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. Table 1. Numerical data regarding the total collected material (A – total number of individuals, B –average density indiv./m2, C- SD, 90 D – maximum density,