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Fluvial sediment samples (river bottom sediment, suspended sediment and active floodplain/overbank) were collected in 2 baseline stations of the Transnational Monitoring Network for each of the 12 countries in the Danube Basin Region: Austria, Bosnia and Herzegovina, Bulgaria, Croatia, Czech Republic, Hungary, Republic of Moldavia, Montenegro, Romania, Republic of Serbia, Slovakia and Slovenia, according to a harmonized methodology. The sediment samples were analyzed for selected hazardous substances (HSs):8 metals, 8 polycyclic aromatic hydrocarbons (PAHs) and 6 pesticides in an accredited laboratory selected as reference laboratory, as well as in national laboratories. Risk ratios were computed in order to compare the results to the Environmental Quality Standards listed in the European Directives, to the national threshold values in the Danube Basin, and to other available international European and American quality standards, and the risk ratios were classified as background, alert and intervention values for readily evaluation. Results show widespread metal(oid) (As, Cd, Cu, Hg, Pb, and Zn) contamination in the Danube Basin, most often associated with historic mining. Concentration values of Ni and Cr exceeding the thresholds are caused by the geological background (ultrabasic and metamorphic rocks in Bosnia and Herzegovina, Republic of Serbia and Romania). Concentration values higher than the thresholds for some of the PAHs (e.g. Fluoranthene) and sometimes for Total PAHs were also noted almost in every countries, caused by industrial activities, wastewater discharges or navigation on the Danube River. Pesticides were below the detection limit or below the legislative intervention thresholds at all national baseline sampling sites.

Discharge of hazardous substances (HSs) in hydrographic basins represent a danger to aquatic biological activity and water supplies and can severely pollute surface water sediments. The increase of pollution in the Danube Basin requires the implementation of systematic monitoring and evaluation of the sediments quality as dictated by the EU Water Framework Directive. For this system development, applicable in mountainous conditions, the Upper Tisa region in the northwest part of Romania on the border with Ukraine, Hungary and Slovakia was selected as a test area. Sampling of overbank (floodplain) sediment, river bottom sediment and suspended sediment was carried out at 10 locations in the test area in order to analyze the concentration and distribution of eight metal(oid)s (Cu, Pb, Zn, Cd, Hg, Ni, Cr and As), in addition to 3 organic components (anthracene, fluoranthene, benzo(e)pyrene) as hazardous substances (HSs). The sediment quality assessment was carried out according to the 2013/39/EU Directive and EU Water Framework Directive standards. Most of the analyzed HS concentrations in river bottom sediment and overbank (floodplain) sediments fall within the limits of environmental quality standards (EQS). As, Cu, Pb, and Zn tend to exceed the EQS at some locations. The highest exceedances were recorded for Pb, for which contents of up to 987 mg/kg were detected. The highest contents were found in the overbank sediments sampled, and the lowest in river bottom sediments, which may indicate historical pollution. Mercury contents in overbank sediment samples exceed all standards, while cadmium content is below the international standards. Anthracene, fluoranthene and benzo(e)pyrene concentrations in overbank and bottom sediments comply with international standards. A few samples in suspended sediments slightly exceed the lowest environmental standard value (i.e., Romanian normal value). HS concentrations remain low in the suspended sediments showing that it is not the main transport route for pollution in this area. HS contents decrease gradually from upstream to downstream due to dilution along the river course. In the last testing point at Someș Aciua, the concentration of metal(oid)s measured in the sediments remain below the EQS limit values, thus there is no risk of transboundary pollution. The main source of metal(oid) contamination is historic base-metal ore mining and the associated mine waste sites in the Baia Mare and other mining areas scattered around the whole region. The main source of the studied organic compounds is the incomplete or low-temperature coal combustion processes that occur in households in rural areas.

The EU Water Framework Directive requires the monitoring and evaluation of surface water sediment quality based on the assessment of risk posed by contamination on the biotic receptors. Fluvial sediments are important receptors of hazardous substances (HSs) pollution from the upstream catchment areas in the Danube River Basin (DRB). For the development of systematic sediment quality monitoring and evaluation, the Drava River region on the border of Hungary and Croatia was selected as a test area representative of lowland hydromorphological conditions. Overbank (floodplain) sediments and river bottom sediments (stream sediments) were sampled at two depths at 9 locations in the test area. Eight heavy metal(oid)s were analyzed As, Cd, Cr, Cu, Hg, Ni, and Zn as hazardous substances. The sediment quality assessment was carried out according to the 2013/39/EU Directive and EU Water Framework Directive standards. Most of the analysed HS concentrations in river bottom sediment and overbank (floodplain) sediments fall within the limits of environmental quality standards (EQS). Results show that there is no significant differences in metal(oid) HS concentrations among the various sediment types and between shallow (0- 5cm) and deeper (stream sediment: 5-10cm ; floodplain sediment: 40-50cm) sediment which suggests that the large lowland Drava River fluvial system is an extensive single fluvial system with homogeneous distribution of sediments and the associated contaminants. Specifically, the studied sediments in the tributaries of the Drava River do not seem to be contaminated with metal(oid) hazardous substances but at certain sites concentrations are elevated above the environmental limit values, especially for As and Zn, and to lesser extent for Cr. The data analysis techniques used enabled the identification of sites with anthropogenic pollution and the recognition of regional pattern in HSs distribution.

A number of Ponto-Caspian Gobiid species have greatly increased their geographical ranges over recent decades. Most expansion studies to date, however, have focused on navigable waterways. In this study, we present a summary of six-years (2008–2013) monitoring of round goby Neogobius melanostomus expansion along two connected non-navigable rivers. Contiguous range expansion was observed in both rivers, with dispersal rate ranging from 1.2 to 3.2 km/year. Gobies at newly invaded sites ranged from 20 to 117 mm, with both juveniles and adult fish observed. Though the data did not allow us to see any consistent pattern in the first years after detection, there was some evidence for a shift to a female-biased, juvenile-dominated population over time. While the abundance of non-native tubenose goby Proterorhinus semilunaris appeared to be negatively influenced by round goby establishment, diversity of nearshore native fish showed no evidence of dramatic decline attributable to round goby.

One measure commonly used for improving water quality in open waters is biomanipulation of fish stock. Previous studies in shallow lakes confirm the importance of reducing cyprinid populations for increasing zooplankton. Empirical results from reservoirs, however, are scarce. The aim of this study was to evaluate the effect of an intensive reduction of cyprinid fishes and perch fry density between 2009 and 2013 on water quality in the Hamry drinking water reservoir. The abundance of common bream broodstock decreased by around 12 000 individuals over the five years, and biomass dropped below 20 kg.ha-1. Stocking with pike and zander proved less effective. In reservoirs similar to Hamry, reduction of bream biomass can be a highly effective means of increasing filtering zooplankton biomass. Phytoplankton dynamics, however, appear to depend on additional factors, with nutrient loading (especially phosphorus) probably playing the primary role.