A Comprehensive Monitoring and Assessment Survey on the Danube - page 02

Fish

The Core Team sampled the near-shore area by electric fishing (EN, 2003) and the river bottom using an electrified benthic frame trawl net. The 'electrified benthic frame trawl' consisted of a stainless steel frame (2m × 1m) with a 5 metre-long drift net. Weighted metal wheels were attached to the frame to keep the device just above the bottom, to prevent the net from filling with material.

In total, 139,866 individuals representing 67 fish taxa were caught which means that the Danube River is an ecosystem with a wide range of fish species. Two species, Alburnus alburnus and Neogobius melanostomus (or Round goby), dominated with 46% and 26% of the total catch, respectively. The electrified benthic frame trawl added valuable information which would have remained hidden using only shoreline surveys.

For example, it was able to detect the monkey goby (Neogobius fluviatilis) for the first time in the Austrian section of the River Danube. It also revealed the common occurrence and high abundance of Zingel species, especially of Zingel streber which occurred at 16 sampling sites with 127 individuals (the JDS2, without the electrified benthic frame trawl, could not prove the occurrence of Z. streber in the Hungarian river section of the Danube).

The three applied national WFD assessment indices of JDS3 indicate a call for action as 50 % of the sites according to FIA, 72,1 % (EFI) and 94,7 % (FIS) respectively show a value worse than "good" and do not meet the requirements of the WFD. In the Upper Danube, fish results mainly reflected hydromorphological alterations and damming as the most important human impacts, as well as the lack of connectivity. The excessive use of waterpower in the Upper Danube, which leads to degraded aquatic habitats, can be detected easily by the absence of sensitive fish species and certain age classes. The Lower Danube seems to be influenced by professional and recreational fishing and poaching.

Zooplankton

In total, in the Danube River and its tributaries, 149 zooplankton taxa were discovered – a bit higher than the total in JDS1 and JDS2. These included 107 Rotifera, 33 Cladocera and 9 Copepoda. Along the Danube River, Rotifera and Copepoda were most numerous.

In the Danube River, the density of zooplankton, influenced often by water velocity and turbidity, varied substantially: an increase in density was observed in the slow-flowing Middle Danube reach.

Invasive Alien Species

Based on the results of JDS3, the Danube River is significantly exposed to non-native species. 25 neophytes (4 aquatic), 34 non-native aquatic macroinvertebrates and 12 non-native fish species were recorded during the JDS3.

The level of biocontamination of the Danube River was estimated as moderate to high, with higher levels for the Upper (high to severe biocontamination) and Middle Danube (moderate to high biocontamination), in comparison to the Lower Danube (low biocontamination).

Comparison with the results of previous Danube Surveys clearly showed a constant impact of invasive alien species on native biota and a considerable increase of the number of non-native aquatic macroinvertebrate species. As a specific example the allochthonous Neogobius fish species can be given which were found in high or even dominating abundance along the rip-rap protected banks in the upper and middle course of the Danube.

Microbiology

Faecal pollution and microbiological contamination from anthropogenic sources have been shown to be a crucial problem throughout the Danube River Basin (Kirschner et al. 2009). Therefore, a thorough analysis of river microbiology has been always on the scientific programme of the Joint Danube Surveys bringing an added value to the survey findings.

Bacterial Faecal Indicators

Escherichia coli and intestinal enterococci are used worldwide as sensitive indicators for the assessment of faecal pollution in the aquatic environment. Faecal pollution can be caused by point sources like discharges of sewage from human sources or livestock enterprises and by non-point sources like pasture, urban and agricultural run-off or water fowl. Fourty-two JDSsampling points (35 Danube samples and 7 tributaries/branches) out of 186 were classified as critically (34), strongly (5) or excessively (3) polluted by Bacterial Faecal Indicators. As hot spots of excessive pollution the tributaries Arges and the Russenski Lom were identified. The highest contamination in the Danube with excessive pollution levels was measured at Kelheim (DE), in the uppermost stretch, with otherwise little to moderate faecal pollution levels. Other hot-spots of faecal pollution in the Danube (strong pollution or high critical pollution levels) were the stretch between Novi Sad and downstream Belgrade (SRB), downstream Budapest (HU, right side) and Dunaföldvar (HU, midstream), downstream Zimnicea (RO, left side) and downstream Arges (RO, left side). A comparison with data from JDS2 revealed very similar median values for both faecal indicators E.coli and Enterococci.

 

Although a slight tendency towards lower values was observed in the Danube, an improvement of the microbiological water quality cannot be deduced from this snap-shot data as it would require long-term observations.

Microbial Source Tracking

The results of the microbial source tracking investigation of JDS3 samples demonstrate quite clearly that human faecal impact is the main driver for faecal pollution levels in the Danube and its major tributaries. Human-associated genetic faecal marker levels could be predicted by the bacterial standard indicator variations, such as E.coli, to a high extent.

Antibiotic Resistance

Antibiotic resistant bacteria are known almost since the use of antibiotics has started. But in recent years the spread of multi-resistance, outside the hospital environment, enhanced this problem. One possible transmission route is via waste water and the water environment. More than 50% of the Escherichia coli isolated during JDS3 showed a modified resistance pattern, but most of them (47 isolates) were only resistant against one or two tested antibiotics. Hence, multi-resistant isolates (with resistance in three or more antibiotic classes) were rare. The frequency of multi-resistance was elevated at the downstream sampling points, (including isolates with resistance against up to seven tested antibiotics). This may reflect the more problematic resistance situation in clinical settings in the downstream countries or could also refer to a cumulative effect. All Escherichia coli isolates were susceptible to last-line antibiotics.

Microbial Ecology

Heterotrophic bacterial production rates and the concentration of large bacterial cells, representing the active part of the bacterial community were significantly inter-correlated and followed a similar trend. The inflow of polluted tributaries and wastewater from point sources was partly reflected in the Danube at the respective river sides and partly contributed to the observed trends. The patterns of bacterial production observed in 2013 were similar to the ones observed for JDS2 in 2007. However, with the exception of only a few samples, heterotrophic production rates in 2007 were lower than in 2013.

 

Results of Chemical Monitoring

Physico-Chemical Quality Elements

Water temperature measured in the Danube River and in selected major tributaries followed the typical pattern for the timing of the survey (August – September), with a larger variation range in tributaries than in the Danube. The longitudinal distribution of conductivity in the Danube River showed a strong decrease in the upper stretch, followed by a constant profile towards the middle and lower stretches. The dilution effect along the Danube was demonstrated by the significant correlation coefficient of conductivity with water discharge values. pH and dissolved oxygen content demonstrated a good balance between primary production and decomposition of organic matter, with most of the oxygen saturation levels situated around the equilibrium value. Several local depletions were found in specific areas (dammed Rackeve-Soroksar side arm, the Iron Gates reservoir) and two tributaries (Tisa and Velika Morava).

Total Nitrogen presented a strong decreasing profile from upper to lower stretch of the Danube (Figure 4), and it was significantly negatively correlated with water discharge. The typical lower profile was noticed in the Iron Gates reservoir, due to the denitrification process from this area. Most of the tributaries presented levels similar to those in the Danube, but elevated concentrations were found in the Timok, Russenski Lom and Arges. No systematic trend in Total Phosphorous concentrations along the Danube River was found; still, a slight decreasing line appeared in the lower stretch, more pronounced in the Iron Gates reservoir area, due to the retention of the suspended material on which this nutrient form is adsorbed. The Total Nitrogen and Phosphorous levels measured in the three arms of the Danube Delta come in good agreement to previous findings which showed that the contribution of the Danube Delta in nutrients retention is negligible, because most of the Danube water passes directly to the Black Sea, almost not reaching the Delta itself. N-ammonium and N-nitrites showed levels below the limit of quantification in most of the sampling sites. Compared with JDS1 and JDS2 results, Total Nitrogen and Total Phosphorous concentrations measured in the Danube River during JDS3 were lower.

The ecological indication given by the general physico-chemical quality elements was assessed based on the intervals for high/good and good/moderate ecological classes as resulted from the environmental quality standards/guiding values reported by the Danube countries. The general view is that most of the sampling sites located on the Danube River belong to either "high" or "good" class, except for the dammed side arm Rackeve-Soroksar and the Iron Gates reservoir area, which fall into the "moderate" class due to the oxygen depletion. "Moderate" class is also present in several tributaries (Morava, Tisa, Velika Morava, Jantra, Russenski Lom and Arges), caused by low oxygen saturation and dissolved nutrients forms.

 

Fig04
Figure 4: Total Nitrogen concentrations in water samples during JDS3 in the Danube River and selected tributaries.