Invasive species and anthropogenic sources of allochthonous trophic subsidies can have substantial eco-logical consequences for freshwater ecosystems, including modifying the diet of consumers and altering food web structure. Here, the diet of an omnivorous cyprinid fish, European barbel Barbus barbus, was assessed in relation to the presence of invasive signal crayfish Pacifastacus le-niusculus and pelletized fish-meal in four rivers in England. Pellets are often used in large quantities by river anglers and thus could provide an important trophic subsidy, not only to the fish but also indirectly via P. leniusculus. Carbon and nitrogen stable isotopes were used to estimate the proportion of diet assimilated from natural sources and from P. leniusculus and pellets by B. barbus of lengths between 420 and 800 mm. Pellets generally made a large contribution to the overall biomass of B. barbus (up to 59 % of population diet) and in the two rivers where they were present, P. leniusculus were also an important resource (up to 30 % of population diet). The proportion derived from macro-invertebrates (excluding P. leniuscu-lus) was substantially lower. Stable isotope mixing models further demonstrated considerable intraspecific variability in B. barbus diet within the rivers, with pellets comprising up to 79 % of the biomass of individual B. barbus in rivers where P. leniusculus was absent. Where present, P. le-niusculus effectively replaced and thus reduced the contribution of pellets to individual fish diet. Thus, isotopic evidence from three of the four rivers indicates that B. barbus populations are heavily reliant ([50 %) upon angler-introduced baits that act as an important allochtho-nous subsidy and will also prey upon invasive P. leniusculus where they are present.

Scales and fin tissue are strong surrogates of dorsal muscle in food web studies as they can provide estimates of muscle values within an acceptable level of error when species-specific methods are used. Their derived fractionation factors can also be applied to models predicting fish diet composition from δ(15) N and δ(13) C values.

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Hatchery‐reared fish are commonly stocked into freshwaters to enhance recreational angling. As these fishes are often of high trophic position and attain relatively large sizes, they potentially interact with functionally similar resident fishes and modify food‐web structure. Hatchery‐reared barbel Barbus barbus are frequently stocked to enhance riverine cyprinid fish communities in Europe; these fish can survive for over 20 years and exceed 8 kg. Here, their trophic consequences for resident fish communities were tested using cohabitation studies, mainly involving chub Squalius cephalus, a similarly large‐bodied, omnivorous and long‐lived species. These studies were completed over three spatial scales: pond mesocosms, two streams and three lowland rivers, and used stable isotope analysis. Experiments in mesocosms over 100 days revealed rapid formation of dietary specializations and discrete trophic niches in juvenile B. barbus and S. cephalus. This niche partitioning between the species was also apparent in the streams over 2 years. In the lowland rivers, where fish were mature individuals within established populations, this pattern was also generally apparent in fishes of much larger body sizes. Thus, the stocking of these hatchery‐reared fish only incurred minor consequences for the trophic ecology of resident fish, with strong patterns of trophic niche partitioning and diet specialization. Application of these results to decision‐making frameworks should enable managers to make objective decisions on whether cyprinid fish should be stocked into lowland rivers according to ecological risk.

Globally, inland waters emit over 2 Pg of carbon per year as carbon dioxide, of which the majority originates from streams and rivers. Despite the global significance of fluvial carbon dioxide emissions, little is known about their diel dynamics. Here we present a large-scale assessment of day- and night-time carbon dioxide fluxes at the water-air interface across 34 European streams. We directly measured fluxes four times between October 2016 and July 2017 using drifting chambers. Median fluxes are 1.4 and 2.1 mmol m−2 h−1 at midday and midnight, respectively, with night fluxes exceeding those during the day by 39%. We attribute diel carbon dioxide flux variability mainly to changes in the water partial pressure of carbon dioxide. However, no consistent drivers could be identified across sites. Our findings highlight widespread day-night changes in fluvial carbon dioxide fluxes and suggest that the time of day greatly influences measured carbon dioxide fluxes across European streams.

The spawning success of lithophilic salmonids is strongly influenced by the fine sediment content ('fines') of spawning substrates, yet knowledge on the impacts of fines on the spawning of non-salmonid lithophiles remains limited, despite their ecological and socioeconomic importance in European rivers. Consequently, the aim here was to use an ex-situ experiment to investigate the impact of sand content on egg survival and timing of larval emergence of the surface-spawning cyprinid European barbel Barbus barbus. Thirty incubator boxes within a recirculating system were filled with one of five experimental sediment mixtures (0 to 40 % sand by mass) that each contained 300 fertilised eggs at a depth of 50 mm. Emerged, free-swimming larvae were captured and counted daily to assess grain size effects on larval survival and emergence. Specifically, total proportion of emerged larvae, cumulative daily proportion of emerged larvae and time required to reach 50 % emergence were measured during the study. Whilst the proportion of sand in the sediments did not have a significant impact on egg-to-emergence survival (mean survival per treatment 75 % to 79 %), it significantly affected the timing of larval emergence to the water column; early emergence was detected in treatments with elevated sand content (on average, 50 % emergence after 12-13 days versus 19 days in the control). Similar to findings from salmonid studies, these results suggest high sand content in spawning gravels can influence timing of larval emergence and potentially cyprinid lithophilic fish survival. 3 1 INTRODUCTION Availability and suitability of spawning habitats are important determinants of fish population viability in freshwater ecosystems (

The crossing of freshwater ecosystem boundaries by marine‐derived nutrients (MDN) is usually associated with migratory salmonid fishes returning to natal rivers. An alternative source of MDN in fresh waters is the widespread use of pelletised marine fishmeal (“pellets”) by freshwater anglers as they target large‐bodied cyprinid fishes, such as European barbel Barbus barbus. Here, the trophic consequences of MDN from pellets for riverine cyprinid fishes were tested. Approaches used stable isotope analyses in controlled and wild scenarios, using B. barbus and chub Squalius cephalus as model species. The isotopic niche, measured as standard ellipse area, was used to assess trophic niche size, and mixing models predicted the extent to which MDN contributed to fish diet. In experimental mesocosms, B. barbus fed low volumes of pellets (c. 3 per fish) for 130 days had isotopic niche sizes that were up to four times larger than a control and “medium” (6 per fish) and “high” pellet (12 per fish) treatments. Somatic growth rates were significantly higher in the “medium” and “high” treatments. In pond enclosure experiments, when juvenile B. barbus and S. cephalus were fed pellets daily for 100 days, there was a substantial and significant shift in the position of their isotopic niche compared to controls with no pellets fed. However, for each species, there were no significant differences in their somatic growth rates in the presence/absence of pellets. In a lowland river, high proportions of MDN contributed to the diet of B. barbus and S. cephalus captured by angling, but with substantial individual variability in those captured by electric fishing. Across all B. barbus >400 mm, MDN dietary contributions ranged between 9% and 71%. This suggested some individual diet specialisations within their population that was associated with feeding on this angler subsidy and that also resulted in a significant increase in the size of their population isotopic niche. These results suggested that when pellets containing MDN are used in freshwater angling, they are consumed and assimilated by cyprinid fishes, influencing individual and population trophic positions, and isotopic niche sizes and dietary specialisations. The results also suggested that the extent to which individuals specialise in feeding on pellets potentially influences their vulnerability to capture by anglers.

Fine sediments can impact river biota, with egg and larval stages of lithophilic fish particularly sensitive to deposition of sand-and clay-sized particles ('fines') in spawning gravels. Mitigation and restoration methods include jetting to cleanse gravels of fines. Despite wide application, impacts of jetting on gravel composition and quality have rarely been quantified. Here, gravel jetting impacts on sediment composition in the River Great Ouse (UK), were tested during an in-situ experiment completed at riffle (55.6 ± 13.4 m 2) and patch (0.3 m 2) scales to determine its magnitude and persistence on surface and subsurface substrate conditions. Before-after (riffle) and control-impact (patch) designs were used, with bedload sediment traps installed downstream of experimental patches to investigate the sediments mobilised during jetting. At the riffle scale, surface grain size was significantly altered; fines were removed resulting in coarser and better-sorted sediments. Similar patterns were detected at the patch scale, although sediment sorting was not significantly altered. Despite reduced fine sediment content of subsurface gravels at the riffle scale, the overall grain size composition was not significantly altered. At the patch scale, no subsurface improvements were detected. Temporally, at the riffle scale, no changes in surface or subsurface sediments lasted more than 12 months; patch scale changes generally persisted for less than 3 months. Thus, whilst gravel jetting could improve spawning gravel quality for surface spawning fishes, including European barbel Barbus barbus, its effects are short-term. However, because subsurface sediments are not affected by gravel jetting, the benefits are limited for redd-building fishes, such as salmonids. Consequently, reducing fine sediment delivery to rivers, such as by changes in agricultural practices, would be more sustainable for managing excessive river sedimentation.