5 Ostracoda (Crustacea) of the River Bed in the Lower Course...Pdf

ZESZYTY NAUKOWE UNIWERSYTETU SZCZECIŃSKIEGO NR 676 ACTA BIOLOGICA 18 2011

AGNIESZKA SZLAUER-ŁUKASZEWSKA* BEATA KOWALUK-JAGIELSKA*1

OSTRACODA (CRUSTACEA) OF THE RIVER BED IN THE LOWER COURSE OF A LARGE LOWLAND RIVER SYSTEM EXEMPLIFIED BY THE ODER RIVER (POLAND)

Abstract The study was carried out in the main branches of the Lower Oder River, differentiated in terms of sediments composition. The aim was performing a comparative studies on the differences in Ostracoda fauna inhabiting various types of sediments. Following bottom sediments were distinguished: hard, sapropel/hard, sapropel and Chironomidae mat. The packet of CANOCO v.4.5 programs was used to investigate the interdependence between the species composition and environmental parameters. Eighteen taxa were found of which 16 were identiﬁ ed to the species level. It was a comparatively high number considering the fact that the samples were collected exclusively from the benthic zone and solely from the main river bed, without ﬂ oodplain. Physocypria kraepelini was an eudominant and Darwinula stevensoni and Cypria ophtalmica were dominants. The samples collected from the sapropel were characterised by the highest density. The case of hard sediments the status of domination was retained as above, but there appeared a new dominant, i.e. Potamocypris unicaudata. The most unique structure of domination was observed in the case of Chironomidae mat, with Limnocythere inopinata as an eudominant and Cypridopsis vidua and P. unicaudata as dominants. As for the species diversity, the hard sediments were characterised by the highest value of Shannon-Wiener Diversity Index, the sapropel/hard sediment was the lowest. The type of river bottom to a great extent affects the density and taxonomic composition of the Ostracoda in rivers.

1 * Department of Invertebrate Zoology and Limnology, University of Szczecin, ul. Wąska 13, 71-415 Szczecin, Poland, e-mail: [email protected]. 86 Agnieszka Szlauer-Łukaszewska, Beata Kowaluk-Jagielska

The waters of the Oder River, rich in oxygen, provide favourable conditions for the development of ostracods even on the surface of sapropel sediment.

Keywords: sediment, habitat, oxygen, river bed, Ostracoda

Introduction In the investigated region, the Oder River is situated in the Toruń-Eberswalde Proglacial Stream Valley. In the 19th and 20th centuries there were conducted numerous works in order to improve the navigability of the river and facilitate agricultural activity in the area of the river valley (Orlewicz & Mroziński 2002). Presently, in the town of Widuchowa the Oder River branches out into the Eastern Oder River and the Western Oder River. The Eastern Oder transports ca. 76% (annual average) and the Western Oder ca. 24% of the Oder River water volume. The two branches of the Oder River are connected by channels situated in Międzyodrze, and the Western Oder is fed with the water from the Eastern Oder (Mikulski & Ostapowska-Bojanowicz 1965) (Fig. 1). The differences in water fl ow in both river branches have a direct infl uence over the types of accumulated bottom sediments. For this reason the part of the river in this region is a convenient ground for comparative studies on the differences in invertebrate fauna inhabiting various types of sediments found in a river bed. The fauna of ostracods in this region has not been investigated so far and the only study that has been conducted in the central and lower part of the Oder River is that by Szlauer-Łukaszewska (2008), which focused on the migration of ostracods in the fl ood zone. Mezquita et al. (1999A and 1999B) studied the ecology and distribution of ostracods associated with fl owing waters, and the latter of the two publications focused on a polluted river. Numerous studies on ostracods have focused on the Rhone River in France: Creuzé des Châtetelliers & Marmonier (1993) investigated the ecology of benthic and interstitial ostracods, Marmonier & Creuzé des Châtetelliers (1992) investigated their biogeography and Claret et al. (1999) studied the effect of management works in the fl oodplain of this river. A study by Kiss (2006) was devoted to the Ostracoda, Cladocera and Copepoda assemblages in different side-arms of the Danube fl oodplain. Higuti et al. (2009) investigated the biodiversity of ostracods in various habitats of the alluvial valley of the upper Parana. Ostracoda (Crustacea) of the river bed in the lower course... 87

Fig. 1. Location of the study transects along the Oder River in the area of the Lower Oder Valley

The aim of the present study was to investigate the fauna of ostracods in the river bed in the lower course of a large lowland river system and to identify the interdependence between the type of bottom sediments and the taxonomic composition of the Ostracoda. 88 Agnieszka Szlauer-Łukaszewska, Beata Kowaluk-Jagielska

Material and methods Sample collection and processing The following transects have been established on the Oder River (Fig. 1): A. The Oder in Widuchowa, before it branches out into the Eastern Oder and the Western Oder. B. The Eastern Oder in Widuchowa. C. The Eastern Oder in Gryfi no. D. The Eastern Oder in Żydowce. E. The Western Oder in Widuchowa. F. The Western Oder in Mescherin. G. The Western Oder in Dziewoklicz. The samples were collected in the end of July 2007, during the potentially worst aerobic conditions. Individual transects were positioned transversely in relation to the river bed. From 5 to 7 samples were collected in each transect, depending on the width of the river and the differentiation of the river bed in the transect area. Water depth near the bank ranged from 1.5 to 3 m, in the middle of the river from 7 to 10 m. The width of the river ranged from 140 to 200 m. Samples were collected using Petersen grab, from the surface of 256 cm2. The samples were rinsed on the spot using a 50 μm mesh net, in order to eliminate the fi nest silt fractions. After the preliminary cleaning, the samples were preserved in 96% ethanol. In the laboratory, the ostracod specimens were selected out of the sediment, using a 4 mm mesh sieve in order to remove coarser sediments, the decantation method to separate sand, and the bubbling technique described by Higgins (1964) to move the Ostracoda onto the surface membrane. The Ostracoda were then sucked from the surface membrane using an exhauster, separated from the sediments on a Petri dish under a Zeiss Discovery V12 stereoscopic microscope and fi nally identifi ed. For permanent slides the soft parts of ostracod bodies were embedded in Hydro-Matrix. The identifi cations were based on determination keys proposed by (Meisch 2000) and Sywula (1974). The Ostracoda collected for the purpose of this study remain in the collection of the author no 1. Following bottom sediments were distinguished: A. Hard, including the sediments containing sand, gravel, shell hash and peat. B. Sapropel/hard, a mixture of sapropel and hard sediment (as above). C. Sapropel, black-coloured silt sediment producing sulphur hydrogen; an oxygen-free sediment. Ostracoda (Crustacea) of the river bed in the lower course... 89

D. Chironomidae mat, differentiated because of its signiﬁ cant speciﬁ city. On sand sediment the larvae of chironomids produced a mat which was 2–10 cm thick.

Analysis of data The packet of CANOCO v.4.5 (ter Braak & Simlauer 2002) programs was used to investigate the interdependence between the species composition and environmental parameters. DCA conformity analysis (Detrended Correspondence Analysis) was used to determine the structure of data and select the type of analysis to investigate the distribution of species with respect to correspondence analysis of environmental variables (Jongman et al. 1987). The interdependence between the species composition of the Ostracoda and the type of sediment was investigated using the RDA (redundancy analysis). In RDA analyses the transformation ln (n + 1) was applied to species data and the forward selection of variables t was conducted (with the Monte Carlo permutation test, for 499 permutations), in order to determine how particular environmental variables affected the Ostracoda species richness. The density, dominance, frequency and Shannon-Wiener (H’) species diversity coeffi cients were calculated. Rombach’s (1999) classifi cation was adopted while calculating the dominance coeffi cient, differentiating: eudominants (32–100%), dominants (10–31%), subdominants (0.32–9%), recedents (1–3.1%), subrecedents (0.32–0.99%), and sporadic species (< 0.31%). Trojan’s (1975) classifi cation was adopted for calculating the frequency coeffi cient, differentiating: euconstants (100–76%), constants (75–51%), accessory species (50–26%), and incidental species (25–0%).

Results In general, 43 samples were studied, in which the total number of 15656 Ostracoda specimens were discovered (Table 1). Eighteen taxa were found of which 16 were identiﬁ ed down to the species level (Table 1). In the Oder River, in the part before it branches out into the Western Oder and the Eastern Oder (i.e. in Widuchowa) the hard sediments and Chironomidae mat were prevalent. In the Eastern Oder the hard sediments prevailed. As for the Western Oder, from the spot where it branches out from the Oder towards its mouth, in the transect E immediately behind the gate sapropel sediment accumulated, in the transect F – hard sediment and in the transect E – sapropel/hard sediment. 90 Agnieszka Szlauer-Łukaszewska, Beata Kowaluk-Jagielska , C- SD, 2 sapropel/hard sapropel mat Chironomidae hard A B C D EF A B C D E F A B C D EF A B C D E F % Frequency % Dominance max SD 2 mean mean density indiv./m indiv. 1.69 1.89 1.39 1.54 1.54 1916390 44567 633333 66845 3714 34767 981286 70092 633333 709199 101314 334827 159061 39765 116250 total numberof Abrev. Herp 28331 659 4254 27902 1 9 0 0 0 0 0 0 430 31 84 313 0 21 27902 3986 10546 27902 4 14 0 0 0 0 0 0 Il sp. 1168 27 70 313 0 19 117 7 20 78 0 11 816 58 98 313 0 36 234 33 89 234 0 14 0 0 0 0 0 0 Pseu 66452 1545 6988 45833 3 60 2383 132 309 1289 4 50 52786 3770 12129 45833 5 71 9593 1370 2060 5580 1 57 1690 422 573 1260 1 75 Il braIl 2706 63 272 1540 0 9 228 13 40 156 0 11 0 0 0 0 0 0 0 0 0 0 0 0 2478 619 756 1540 2 50 Il decIl 6841 159 536 3125 0 26 597 33 96 308 1 17 5776 413 898 3125 1 43 313 45 118 313 0 14 156 39 78 156 0 25 Br retBr 78 2 12 78 0 2 0 0 0 0 0 0 78 6 21 78 0 7 0 0 0 0 0 0 0 0 0 0 0 0 Li inoLi 105000 2442 11548 75938 5 51 3398 189 495 2070 5 39 17578 1256 1686 5000 2 71 7813 1116 1748 4688 1 43 76211 19053 37923 75938 48 50 Fa lev 6225 145 850 5580 0 16 137 8 20 78 0 17 195 14 42 156 0 14 5893 842 2093 5580 1 29 0 0 0 0 0 0 Is bea 78 2 12 78 0 2 0 0 0 0 0 0 78 6 21 78 0 7 0 0 0 0 0 0 0 0 0 0 0 0 Da ste 438284 10193 34001 206473 23 77 21410 1189 3027 12305 32 61 186050 13289 24175 92500 19 100 222760 31823 77063 206473 31 86 8063 2016 3320 6930 5 50 Il monIl 500 12 38 208 0 12 116 6 19 72 0 11 384 27 61 208 0 21 0 0 0 0 0 0 0 0 0 0 0 0 Po uni 44913 1044 3770 24688 2 77 7856 436 695 2930 12 83 5738 410 485 1484 1 79 2578 368 575 1563 0 57 28741 7185 11794 24688 18 75 Ph kre 815932 18975 69540 423333 43 77 13258 737 1777 6875 20 56 550382 39313 110918 423333 56 100 246495 35214 68629 189732 35 86 5796 1449 1194 2813 4 75 Cy vid 65843 1531 3897 16741 3 74 3930 218 401 1289 6 72 11875 848 1429 5000 1 79 21975 3139 6203 16741 3 57 28063 7016 8198 16696 18 100 Ca neg 3477 81 392 2500 0 9 0 0 0 0 0 0 2539 181 667 2500 0 14 938 134 246 625 0 29 0 0 0 0 0 0 Cy oph 274009 6372 16200 89286 14 74 10641 591 1482 5859 16 67 105000 7500 10915 33333 11 86 153137 21877 34201 89286 22 86 5231 1308 2104 4410 3 50 ypb030205060 000000000000000 pub2003200220152006000 Cy Ca neg j 56534 1315 4099 26667 3 67 2754 153 402 1719 4 44 41580 2970 6927 26667 4 93 9568 1367 1451 3125 1 71 2632 658 865 1890 2 75 G.W . (Paris (Brady Masi, Masi, (O.F. (O.F. (Jurine, Sars, 1887 Sars, 1890 O. F. Müller, F. O. D – maximum density, E – dominance, F frequency) D – maximum density, 1905 1776 1820) 1920) Total Species (Hirschmann, 1912) Müller, 1903 (Baird, 1843) Müller, 1776) Schäfer, 1943 (Norman, 1862) (Zaddach, 1844) (Zaddach, Ilyocypris sp. juv. sp. Ilyocypris Herpetocypris sp. & Robertson, 1870) H (Shanon-Weaver) H Pseudocandona juv Pseudocandona Fabaeformiscandona Fabaeformiscandona Ilyocypris monstrifica monstrifica Ilyocypris Candona neglecta juv Candona neglecta Cypridopsis vidua Cypridopsis Limnocythere inopinata Limnocythere Potamocypris unicaudata Ilyocypris decipiens Ilyocypris Cypria ophtalmica ophtalmica Cypria Cypris pubera pubera Cypris Ilyocypris bradyi bradyi Ilyocypris Bradleystrandesia reticulta reticulta Bradleystrandesia Isocypris beauchampi beauchampi Isocypris levanderi Darwinula stevensoni Candona neglecta Candona neglecta Physocypria kraepelini Physocypria 9 8 5 6 7 1 2 3 4 11 10 12 13 14 15 16 18 17 No Table 1. – total number of individuals, B –average density indiv./m Numerical data regarding the total collected material (A Table Ostracoda (Crustacea) of the river bed in the lower course... 91

A DCA analysis for the Ostracoda samples from the Oder River bed showed gradient length represented by the ﬁ rst ordination axis amounting to 2.78 SD. (Table 2). Such a result allowed for the application of RDA-type linear ordination techniques in order to determine the relationship between the species composition of the Ostracoda and various types of river sediment composition. Four ordination axes accounted for 50.3% of the total variation of species composition.

Table 2. Summary of DCA for samples from River Odra bed

Axes 1 2 3 4 Total inertia Eigenvalues: 0.659 0.146 0.035 0.022 1.714 Lengths of gradient: 2.780 1.855 1.819 2.522 Cumulative percentage variance of species data: 38.4 47.0 49.0 50.3 Sum of all eigenvalues: 1.714

The samples on the DCA diagram formed two clear clusters: in the right side of the diagram there could be found those samples in which sapropel dominated, and on the right side those samples in which hard sediments dominated, such as sand, gravel, and shell hash (Fig. 2). The obtained results of the RDS analysis for samples show that the variables applied in the ordination account for about 18% of the total species diversity of the Ostracoda (Table 3). The results of the forward selection of environmental variables showed that only one environmental axis, i.e. sapropel (p = 0.002) was signifi cant and accounted for the interdependence between the taxonomic composition of the Ostracoda and their habitat (Table 4). The RDA ordination diagram displays correlations between particular ostracod species and particular types of the river bed sediment composition (Fig. 3). It is signifi cant to the relative position of each axis. Between the axes: peat, sand and shell hash is the acute angle, which suggests the mutual similarity of these environmental factors, which, moreover, have been classifi ed as a hard sediments. The remaining axes form an obtuse angle between them which shows the diversity represented by environmental factors. The strongest positive correlation with reference to the sapropel axis can be observed in the case of the following ostracod species: Darwinula stevensoni, Physocypria kraepelini, Cypria ophtalmica and young individuals of Candona neglecta. A negative correlation with reference to the 92 Agnieszka Szlauer-Łukaszewska, Beata Kowaluk-Jagielska sapropel axis can be observed in the case of Ilyocypris bradyi and Potamocypris unicaudata and is tantamount to the association of these species with the remaining environmental axes, characterised by hard sediment (Fig. 3).

Fig. 2. DCA ordination diagram of samples from the Oder River bed. Sediments categories: black dot – sapropel, white dot – sapropel-hard, white square-hard sediments, X- Chironomidae mat). Abbreviations of sediments composition: san- sand, sap- sapropel, pea- peat, she- shell hash, gra – gravel, Chir – Chironomidae mat Ostracoda (Crustacea) of the river bed in the lower course... 93

Fig. 3. RDA ordination diagram of Ostracoda species ordinated with composition of sediments as an environmental factors. Arrow abbrev. : saprop – sapropel, shell – hash shell, Chiro – Chironomidae mat

Table 3. Summary of RDA results for composition of sediments from River Odra bed

Axes 1 2 3 4 Total variance Eigenvalues: 0.142 0.021 0.012 0.006 1.000 Species-environment correlations: 0.612 0.465 0.363 0.401 Cumulative percentage variance -of species data: 14.2 16.2 17.4 18.1 -of species-environment relation: 78.4 89.9 96.4 100.0 Sum of all eigenvalues 1.000 Sum of all canonical eigenvalues 0.181 94 Agnieszka Szlauer-Łukaszewska, Beata Kowaluk-Jagielska

Table 4. Forward selection results with test of signiﬁ cance for composition of sediments from River Odra bed

Variable LambdaA P F sapropel 0.14 0.002 6.47 Chironomidae mat 0.02 0.340 1.07 shell hash 0.01 0.646 0.66 sand 0.01 0.930 0.36

Taking into account the average values for samples collected from all habitat types, the total average density amounted to 44 567 indiv./m2, with Physocypria kraepelini as an eudominant and such species as Darwinula stevensoni and Cypria ophtalmica as dominants; with regard to frequency of occurrence these species were euconstants (Table 1). The domination structure was similar for various types of sediment and usually the dominance of the three above-mentioned species was signiﬁ cant. The samples collected from the sapropel sediment were characterised by the highest average density of 101 314 indiv./m2, with a similar domination structure to the one described above. The domination of D. stevensoni was the most remarkable; the species obtained the eudominant status being characterised by record species density of 31 826 indiv./m2. In the remaining types of sediment the average density was lower, and amounted to 70 092 indiv./m2 for sapropel/hard type, 39 765indiv./m2 for Chironomidae mat and only 3 714 for the hard sediment. In the case of the sapropel/hard type there was observeda strong domination of P. kraepelini (56%). The structure of domination was slightly different in the case of hard sediment and Chironomidae mat. In the case of hard sediment the status of P. kraepelini, D. stevensoni and C. ophtalmica was retained, but there appeared a new dominant, i.e. Potamocypris unicaudata. The most unique structure of domination was observed in the case of Chironomidae mat, with Limnocythere inopinata as an eudominant and Cypridopsis vidua and P. unicaudata as dominants. As for the species diversity, the hard sediment was characterised by the highest value of Shannon-Wiener Diversity Index amounting to H’ = 1.89. The sapropel/hard sediment was characterised by the lowest value of the species diversity index amounting to H’ = 1.39. Ostracoda (Crustacea) of the river bed in the lower course... 95

Discussion The quantitative and qualitative composition of ostracods in rivers is conditioned by the degree of pollution, the river fl ow (sediment morphology), location in a river system (main branch or fl oodplain), the depth at which a sample is collected (benthic or interstitial zone). At a 50 km long part of a heavily polluted river in Spain Mezquita (1999B) discovered 17 species, even though the Ostracoda were totally absent from the most heavily polluted localities. In three rivers of the Iberian Peninsula Mezquita (1999A) recorded 30 species, while Claret et al. (1999) recorded 10 species on the fl oodplain of the Rhone in restored backwater and drainage canals. Kiss & Schöll (1999) found 15 species in the system of the Danube River, out of which number 5 were recorded in the main branch of the river. Furthermore, Creuzé des Châtetelliers & Marmonier (1993) discovered 24 ostracod species on sand and gravel sediments in the main branch of the Rhone River, collecting samples from both benthic and interstitial habitats. In various rivers of Belarus (9 localities in total), Nagorskaya & Keyser (2005) identifi ed 22 species. In the present study, in the main river bed of the lower course of the Oder River, along the total length of 60 km there have been identifi ed 18 taxa represented by 16 species. It was a comparatively high number considering the fact that the samples were collected exclusively from the benthic zone and solely from the main river bed. The species composition of ostracods found in various European rivers is very diversifi ed. According to Marmonier & Creuzé des Châtetelliers (1992), dominant species in the Rhone River included: Cypridopsis vidua, Limnocythere inopinata and Potamocypris cf variegata. Kiss & Schöll (1999), in the main branch of the Danube River recorded such species as: Cyclocypris ovum, Cypria ophtalmica, C. vidua, L. inopinata and Notodromas monacha, while Physocypria kraepelini was recorded only in the fl oodplain areas. In three rivers of the Iberian Peninsula, Mezquita et al. (1999A) discovered, similarly to the authors of the present study, such species as D. stevensoni, Ilyocypris spp., L. inopinata, Candona neglecta, C. vidua and P. variegata. However, apart from these, 24 other species were identifi ed. On the other hand, similarly to the authors of the present study, in a polluted river in Spain Mezquita 1999B discovered only D. stevensoni, C. vidua and Ilyocypris bradyi and 14 other species. Such considerable differences were due to the climatic factor (Mediterranean climate in Spain), the height above 96 Agnieszka Szlauer-Łukaszewska, Beata Kowaluk-Jagielska sea level and a degree of pollution. Taking into account average values for the Oder River, the dominant species in the investigated part of the river included: Physocypria kraepelini, Darwinula stevensoni and Cypria ophtalmica. This tendency was also observed in the case of sapropel (mud) sediment. C. ophtalmica is a freshwater species that is known to occur in various types of waters, including those characterized by high organic pollution and in the profundal zone of lakes. Its populations are abundant in ponds fi lled with leaf-litter (Meisch 2000). As for P. kraepelini, it is found in various waterbodies such as ponds, the littoral zone of lakes, streams, canals and ditches. Meisch (1990) collected the species in question from an eutrophic gravel pit located over anoxic, H2S – producing mud. Darwinula stevensoni prefers ponds, lakes and slow fl owing streams. It can be encounteredin both muddy and sandy substrates. However, in the case of hard sediments the list of dominant species included also Potamocypris unicaudata. This fact was confi rmed by the results of the RDA analysis, where a negative correlation to sapropel (i.e. positive correlation to hard sediments) was observed in the case of Ilyocypris bradyi and P. unicaudata dwells in both pure freshwater and in slightly brackish habitats (Meisch 2000). The structure of domination was the most unique in the case of Chironomidae mat, where such species as Limnocythere inopinata, Cypridopsis vidua and P. unicaudata were dominants. According to Meisch (2000), I. bradyi and L. inopinata are species encountered in both muddy and sandy habitats. Marmonier & Creuzé des Châtetelliers (1992) described the species C. vidua, L. inopinata and Potamocypris cf variegata, which they encountered in the Rhone River, as typical ones for slow fl owing waters characteristic for the main river bed of a large river. What is more, they described L. inopinata and P. cf variegataas typical riverine species. Unfortunately, few authors provide data referring to the density of the Ostracoda at the bottoms of the investigated rivers. In the main river bed of the Rhone River, Creuzé des Châtetelliers & Marmonier (1993) observed average density of the Ostracoda ranging from 10 to over 19 000 indiv./m2, with lower values characteristic for oligotrophic waters. McGregor (1969) reported maximum density of Darwinula stevensoni encountered in a lake amounting to 7277 indiv./m2, while Ranta (1979) encountered the species in soft mud in front of the macrophyte belt and reported density values up to 160 000 indiv./m2. In the present study, the average density amounted to 44 567 indiv./m2, with the maximum value in the case of sapropel sediments – 101 314 indiv./m2, and the minimum value in the case of hard sediments – 3 714 indiv./m2. Ostracoda (Crustacea) of the river bed in the lower course... 97

On the basis of the data given above a conclusion can be drawn that the type of river bottom to a great extent affects the density and taxonomic composition of the Ostracoda in rivers and other habitats. A similar conclusion was drawn by Tórz & Chojnacki (2002), who identifi ed the bottom type as a major factor determining the taxonomic structure of the benthic zone in the Oder River in the same area as the one investigated in the present study. The researchers observed that on the sandy bottom and mixed bottom with the prevalence of sand, gravel and shell hash, the bottom fauna was considerably richer and more diversifi ed than in the case of the silt bottom, with the number of recorded taxa amounting to 18 and 3 in the two bottom types, respectively. Also in the present study, with reference to species diversity the value of Shannon-Wiener Index was the highest for the hard sediments and amounted to H’ = 1.89. The least diversity was observed in the case of the sapropel/hard sediment, with the value of Shannon- -Wiener Index amounting to H’ = 1.39. On the other hand, as has been mentioned earlier, the situation was exactly opposite with regard to density: maximum density was observed in the case of the sapropel sediments and the minimum density – in the case of the hard sediments. Creuzé des Châtetelliers & Marmonier (1993) arrived at the conclusionthat geomorphological and hydrological features might control the microdistribution of ostracods, and these factors have a direct infl uence over the bottom type. Apart from that, the microdistribiution of epigean ostracods seems to be infl uenced by the stability of the substrate. In the present study the Chironomidae mat differed from the other investigated sediments types with reference to dominance structure, featuring unique dominant species. It is an interesting and uncommon fact that Cypridopsis vidua obtained the dominant status in this habitat. According to Meisch (2000), this species inhabits various water bodies with rich vegetation. The mat made up from the dwellings of the Chironomidae had a spacious, porous structure and as a bottom type somewhat resembled e.g. the dense growth of Ceratophyllum. On the other hand, according to Marmonier & Creuzé des Châtetelliers (1992) the species in question is typical for stagnant waters. There is no unanimity among the researcherswhether oxygen is a factor conditioning the occurrence of ostracods in rivers. Mezquita et al. (1999B) observed the total absence of Ostracoda in the most polluted areas of the 3 investigated river due to low oxygen content (4 mgO2/dm ) and high concen- trations of toxic pollutants such as ammonium. They also observed that the highest species diversity in the benthic zone, equaling H’ = 3.27, was recorded while the 98 Agnieszka Szlauer-Łukaszewska, Beata Kowaluk-Jagielska

3 oxygen content amounted to 87 mgO2/dm . In the main river bed of the Rhone River, Creuzé des Châtetelliers & Marmonier (1993) observed no correlation between the oxygen content in the water and the distribution of ostracods. In the same part of the Oder River as the part investigated in the present study, in the zone over the bottom, Tórz (2002) recorded the following values of average oxygen concentration and percent oxygen saturation: for the Eastern Oder – 10.6 mg 3 3 O2/dm and 97% respectively; for the Western Oder – 10.2 mgO2/dm and 94.1% respectively. What is more, in summer there was observed over-oxygenation of the Oder waters, even in the zone near the bottom. On the other hand, the authors of the present study discovered the presence of sapropel sediments at the bottom of the Oder River. Inside this type of sediment oxygenless conditions are found and sulphur hydrogen is produced, which is toxic for living organisms. In spite of this, in habitats with the sapropel sediment the greatest abundance of the Ostracoda was recorded. Thus, the waters of the Oder River, rich in oxygen, provide favourable conditions for the development of ostracods even on the surface of sapropel sediments. The continuously ﬂ owing river waters do not allow the bottom sediments to run out of oxygen as is the case with stagnant waters. Furthermore, organic matter which constitutes a basic element of a silt bottom provides an excellent food for the Ostracoda. Finally, Physo- cypria kraepelini which is a dominant species on a silt bottom, is comparatively resistant to the conditions found in this type of habitat: Meisch (1990) collected it from a eutrophic gravel pit situated over anoxic, H2S- producing mud.

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