Previous studies on plastic pollution of aquatic ecosystems focused on the world's oceans. Large rivers as major pathways for land-based plastic litter, has received less attention so far. Here we report on plastic quantities in the Austrian Danube. A two year survey (2010, 2012) using stationary driftnets detected mean plastic abundance (n = 17,349; mean ± S.D: 316.8 ± 4664.6 items per 1000 m−3) and mass (4.8 ± 24.2 g per 1000 m−3) in the river to be higher than those of drifting larval fish (n = 24,049; 275.3 ± 745.0 individuals. 1000 m−3 and 3.2 ± 8.6 g 1000 m−3). Industrial raw material (pellets, flakes and spherules) accounted for substantial parts (79.4%) of the plastic debris. The plastic input via the Danube into the Black Sea was estimated to 4.2 t per day.

Dispersal of fluvial freshwater fish larvae occurs commonly in heavily regulated rivers. Studies increasingly indicate that drifting young fish have an active component to their movement and so are capable of coping with the dynamic hydraulic forces typical of rivers. We investigated hydraulic–drift relationships of the young stages of fish over one breeding season along a gravel shore of the Austrian Danube using stationary drift nets from the first occurrence until the end of the seasonal peak (April–June 2008). Observed densities of families and developmental stages were related to the hydraulic parameters flow velocity (in three spatial directions), turbulent kinetic energy and water depth, derived from a three‐dimensional hydrodynamic model of the sampling site. We detected distinct responses of drifting young fish to several hydro‐physical factors under conditions (low light level, over‐critical currents), which were considered to cause washouts and passive drift. In general, drift densities decreased with water depth and flow velocity. Weak swimmers (e.g. bull head Cottus gobio and early‐stage cyprinid larvae) avoided turbulent flows, as their abilities to orientate and hold position may be limited. Early larval as well as early juvenile stages of cyprinids used lateral currents directing to the bank, potentially to drop out of the flow and reach inshore areas. Our study indicates that there are family‐specific and stage‐specific responses to hydraulic variables and that fish actively disperse, probably to minimize mortality and maximize successful dispersal. Copyright © 2013 John Wiley & Sons, Ltd.

Little information is available on governing factors of larval fish dispersal in natural river systems. Therefore, we aimed to describe dispersal and retention of marked larval nase carp, Chondrostoma nasus, along a shoreline nursery habitat of the River Danube. Based on a three-dimensional hydrodynamic model, we analyzed the influence of the hydraulic conditions on larval dispersal. We also related observed larval pathways to numerical particle tracing. Clear differences in the temporal drift pattern were due to significant differences in the hydrodynamic characteristics of the release stations. Some larvae remained in the study reach, most upstream of the release point. These were significantly larger than drifting larvae. We conclude that larval dispersal has an active component and that dispersal and retention patterns are dependent on the habitat structure and hydrodynamic characteristics of the releasing points. This emphasizes the importance of links between the location of spawning sites within the river and variation in flow during early development, the combination of which may contribute to successful recruitment of fluvial fish species.Résumé : Peu de données sont présentement disponibles sur les facteurs qui régissent la dispersion des larves de poisson dans les systèmes fluviaux naturels. Aussi, nous avons tenté de décrire la dispersion et la rétention de larves marquées de hotu, Chondrostoma nasus, le long d'un habitat de nourricerie littorale du fleuve Danube. À la lumière d'un modèle hydrodynamique tridimensionnel, nous avons analysé l'influence des conditions hydrauliques sur la dispersion des larves. Nous avons également relié les voies de dispersion des larves à des modèles numériques de suivi de particules. Des différences nettes notées dans les motifs de dérive dans le temps découlaient de différences significatives des caractéristiques hydrodynamiques des sites de lâcher. Certaines larves sont demeurées dans le tronçon à l'étude, principalement en amont du point de lâcher. Ces larves étaient significativement plus grandes que les larves dérivantes. Nous concluons que la dispersion des larves comprend une composante active et que les motifs de dispersion et de rétention dépendent de la structure des habitats et des caractéristiques hydrodynamiques des points de lâcher. Ces résultats soulignent l'importance des liens entre l'emplacement des sites de fraie dans un cours d'eau et les variations du débit durant le développement précoce, la combinaison de ces deux facteurs pouvant contribuer au recrutement efficace d'espèces de poissons fluviaux.[Traduit par la Rédaction]

The dispersal patterns of marked larvae of the nase carp (Chondrostoma nasus L.) were observed alongside dissimilar shoreline configurations in the main channel of the free-flowing Austrian Danube and compared with those of floating particles to investigate the mode of dispersal (active–passive). Individuals of different larval stages and floats at similar densities were released at an artificial rip-rap with groynes and a rehabilitated gravel bar. In both habitats, marked individuals were recaptured during the sampling period of 4 d after release. Relevant shoreline attributes for larval dispersal, such as the accessibility of nursery habitats, connectivity between adjacent habitats, and retention potential, were more pronounced at the gravel bar than at the rip-rap. At the gravel bar, larvae moved upstream and downstream within the connected bankside nurseries and displayed longer residence times. Larvae settled in groyne fields along the rip-rap as well; however, longitudinal dispersal was disrupted by groynes, forcing larvae to enter the main channel. Rather than settling in subsequent groyne fields, we assume that these larvae are displaced downstream and potentially lost from the local population.

This paper investigates the variability of mesohabitat characteristics in various riffle-pool reaches. The tested river sections (n ¼ 13) feature clear variation in slope (0.0004-0.0132) and low flow discharge (0.05-915 m 3 s À1 ) in different river types (straight to meandering). Mesohabitat characteristics (water depth, flow velocity, bottom shear stress) were calibrated according to the MEMconcept (MEM-Mesohabitat Evaluation Model). Statistical analysis clearly revealed significant differences ( p < 0.001) for the same mesohabitats (e.g. riffles) in different rivers concerning the tested abiotic habitat parameters. A comparative analysis of hydromorphological parameters (width-depth variance, Froude number) showed no correlation to mesohabitat variability based on 2D/3D numerical modelling related to a range of flows (n ¼ 10) (low flow to annual flood). Only an increasing hydraulic radius (R hy ) was correlated to an increase in fast run and a decrease in run habitats. In a case study, a Fish Guild Concept (FGC) is presented which links mesohabitats to the rheophilic fish guild (12 fish species grouped) at the Sulm River. Mesohabitat suitability (preferred, useable and avoided) for the FGC was determined for spawning, juveniles (0þ, 1þ), sub-adult and adult stages based on meso-unit and point abundance electro-fishing.