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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

Non‐native freshwater fish species can have adverse ecological impacts on native populations. However, the mechanisms determining the success or otherwise of their invasion and their role in invaded communities remain largely unknown. This is particularly true for the Mediterranean region, where endemic species characterised by restricted natural ranges may be at high risk of extinction. The largemouth bass (Micropterus salmoides) is native to North America but is invasive in the Italian Peninsula. The aim of this study was to explore the trophic position of M. salmoides, its diet and niche overlap with native fish species in the littoral areas of a Mediterranean lake. Our study was supported by analysis of stable C and N isotopes in the tissues of fish and their potential food sources, twenty years after the introduction of the largemouth bass to Lake Bracciano (Italy). Samples were collected in locations varying in terms of physical structure and resource availability at lower trophic levels in the food web, which was greater in the southern (hereafter: South) than in the northern (hereafter: North) area of the lake. These differences made it possible to explore the mechanisms linking environmental conditions and the role of alien predators in the invaded food web. The abundance of M. salmoides was higher, and the diversity of native fish species was lower, in North than South. In North, M. salmoides had a piscivorous diet and occupied a higher trophic position in the food web than in South, where invertebrates constituted an important part of its diet. As a consequence, trophic niche interference with other fish species at intermediate trophic levels was higher in South. In contrast, in North, M. salmoides showed stronger trophic interference with the percid Perca fluviatilis, a native top predator in the food web, but weaker interference with remaining fish species. Our results help to understand the role of alien species in the food webs of Lake Bracciano, which primarily depends on the habitat and the availability of prey across trophic levels. Physical and ecological variations in the habitat were associated with differences in predatory interactions among native and alien fish species. This suggests that a reduction in productivity and biodiversity at lower trophic levels in lake food webs may favour the success of opportunistic invasive fish species, given the ability of the invaders to maintain some of their characteristics silent, and to fully express their genotype under favourable environmental conditions.

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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.