Nitrogen dynamics in rural streams : differences between geomorphologic units

Nitrogen dynamics were investigated in the top centimeters of bottom sediment in three different geomorphologic units (riffles, sand bars, pools) and along two contrasted reaches (nitrogen-rich, nitrogen-poor) of a rural stream. Some predictions of links between geomorphology and processes were verified: (i) hydrological exchanges and nitrification were lower in pools than in riffles and bars, whereas (ii) ammonification and denitrification were higher in pools. Studies of denitrification potential in six other reaches of three N-rich streams support these conclusions and demonstrate that pools are spots of high microbial activity in streams

The distribution of macroinvertebrate assemblages in a reach of the River Allier (France), in relation to riverbed characteristics

International audienceMacroinvertebrate assemblages of large alluvial streams are poorly documented. This study identified the physical characteristics affecting the macroinvertebrates community distribution in a large alluvial river devoid of major anthropogenic impacts. It was oriented towards the influence of the characteristics of the physical habitat (velocity, depth, grain-size classes of mineral substrates, macrophytes) on macroinvertebrates (richness, density, body size, feeding habits), with particular attention to the pollution-sensitive taxa. The study was carried out in June during a period of hydrological stability. The effects of water velocity, depth and substrates on taxa were evaluated with multivariate analyses. Mineral substrates were most abundant while macrophytes accounted for only 3% of sampled habitat. Invertebrates that were present were those characteristics of the transition zone between upper and middle life reaches. Among the 63 taxa sampled, 14 were abundant. In relation to the characteristics of the physical environment, the macroinvertebrate assemblages were discriminated by substrate, velocity, and depth. Habitat exploitation, however, appeared complex. The highest community richness, EPT richness, and density were found in various substrates where the velocity ranged between 30 and 120 cm s)1, depths ranged from 16 to 50 cm. The most pollution-sensitive taxa preferred riffle habitats with velocities >70 cm s)1 and substrate >64 mm. This suggest that rapid bioassessment programmes should be carried out in the mineral substrates of the geomorphological unit riffles where richness is high and density sufficient to represent the macroinvertebrate community, including pollution-sensitive taxa

The life-story of plastic butter tubs.

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