The fourth part studies time scales. Whether for layering, zoning, or cubing, the pond is distinguished by response times shorter than the annual (seasonal) rate, which is specific to the lake, but longer than the diurnal rhythm, which is the specificity of marshes and pools. The pond can be best characterized by interdiurnal variability. To show in a didactic way that the pond forms the only body of water whose functioning is determined by an acyclic time scale, it is suggested to draw "the grimace of the pond" (fig. 2). In conclusion, it is proposed to define a pond as a continental body of water whose intermediate size allows an irregular and temporary structuring of water masses on the scale of a few days and an inertia developing in relationship with its immediate environment (slope limnosystem). In order to maintain cohesive reasoning in spatiotemporal scales, it is proposed to define a lake as a continental body of water, the large size of which allows a long-terme (at least seasonal) or permanent structuring of water masses and an inertia developing in the in relationship with its drainage basin (basin limnosystem).
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Laurent Touchart. La définition de l'étang en géographie limnologique. Laurent Touchart. Géographie de l'étang: des théories globales aux pratiques locales, L'Harmattan, pp.13-53, 2007, 9782296029361. ⟨hal-02076272⟩



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