YOUNES (Y.), GARCIA (X.F.), & GAGNEUR (J.). 2002.- Study of the mountain tourism impact on water quality and assemblages of macrobenthic populations in the Andorran rivers. Journal of Water Science , 15(1) : 421-424.

Abstract

The important development of tourism during winter and summer disturb the main Andorran streams. The impact of these disturbances were studied in high altitude streams characterised by particular environmental conditions (e.g. low temperature - steep slope) as little is known about water quality and associated benthic fauna in Andorran streams (PUIG, 1979 ; PEÑA, 1983).

Thirteen sites were seasonally studied during one year, from autumn 1998 to autumn 1999, in each site, eight macroinverbrates samples were collected using a Surber sampling net (mesh size 200 µm - sampling area : 1/20e m2). In addition, nine physicochemical parameters (temperature - pH - conductivity - DCO - DBO5 - nitrate- nitrite - ammonia - phosphate) were weekly measured in each site during the entire sampling period.

A discriminant analysis performed on the physicochemical data reaveled and upstream-downstream gradient of the water quality, with highest disturbance in the downstream sites. From a temporal point of view, the water quality follows a seasonal cycle highlighted by the opposition between on the one hand winter-spring and summer 1999 , and on the other hand autumn 1998-1999.

The spatio-temporal distribution of the benthic fauna, based on the discriminant analysis, highlighted the disappearance of polluo-sensitive taxa in intermediate and downstream sites such as filtering collectors (Simuliidae), predators (Perlidae - Perlodidae - Rhyacophilidae) and shredders (Nemouridae - Leuctridae). In these areas, sensitive taxa were replaced by polluo-tolerant ones such as Chironomidae and Oligocheta. These patterns were already observed in other mountain streams subjected to high organic disturbances (DECAMPS ET PUJOL, 1977).

As a consequence, polluo-tolerant taxa replace polluo-sensitives taxa with similar functionnal status. For example Oligocheta, proliferate in the downstream (Gran Valira), whereas upstream, they were only abundant on a point source pollution site located on the Ariege river. The same phenomenon was also observed for predators: High unpolluted altitude sites were characterised by stoneflies and caddisflies (Perlidae, Perlodidae, Rhyacophilidae) which were replaced downstream by leeches (Glossiphoniidae). As a consequence, the calculation of the several biological indices of water quality (I.B.G.N - B.M.W.P'), and diversity descriptors (taxa richness - Shannon index - Margalef index), indicate a drastic reduction of the taxa diversity, evenness and water quality from upstream to downstream sites.

Disturbances associated with tourism development appeared in the medium altitude sites. In these sites, the water quality benefits from the good water quality from the upstream sites during high discharge periods (i.e. in autumn). This phenomena explained the observed temporal variability of both physicochemical conditions and benthic fauna structure. In the downstream sites, the pollution can be considered as chronic and hence, low environmental variability was observed. These results show that Andorran stream ecological integrity is low form both physicochemical diversity and population assemblages point of views. Moreover, the impact of anthropic disturbances followed an annual cycle according to tourism activities with pollution peaks in winter and summer corresponding respectively to ski resort and hiking tourism. The only partial recovery period is represented by the autumn high drainage period.

From a theoretical point of view, these results allowed to test the intermediate disturbance hypothesis (CONNELL, 1978 ; RESH, 1988 ; TOWNSEND et al., 1997) and to search the threshold of resistence/resilience characteristics of a stream ecosystem considering stream benthic assemblages (EDWARD et RYKIEL, 1985 ; REICE et al., 1990 ; PICKETT et al., 1989).

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