No abstract

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 9662To link to this article : AbstractTo test the hypothesis that decomposition of leaf species in streams is influenced by afforestation with Eucalyptus globulus, we compared decay rates, nutrient levels, fungal biomass and macroinvertebrate assemblages on alder and eucalyptus leaf litter in three streams (two headwaters under different forests, and a mid reach) of the Agüera catchment (northern Spain).Whatever the reach, alder always decomposed significantly faster than eucalyptus. Litter contents in nitrogen and phosphorus rose during breakdown at the mid reach, but not at the headwaters. No differences in fungal biomass were found between alder and eucalyptus leaves at the headwater reaches; however, at the mid reach, eucalyptus showed the highest values.Alder litter, a high quality substratum, was readily colonized by shredders, and decayed rapidly at all sites. Eucalyptus, a low quality species, had lower nutrient contents and was less favoured by shredders. Under high nutrient levels (particularly phosphorus), however, it was readily colonized by fungi, thus shifting from medium to high breakdown rates. The potentially negative impact of afforestation with eucalyptus on streams can thus be reduced in situations of high concentrations of dissolved nutrients.

Aquatic hyphomycete assemblages on decomposing leaf litter of the exotic species Eucalyptus globulus Labill. were compared with those on the native riparian species Alnus glutinosa (L.) Gaertn. in three low-order streams of the Agüera watershed (Basque Country – Cantabria, Spain). These streams contrast by the importance of eucalypt in the riparian vegetation and by the nutrient contents in the water. Neither the total conidial production nor the number of fungal species differed between the two leaf species in any of the streams. Fungal colonization of eucalypt leaves appeared to be delayed by about 2 weeks, probably owing to their high content in inhibitory constituents. While Flagellospora curvula Ingold largely dominated pioneer assemblages on both leaf species, the second most important species, Lunulospora curvula Ingold, exhibited a preference for eucalypt. In the stream surrounded by eucalypt, the fungal diversity was surprisingly lower on eucalypt than on alder. In the stream with higher orthophosphate concentrations, the conidial production was similarly increased on both leaf species. The present data together with recent results from a similar investigation in Portugal suggest a minor impact of eucalypt on the activity and diversity of aquatic hyphomycete leaf-associated assemblages in southern European streams. Key words: aquatic hyphomycetes, eucalypt, alder, leaf litter, stream.

We studied epilithic biomass and stream metabolism along the Agüera stream (northern Spain) for one year. Epilithon was most abundant in mid-reaches; however, stream metabolism shifted there from autotrophy to heterotrophy during the year. The spatial distribution of epilithon depended mostly on light availability, nutrients and substrate characteristics. Spates were the main source of temporal variation, and resilience of epilithon was controlled by nutrient concentrations. A certain level of disturbance (by sloughing) could be important in maintaining the high capacity of selfpurification of this stream.

/ Water quality levels and loads of nutrients transported by the AgL~era stream (northern Spain) were studied for a year to assess the self-purification capacity of this system. The main villages produce an increase of nutrient concentrations and a degradation of water quality. Nevertheless, the high retention capacity, especially for phosphate, allows the stream to recover its previous levels of quality after a short reach. The retention of nutrients depends on a complex combination of the flow level, hydrologic stability and the development of periphytic communities.Increasing concern about the conservation of rivers, and mainly about the quality of stream waters, led to the development of several quality criteria and quality indexes (Tuffery and Verneaux 1967, Nisbet and Verneaux 1970, Janardan and Schaeffer 1975, Hellawell 1977, as well as to the assessment of the role played by fluvial systems in the dilution and purification of the nutrient and pollutant inputs (Margalef 1983). Rivers are not just sewers that channel the materials exported out of the terrestrial ecosystems, but centers of intense biological activity, with an important capacity for processing the transported materials (Hearne and Howard-Williams 1988, Cooper 1990). The capacity of purification is often .exceeded by nutrient inputs, and rivers all around the world show strong eutrophication and pollution. Therefore, in order to manage our environment in a sound way, it is necessary to assess the ability of self-purification of any running waters and to determine the maximum permissible levels of inputs, as well as the ways to attain those levels. Similarly, it is important to take into account any changes in the self-purification capacity of river systems as a result of human-caused impacts such as canalization, modification of river margins and riverbed or clear-cut of riparian vegetation (Pinay and others 1990).