Impact of water abstraction on storage and breakdown of coarse organic matter in mountain streams.

Water abstraction is a prevalent impact in streams and rivers, which is likely to increase in the near future. Because abstraction reduces discharge, the dimensions of the wetted channel and water depth and velocity, it can have strong influence on stream ecosystem functioning. Although the impacts of large dams on stream and river ecosystems are pretty well known, the effects of diversion schemes associated with low dams are still poorly understood. Furthermore, the remote location of many diversion schemes and the lack of collaboration by power companies often make it difficult to know the volume of water diverted and its environmental consequences. To assess the impact of water abstraction on the storage and breakdown of coarse particulate organic matter in streams we compared reaches upstream and downstream from five low dams that divert water to hydropower plants in mountain streams in N Spain. We measured the storage of organic matter and the breakdown of alder leaves in winter and spring, and calculated the results at the patch (i.e., per square meter of bed) and at the reach scale (i.e., per lineal meter of channel). Water diversion significantly reduced discharge, and the width and depth of the wetted channel, but did not affect water quality. Diversion significantly reduced the storage and breakdown of organic matter in winter but not in spring. The number of shredders colonizing litter bags was also significantly reduced. The results point to an important effect of water abstraction on the storage and breakdown of organic matter in streams at least in some periods, which could affect downstream reaches, global carbon fluxes, and associated ecosystem services.

Irrigation agriculture affects organic matter decomposition in semi-arid terrestrial and aquatic ecosystems.

Many dryland areas are being converted into intensively managed irrigation crops, what can disrupt the hydrological regime, degrade soil and water quality, enhance siltation, erosion and bank instability, and affect biological communities. Still, the impacts of irrigation schemes on the functioning of terrestrial and aquatic ecosystems are poorly understood. Here we assess the effects of irrigation agriculture on breakdown of coarse organic matter in soil and water. We measured breakdown rates of alder and holm oak leaves, and of poplar sticks in terrestrial and aquatic sites following a gradient of increasing irrigation agriculture in a semi-arid Mediterranean basin transformed into irrigation agriculture in 50% of its surface. Spatial patterns of stick breakdown paralleled those of leaf breakdown. In soil, stick breakdown rates were extremely low in non-irrigated sites (0.0001-0.0003 day(-1)), and increased with the intensity of agriculture (0.0018-0.0044 day(-1)). In water, stick breakdown rates ranged from 0.0005 to 0.001 day(-1), and increased with the area of the basin subject to irrigation agriculture. Results showed that irrigation agriculture affects functioning of both terrestrial and aquatic ecosystems, accelerating decomposition of organic matter, especially in soil. These changes can have important consequences for global carbon budgets.

The use of wooden sticks to assess stream ecosystem functioning: comparison with leaf breakdown rates.

Breakdown of organic matter is a key process in streams and rivers, and thus, it has potential to assess functional impairment of river ecosystems. Because the litter-bag method commonly used to measure leaf breakdown is time consuming and expensive, several authors proposed to measure breakdown of wooden sticks instead. Nevertheless, currently there is little information on the performance of wooden sticks versus that of leaves. We compared the breakdown of tongue depressors made of untreated poplar wood, to that of six common leaf species in two large streams in the Basque Country (northern Spain), one polluted and the other unpolluted. Breakdown rates ranged from 0.0011 to 0.0120 day(-1), and were significantly lower in the polluted stream. Wooden sticks performed very similarly to leaves, but were less affected by flood-induced physical abrasion. The ranking of the materials according to their breakdown rate was consistent, irrespective of the stream. The experiments with leaves were 10 times more costly for breakdown rate, 4 times if we include the rest of the variables measured. Therefore wooden sticks offer a promising tool to assess river ecosystem functioning, although more research is necessary to define the thresholds for ecosystem functional impairment.