Oligochaeta community of the main Serbian waterways - Atanackovic et al. page 04

Discussion

During this investigation, a rich Oligochaeta community was detected, with 52 identified taxa, belonging to 9 families. A total of 37 taxa were recorded in the Danube, while fewer species were detected in its tributaries (Tisa 27, Sava 21), mostly due to a smaller number of examined sites.
Aquatic worms were one of the principal members of the macroinvertebrate community in the investigated rivers, with regard to species richness. In previous investigations of these rivers, Oligochaeta were also recognized as one of the most distinguished members of the macroinvertebrate community (Djukic and Karaman, 1994; Jakovcev, 1987, 1988; Jakovcev-Todorovic et al., 2005; JDS-ITR National Report, 2002; JDS-ITR National Tisa Report, 2002; Jurca and Miljanović, 2006; Martinovic-Vitanovic et al., 1999, 2006; Paunovic et al., 2005, 2007, 2008; Simic et al., 1997; Simic and Simic, 2004; Simonovic et al., 2010; Vojinović-Miloradov et al., 2003).
Oligochaeta were found to be one of the principal members of the benthic community, not only in the Serbian stretch of the Danube, but also along a considerable portion of the river (Csányi and Paunović, 2006; Elexová, 1998; Russev,1970; Sommerwerk et al., 2009), as well as in the investigated tributaries: the Sava (Mihaljevic et al., 1998; Paunovic, 2004; Paunovic et al., 2008) and the Tisa (Csanyi, 2002).
Most of the observed species are typical of potamon-type rivers in the region and well adapted to high organic load and soft sediment. Representatives of the Tubificidae family, adapted to high organic load (Moog, 2002), which are taxa characteristic of a soft substrate (pelophilous taxa, Timm et al., 2001), were observed in all of the investigated river stretches: Branchiura sowerbyi (alien invasive species), Limnodrilus hoffmeisteri, L. claparedeanus, L. udekemianus, L. profundicola, Potamothrix hammoniensis, Psammoryctides albicola and Tubifex tubifex. Apart from pelophilous species, taxa characteristic of other substrates were found to be the psammophilous Psammoryctides barbatus (Danube and Tisa rivers) or phytophilous representatives of the Naididae family.
A similar community structure was observed in the Slovakian stretch of the Danube (Elexová, 1998; Sommerwerk et al., 2009; Šporka and Nagy, 1998). The thermophilous species A. furcatus was recorded in the Serbian stretch of the Danube (Timm, 1999), which could be indicative of a local (intra-microhabitat) disturbance, resulting from altered hydrological conditions along a considerable portion of the stretch due to damming, and the subsequent change in thermal conditions.
Most of the macroinvertebrates in the Serbian stretch of the Sava River have been observed in the Belgrade area (Jakovcev, 1988, 1989, 1991; Martinovic-Vitanovic et al., 1999; Paunovic, 2004). The identified Oligochaeta community is typical of large lowland rivers in the region. The detection of H. gordioides in the Sava River is especially interesting because this species is not typical of lowland rivers (Csanyi, 2002; Jakovcev-Todorovic et al., 2006). Šporka (1998) reported finding this species in a gravel substratum in Slovak/Hungarian portions of the Danube River. In Serbia, H. gordioides has previously been collected only in highland streams (Kalafatic et al., 1999; Paunovic et al., 2003; Simić et al., 1993).
Several rheophilous species of Ponto-Caspian origin have spread upstream, most likely with ships: Isochaetides michaelseni, P. moldaviensis, and Psammoryctides moravicus. In addition, Paranais frici, a brackish-water species (Timm et al., 2001),
was found in the Danube and the Sava. These findings support the belief that the Ponto-Caspian area is an important spreading centre for alien species in Europe (Arbaciauskas et al., 2008; Panov et al., 2009).
Although the investigated stretches are considered to be of similar water types (belonging to large lowland rivers), certain differences within the Oligochaeta species composition have been identified. The positions of the investigated stretches shown in Figure 2 suggest that the Tisa River is distinct in some characteristics associated with the composition of the Oligochaeta fauna. Representatives of the Lumbriculidae family were recorded in the Tisa River only. The finding of Trichodrilus sp. was interesting, because it is comprised of taxa typical of cold-water streams (Timm et al., 2001).
The greater species richness recorded in the Danube River could be attributed to different lengths of the investigated river stretches, and consequently different numbers of studied sampling sites.
Further, 11 taxa not found in the other rivers are present in the Tisa River, suggesting a certain peculiarity of the investigated stretch. This was confirmed by previous investigations; P. moldaviensis was found to be one of the principal taxa within the benthic community of the Tisa River (Vojinović-Miloradov et al., 2003).
The difference between the stretches explained by CA (Figure 2) and correlation analyses is mostly generated by the presence of rare taxa. The species that were recorded in only one out of three investigated stretches (SG 1, 4 and 7) comprise species that are, according to previous investigations (JDS-ITR National Report, 2002; JDS-ITR National Tisa Report, 2002; Jurca and Miljanović, 2006; Paunovic et al., 2005, 2007; Vojinović-Miloradov et al., 2003), rare (e.g. Trichodrilus sp., Tubifex montanus, Pepsidrilus pusillus), or not previously recorded (T. montanus, H. gordioides.) in the investigated area.

Uncommon taxa for this river type, and also rare in general (Timm, 2009), have reached the Tisa River from its Carpathian Basin tributaries. The finding of these taxa as well is indicative of the influence of the tributaries which, according to our data, is more pronounced in the case of the Tisa River, compared to the Danube and the Sava.

Conclusions

Oligochaetes are one of the principal members of the macroinvertebrate community in Serbia’s major watercourses with regard to species richness. The observed community is typical of large lowland rivers in the region. The recorded species, dominated by the Tubificidae family, are well adapted to high organic load and soft sediment. The observed differences in species composition between the investigated rivers are caused by the presence of uncommon and rare taxa. Thus, the Tisa River is under the influence of the Carpathian Basin, and the Danube is under the influence of the Ponto-Caspian region. This paper indicates that Oligochaeta, which is one of the principal members of macroinvertebrate community, should be part of ecological status assessment.

Acknowledgements

The work reported in this paper is supported by the Ministry of Science and Technological Development of the Republic of Serbia, Project No. TP 037009.