Heavy metal accumulation of Danube river aquatic plants — indication of chemical contamination

In this paper, the ecological status of a section of the Danube River flowing through Serbia from Bezdan to Djerdap was evalutated. Using the chemical composition of water, sediment samples from the littoral zone and dominant aquatic macrophytes, the level of chemical pollution was ascertained. Chemical analyses of the water and sediment indicated that the tributaries flowing into the Danube significantly influenced the chemical load of the water and as a direct consequence, the sediment. The concentration of heavy metals including Cu, Mn and Cd found in plants of the Potamogeton genus, further indicated significant chemical pollution, establishing a clear link between the chemical composition of plant tissues and the chemical composition of water and sediment. This paper therefore describes how the chemical composition of aquatic plants can be used as a reliable indicator for heavy metal pollution of aquatic ecosystems.     

Nutrient uptake and benthic regeneration in Danube Delta Lakes

We investigated the nutrient uptake capacity of three lakes (Uzlina, Matita and Rosu) within the Danube Delta during high water level in June and low water level in September 1999. Special emphasis was placed on nutrient cycling at the sediment-water interface and on the self-purification capacity of the lakes in the Danube Delta. In order to estimate the nutrient uptake of selected lakes we present in this paper the results of water analyses, benthic flux chamber experiments and deck incubation experiments of ¹⁵N-labeled sediment cores at the inflow and the outlet of the lakes. The external input of dissolved inorganic nitrogen and silica into the lakes decreases with increasing distance to the main Danube branches whereas the total dissolved phosphorus input is independent of the hydrological distance to the main branches. The nutrient loading is highest in the inflow channels, and decreases towards the outflow of the lakes. In June, the uptake of NO₃ ^–, TDP and Si(OH)₄ in the lakes was higher than in September. In contrast, NH₄ ⁺ uptake was more intense in September, when benthic release was more intense as well. On average, about 76% of the external plus internal nitrogen and phosphorus input into the lakes was taken up by macrophytes and phytoplankton during the growing season, whereas the uptake of external nutrient input amounted to about 43%. The benthic release of ammonia and silica increases from June to September and indicates, that part of the nutrients taken up during the growing season might be released during winter. We estimate the net impact of the Delta on the nutrient reduction of the Danube during the growing season is about 4.3%, assuming 10% of the Danube water is flowing through the Delta.     

Observations on the population dynamics of Potamothrix hammoniensis (Michaelsen, 1901) (Tubificidae,Oligochaeta) in Lake Isacova in the Danube Delta

The Danube Delta is one of the widest wetland systems in Europe and Lake Isacova is one of the biggest lakes in the Delta. The oligochaete community in the Lake comprised 52% of the total benthic fauna and Potamothrix hammoniensis (Michaelsen, 1901) (Tubificidae, Oligochaeta) represented more than 90% of the oligochaetes sampled. During September 1991 – August 1994, the proportion of ovigerous individuals of P. hammoniensis represented less than 20% of the total population, with localised differences in the Lake occurring over time. Breeding and cocoon production in the study period started in March of the year, when the water temperature was about 7 °C, and lasted until May 1992 and June 1993, respectively. The newly hatched individuals reached the highest proportion in May. Although large individuals were found again by the end of September (1991) and the beginning of October (1993), no spermatozeugmata in their spermathecae or cocoons were observed during November and October, respectively. Member of this cohort bred and started cocoon deposition during the following spring, after about 11 months of maturation (from April to March). A second, less intensive period of breeding was recorded during August 1992 and this lasted until the beginning of September 1993. However, cocoons from this latter period did not show embryonic growth and hatching until the autumn floods bring oxygenated water to the bottom level. This resulted in a period of rapid development followed by the mass hatching of cocoons. These specimens needed a maximum of 9 month (from October to July) to reach sexual maturity.     

Importance of flood zones for nitrogen and phosphorus dynamics in the Danube Delta

The change of concentration of total reactive phosphorus (TRP) and dissolved inorganic nitrogen (DIN) was studied in the lower Danube river and in selected lakes situated in the wetland area of the Danube Delta. The differences Danube Delta in nutrient concentration in the river waters entering the delta and the delta in different sites (especially lakes) of the wetland area are considered to reflect retention in the system. The highest retention was found in periods of moderate and low water level when the surface-to-volume ratio of the lakes was high. In these periods the in-lake concentration of TRP and DIN could be as low as 11 and 23% of the values found in the inflowing river.     

Phytoplankton and submerged macrophytes in the aquatic ecosystems of the Danube Delta during the last decade

The dynamics are presented of the main primary producers in seven representative Danube Delta lakes. Generally, the primary productivity developed from the macrophyte-epiphyte complex towards the phytoplankton. Species changes occurred in the two main compartments with increasing relative abundance of colonial blue-green algae in the phytoplankton and in submerged macrophytes of species with a vertical growth strategy. These changes are linked to accelerated eutrophication of the lakes, with increased phosphorus loading and a reduction in N:P ratio.     

The significance of side-arm connectivity for carbon dynamics of the River Danube, Austria

1. Side‐arms connected to the main stem of the river are key areas for biogeochemical cycling in fluvial landscapes, exhibiting high rates of carbon processing. 2. This work focused on quantifying autochthonous and allochthonous carbon pools and, thereby, on comparing transport and transformation processes in a restored side‐arm system of the River Danube (Regelsbrunn). We established a carbon budget and quantified carbon processing from March to September 2003. In addition, data from previous studies during 1997 to 1999 were assessed. 3. Gross primary production (GPP) and community respiration were estimated by diel oxygen time curves and an oxygen mass balance. Plankton primary production was determined to estimate its contribution to GPP under different hydrological conditions. 4. Based on the degree of connectivity, three hydrological phases were differentiated. Most of the organic matter, dominated by allochthonous carbon, was transported in the main channel and through the side‐arm during floods, while at intermediate and low flows (and thus connectivity), transformation processes became more important and autochthonous carbon dominated the carbon pool. The side‐arm system functioned as a sink for particulate matter [total suspended solids and particulate organic carbon (POC)] and a source of dissolved organic carbon (DOC) and chlorophyll‐a. 5. Autochthonous primary production of 4.2 t C day^?1 in the side‐arm was equivalent to about 20% of the allochthonous inputs of 20 t C day^?1 (POC and DOC) entering the area at mean flow (1% of the discharge of the main channel). Pelagic photosynthesis was generally high at mean flow (1.3–3.8 g C m^?2 day^?1), and contributed up to 90% of system productivity. During long stagnant periods at low discharge, the side‐arm was controlled by biological processes and a shift from planktonic to benthic activity occurred (benthic primary production of 0.4–14 g C m^?2 day^?1). 6. The transformation of the organic matter that passes through the side‐arm under different hydrological conditions, points to the importance of these subsystems in contributing autochthonous carbon to the food web of the main channel.     

Macroinvertebrates along the Serbian section of the Danube River (stream km 1429–925)

Results of the investigation of the aquatic macroinvertebrate fauna along a 504 km stretch of the Danube River in Serbia are presented. A total of 74 macroinvertebrate taxa were observed during a 2001 survey. Oligochaeta and Mollusca were the principal components of the community with regard to species richness and abundance. Based on data on the qualitative composition of the macroinvertebrate fauna, a correspondence analysis divided the investigated stretch in three sectors — upper (Pannonian), Iron Gate sector and entrance sector to the Iron Gate stretch. The distribution patterns of certain species supported the division of sectors defined by correspondence analysis.     

A new procedure for comparing class boundaries of biological assessment methods: A case study from the Danube Basin

For the implementation of the European Water Framework Directive class boundaries of biological assessment methods need to be intercalibrated. The most commonly applied intercalibration approach of national river assessment methods in Europe requires data on near-natural reference sites; however, these data are generally scarce. We developed an alternative approach based on sites impacted by similar levels of disturbance and tested it with national assessment methods based on diatoms and benthic invertebrates from countries in the Danube River Basin. Using environmental variables we screened for sampling sites of at least good environmental status. Relations between different assessment methods were established by common metrics, and we standardized these metrics by “biological benchmarks” obtained from the screened datasets. This approach allows for intercalibration even if near-natural reference sites are absent; relatively few and easily available data are required.     

New records of non-indigenous gobies (Neogobius spp.) in the Austrian Danube

Beginning in the early 1990s, species of the genus Neogobius rapidly expanded their native range from the Black Sea and the lower reaches of the River Danube upstream to include the middle and upper Danube. In 2002, 83 rip‐rap sites at 25 locations along the Austrian section of the River Danube were sampled by catch per unit effort (CPUE) with electrofishing to assess goby distribution. Bighead goby N. kessleri was found in all locations and at 76% of these sites, it was the third‐most abundant species (17% of CPUE). Densities of Neogobius spp. were highest in industrial harbours (IH), whereas the native species European bullhead Cottus gobio was found primarily in the main channel and tubenose goby Proterorhinus marmoratus in non‐industrial harbours. The history of records and the present distribution of Neogobius spp. suggest that IHs are the main dispersal points.     

Spatial heterogeneity of spring phytoplankton in a large tropical reservoir: could mass effect homogenize the heterogeneity by species sorting?

Dredging Austrian Danube harbours becomes necessary to keep the navigation channel free from bed sediment and to insure safe navigation, especially after major floods. In most cases, the excavated substrate is deposited in the free-flowing river section in the vicinity of the mouth of a harbour. The effects of harbour dredging on the biota need to be studied to minimize possible outcomes that may endanger the ecological status of the river system in order to implement the aims of the European Water Framework Directive. This study presents the first quantitative information on the colonisation of benthic invertebrates with respect to Danube harbours. One sampling at Linz Harbour was conducted before dredging and four samplings were carried out after dredging. Four transects were documented in the affected river section. The study used an air-lift sampler and a grab sampler to investigate if and to what extent the benthic invertebrate community is disturbed. It found that the sediment dredging heavily affected the benthic invertebrates (decline of 82% of biomass), while the dumping of the material had no effects on the benthic Danube biota. As a rough estimate (conferring to a simple trend line analysis) the harbour bottom fauna would recover after 235 days.     

Colonization processes of diatoms on artificial substrates in the River Danube near Budapest (Hungary)

Periphyton was collected from sand-blasted slides exposed in the main channel of River Danube at Gö (1669 rkm). Samples were taken from 21 May to 23 November in 1984. Three to four days after placing the substrates into the river the first traces of an observable coating appeared. The formation of the algal coating in the Danube presented certain periodical features. From time to time, an essential decrease in the number of individuals could be observed, which generally coincided with changes in the struc- ture of the community. This could be traced well, by grouping the algae according to morphological categories and following the temporal changes affecting these categories. The early phase of the colo- nization process is dominated by relatively large araphid and biraphid diatoms. In the second phase small bi- and monoraphid species are dominant. Finally, the medium size mono- and biraphid algae are dominating.     

Assessing the efficiency of connectivity measures with regard to the EU-Water Framework Directive in a Danube-tributary system

The effect of restoring connectivity for fish by the construction of 11 fish ladders in the Pielach and Melk rivers, both tributaries to the Danube in Austria, was monitored using electric fishing and fish traps between 1999 and 2004. In order to assess the efficiency of connectivity rehabilitation measures pre- and post-project data combining electric fishing and trap catch data were analyzed by means of three fish-based assessment methods: a MUlti-Level concept for a Fish-based, river-type-specific Assessment of ecological integrity (MULFA), the Fish Index Austria (FIA) and the European Fish Index (EFI). The effect of adding qualitative trap catch data to electric fishing data on metrics and indices was also tested and the magnitude of the effect was related to the distance of the sites from the river mouth. The results clearly demonstrated the significant contribution of connectivity rehabilitation measures to the ecological integrity of rivers like the River Pielach where morphological conditions are good, whereas remaining channelization still limits the success of connectivity measures in the River Melk. Trap catch data were found to represent an essential source of additional information to assess the efficiency of connectivity measures shortly after their implementation. The negative correlations of the magnitude of the effect of different indices and metrics with the distance of assessment sites from river mouths obviously underline the importance of the river Danube as a source for the re-colonization process. While the indices tested were found to have limited ability to reflect short-term response of fish assemblages to continuum rehabilitation, guild metrics were able to detect improvements of the ecological status shortly after the implementation of connectivity measures. Six metrics showed significant differences between pre and post-project data reflecting the expected increase of the ecological integrity: (1) Fish Region Index (FRI; FIA, MULFA), (2) number of subdominant species and (3) number of flow-guilds (FIA), (4) number of type specific species (MULFA), (5) number of benthic species and (6) number of potamodromous species (EFI); the FRI differences were only significant when trap catch data were added. The EFI indicated a decline of ecological integrity through increases in the density of omnivorous species and the relative number of tolerant species as well as a decrease in the relative number of intolerant species. Significantly decreasing responses with the distance from the river mouth were documented by the EFI and MULFA-index, the FRI (FIA, MULFA), total biomass and for the number of type specific species (MULFA). Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

A Comparative Study of Two Corbicula Morphs (Bivalvia,Corbiculidae) Inhabiting River Danube

In Europe, at least two Corbicula morphs (generally referred as C. fluminea and C. fluminalis) are known. However, their taxonomic and systematic position is controversial. In order to ascertain their taxonomy and systematics, we studied their characteristics in River Danube. Morphometric analysis confirmed the existence of two distinguishable morphs. Based on mtCOI sequence, morph‐1 is identical to C. fluminea, while morph‐2 is an undefined species. According to the median joining analysis, morph‐1 belongs to the C. fluminea clade, whereas morph‐2 forms a distinct clade, which is known only from invasive populations. Differences were recorded in population structure and reproductive biology, too. It is probably correct to treat the two morphs as distinct taxa, but as long as the African and western Asian parts of the genus’ range are poorly studied, taxonomic position of morph‐2 remains unclear. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Long term functional changes within the Oligochaeta communities within the Danube River Delta,Romania

Within the Danube River delta's lakes the Oligochaeta communities comprise between 7.9% and 36.2% of the total biomass of benthic fauna. Their importance in energy flow at the ecosystem level changed in relation to fast trophic transition of all shallow lakes to the hypertrophic state. The parameters of the energy budget of the dominant populations and the potential production of benthivorous fish species assessed during 1976–1994 interval support this conclusion. P/B ratio, K ₁ and K ₂ coefficients assessed for both the 1976–1980 and 1991–1994 intervals revealed different functional patterns of response of Potamothrix hammoniensis (Michaelsen, 1901) and Limnodrilus hoffmeisteri (Claparede, 1862) to varying trophic conditions. Changes in communities structure, size of the constituent populations and their age distribution, as well as the quantity and quality of food supply and level of hypoxia at the sediment/water interface were the main factors that affected the role of these populations as energy carrier from the huge energy pool represented by sedimented organic carbon to benthivorous fish species.

     

The estimation of production and mortality ofBosmina longirostris (O. F. Müller) in the River Danube (Danubialia Hungarica,CIX)

Produced and eliminated biomass ofBosmina longirostris in the River Danube (Hungary) was estimated in 1982. An attempt was made to calculate the rate of immigration and mortality, taking into consideration the hydrological characteristics of the river. Production and immigration influence population change to about the same extent.     

Taxonomic status of salmonids in the Bulgarian stretch of the Danube River and their bionomic strategy

Analysis of salmonid fish samples from the Bulgarian stretch of the Danube resulted in the conclusion that this river segment is inhabited by the Black Sea salmon Salmo labrax Pallas, 1814 and not by the huchen Hucho hucho (Linnaeus, 1758), as originally believed. Occurrence of huchen in this stretch of the Danube River is improbable. It has also been suggested that the anadromous Black Sea salmon population does not exist. It remains to be determined if the salmon occurring in the Danube River and along the coast of the Turkish, Bulgarian, Romanian and Ukrainian coasts of the Black Sea represent the black trout, i.e. the brook form of the Black Sea salmon washed down from the tributaries or the specific potamodromous river form.     

Zooplankton abundance in the River Danube, Austria: the significance of inshore retention

1. Zooplankton density was examined in a free-flowing section of the River Danube, Austria. Spatial and temporal patterns were used to identify mechanisms regulating zooplankton dynamics. 2. Zooplankton abundance differed significantly between sampling sites. These differences were the result of variations in water residence time within the habitats. Inshore habitats with low flushing rates act as storage zones for zooplankton. 3. The hydrological conditions govern the number and size of storage zones. In the study area, the highest availability of storage zones was observed at medium water level. 4. Total zooplankton numbers, total rotifer numbers and the densities of the most abundant rotifer taxa were significantly positively related to the availability of adjacent storage zones. 5. The present authors propose that inshore retention capacity, and the hydrological processes and mechanisms controlling the extent of inshore retention may be of major significance in regulated rivers with high water velocities, preventing substantial zooplankton growth in the main channel.     

Mussels as bioindicators of trace metal pollution in the Danube Delta of Romania

Specimens of mussels species Anodonta anatina, Unio pictorum, U. tumidus and surfical sediment samples were collected in the summers of 1994 and 1995 from twelve lakes in the Danube Delta, Romania. Whole mussel tissues were analyzed for metals (Ag, As, Cd, Co, Cu, Cr, Ni, Pb, Se and Zn), and sediment samples were subjected to weak acid extraction and to a sequential extraction procedure, and analyzed for Cr, Cu, Ni and Zn. Total mean Cu and Ag concentrations were statistically greater in Unio species than in Anodonta species, but other metal levels did not significantly differ between mussel species. There was a ten fold variation for Cd and Pb and five fold variation for Zn, Ag and Co concentrations in mussels among lakes. Mean concentrations of As, Cu, Se and Zn in mussel tissues varied two fold among lakes. Zn, Co and Cd concentrations in mussels were significantly negatively correlated with distance gradients from the origin of the Delta and with distance from main channels of the Danube. Other metal concentrations in mussels did not correlate with either distance gradient. Trace metal levels in mussel tissues were correlated with those in weak acid and in sequential extractions of sediment. Zn concentrations in the carbonate fraction was the best predictor of Zn concentrations in mussels. Cu and Ni levels in the organic matter and sulfide fraction were the best estimators of Cu and Ni concentrations in mussels. This revised version was published online in July 2006 with corrections to the Cover Date.     

mtDNA sequence data supports an Asian ancestry and single introduction of the common carp into the Danube Basin

Common carp Cyprinus carpio ( n =24) from the upper Danube (wild or feral forms, scaleless varieties) as well as Japanese koi shared the identical mtDNA haplotype (565 bp). In contrast, Amur River carp ( n =5) exhibited unique haplotypes, 1–12 bp divergent from the Danubian one. These results support an Asian origin and single introduction or domestication event leading to the present diversity of common carp in Europe, as well as the ornamental Japanese koi.