1. Natural and anthropogenic disturbances are influential factors in river ecosystems. Lowland rivers that cross urbanised areas experience direct disturbance due to urbanisation, affecting suitability for many vertebrate and invertebrate species and with important implications for ecosystem functioning and stability. In addition, lowland rivers are potentially subjected to major flood events, whose effects may be exacerbated by urbanisation in the river basin. A greater understanding of the combined effects of flood disturbance and urbanisation is needed if we are to preserve riverine ecosystems and improve management strategies. 2. We characterised macroinvertebrate, detritus-based food webs upstream and downstream of Rome (Italy) before a period of exceptional flooding and at intervals until 71 days following the final flood event. We studied community and food-web characteristics, combining carbon and nitrogen stable isotope analysis with macroinvertebrate population census data. 3. The flood altered the species assemblage at both upstream and downstream locations. The food webs shifted towards a more complex configuration, with increased trophic generalism, linkage density and mean chain length. However, the food web at the upstream location gradually recovered its pre-flood configuration, whereas at the downstream location, where flood effects were stronger, the food web showed little capacity for recovery within the time span of the study. The contrasting food-web responses can be explained with reference to differences in resource availability for detritivores and predators and in water turbidity, which in turn reflect an interaction between flood disturbance and river basin urbanisation. 4. The description of short-term variations in food-web properties may help to improve ecosystem monitoring and the management of freshwater bodies and can be used to assess short-term and long-term effects of disturbance. A better understanding of the negative effects of urban pressure on macroinvertebrate community stability should help to define appropriate measures for the conservation and restoration of urbanised river areas, especially given that extreme flood events are expected to increase due to climatic change

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The high numbers of primary producers represent multiple sources of organic matter accumulating onto lake bottoms. The difficulty of distinguishing the relative contribution to the mixture presents considerable challenges to the analysis of these organic deposits. In this study, dual-stable isotope analysis and IsoSource model were used to identify allochthonous and autochthonous components of detritus deposits (Particulate Organic Matter: POM) at two different bottom slope sites of a volcanic lake (lake Bracciano). Experiments were carried out to calibrate IsoSource on constructed plant mixtures and assess changes in isotope ratios during plant decomposition. IsoSource satisfactorily discriminated the constructed mixture sources with a few exceptions. Changes in isotopic enrichment during decomposition were low, and thus did not represent a confounding variable in the isotopic analysis. By contrast, chemical and geological differences of the study sites were associated with differences in plant delta(13)C and delta(15)N values (more than 2aEuro degrees within single plant species). At both sites, the isotopic signals of POM fell between polygons delineated by source end members with an evident shift of delta(13)C toward allochthonous sources. POM amount and diversity were greater at the flatter bottom site, where allochthonous contributions were larger than at the other site. In particular, IsoSource ranked species contributions as follows: A. glutinosa > P. australis > A. donax > S. alba > P. nigra > the benthic macroalga Chara sp. at the first site, and A. glutinosa > P. nigra > the aquatic macrophyte C. demersum at the latter. The composition of littoral POM was determined by allochthonous sources in proportion to their relative abundances (as percent land cover) with differences between sites due to bottom slope

We tested the capacity of Ulva lactuca to mark N sources across large marine areas by measuring variation in its δ(15)N at several sites in the Gulf of Gaeta. Comparisons were made with the macroalga Cystoseira amentacea. Variation of δ(15)N values was assessed also in the coastal waters off the Circeo Natural Park, where U. lactuca and C. amentacea were harvested, as these waters are barely influenced by human activities and were used as reference site. A small fragment from each frond was preserved before deployment in order to characterize the initial isotopic values. After 48 h of submersion, U. lactuca was more responsive than C. amentacea to environmental variation and δ(15)N enrichment in the Gulf of Gaeta was observed. The spatial distribution of δ(15)N enrichment indicated that different macro-areas in the Gulf were affected by N inputs from different origins. Comparison of the δ(15)N values of fragments taken from the same transplanted frond avoided bias arising from natural isotopic variability.

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