Available online at www.sciencedirect.com SCIENCE ^DIRECT" ^__ LIMNOLOGICA ELSEVIER Limnologica 35 (2005) 274-282 www.elsevier.de/limno Long-term changes in cladoceran assemblages in the Danube floodplain area (Slovak-Hungarian stretch) Marta Illyováa'*, Dánka Némethováb'c ^Institute of Zoology, Slovak Academy of Sciences, Dúbravská cesta 9, 845 06 Bratislava, The Slovak Republic b'Research Centre for Environmental Chemistry and Ecotoxicology, Masaryk University, Brno, Czech Republic ^Centre of Biostatistics and Analysis, Masaryk University, Brno, Czech Republic Received 12 April 2005; received in revised form 12 August 2005; accepted 16 August 2005 Abstract In this paper, the distribution of Cladocera species in the different sampling sites: the main channel/old river bed, parapotamal type side arms and plesiopotamal side arms, is described. The structure of cladoceran assemblages in the by-passed Danube section and in the adjacent floodplain water bodies has changed since the Gabčíkovo hydropower plant was put into operation. Great changes have been observed in the previous parapotamal side arm situated between river km 1840 and 1820, artificially fed with water from the head-race canal. The dominance of tychoplanktonic (benthic and phytophilous) species has increased, while the typical euplanktonic species have disappeared. Three characteristic groups of cladoceran assemblages were recorded when a different type of habitat was taken into consideration. Euplanktonic cladocerans prevailed on all sampling sites before damming. In periods after damming, littoral species, and later also a euplanktonic forms, dominated on the main channel sampling sites. In parapotamal and plesiopotamal side arms with rich littoral macrovegetation during periods after damming, phytophilous cladoceran species were the ones with the highest occurrence. The samplings from the first time period were rather homogenous. The samplings from the second and third period were more similar when considering the sample site than regarding the time period. In total, 64 cladoceran species were recorded in the course of 13 years (from 1991 to 2004). The increase in number of Cladocera species from 1991 to 2004 was significant. Chydorus sphaericus was found to be the most widely distributed species in the study area. The finding of Disparalona hamata is the first faunistic record from the central part of the Danubian watersheds. © 2005 Elsevier GmbH. All rights reserved. Keywords: Crustacea; Cladoceran assemblages; Inundation; Habitat requirements; Gabčíkovo Introduction In 1992 the Gabčíkovo hydroelectric power plant was put into operation. After the Danube River being dammed at Cuňovo (river km 1851.7) an active "Corresponding author. Tel.: +4202 59503611. E-mail address: marta.illyova@savba.sk (M. Illyová). connection between the abandoned Danube stretch and the side-arm system in the floodplain was abolished. The water supply to the protected floodplain was realized through an artificial water recharge system. As a result of a barrage system implementation at Gabčíkovo - Holčík, Bastl, Ertl, & Vranovský (1981) predicted a number of ecological changes occurring in aquatic communities in the affected area. 0075-9511/$-see front matter © 2005 Elsevier GmbH. All rights reserved. doi:10.1016/j.limno.2005.08.004 M. Illyová, D. Némethová / Limnologica 35 (2005) 274-282 Investigation of Cladocera in the Danube floodplain has a long tradition. The first list of species living in the water bodies of Žitný ostrov Island presented Vranovský and Ertl (1958). The most important data on zooplankton of the Danube River and adjacent water bodies are included in the papers of Ertl (1966) and Vranovský (1969, 1972, 1974, 1985, 1991, 1995), but did not aim to provide the analysis of the cladoceran fauna. Later, Illyová & Némethová (2002) studied the relationship between cladoceran and copepod communities and the different types of macrovegetation in the Danube floodplain area. The crustacean assemblages on the right side of this section (the Szigetkoz floodplain area in Hungary) had been investigated for a long time by Bothár (1973, 1979), Bothár & Ráth (1994) and Gulyás (1994). Intensive hydrobiological investigation in the region started in 1990 and has continued up to now in order to monitor environmental impacts of the river regulation. First changes in planktonic crustacean assemblages as a result of intensive water engineering activities were observed as early as in the first years after damming (Illyová, 1996; Vranovský, 1997). On the right side of the Danube (Szigetkoz) Bothár (1994) and Kiss (2004) also observed long-term changes in crustacean assemblages. The aim of this paper is (i) to resume the 13-year-long monitoring of the species composition and relative abundance of cladoceran assemblages (Ctenopoda, 275 Anomopoda, Onychopoda and Haplopoda) in the Danube River and adjacent water bodies on the left-bank of the floodplain (r. km 1840.5-1804); (ii) to detect long-term changes in the composition of these assemblages. Study area The study section is situated in the Danubian lowland area in Slovakia (Fig. 1). Svobodová (1994), Illyová (1996) and Vranovský (1997) have already published a comprehensive characteristic of these sampling localities. The sites were selected because they represent a basic type of the local aquatic environment, influenced by the operation of the Gabčíkovo hydropower plant. The classification of water bodies proposed by Ward, Trockner, Arscortt, & Claret (2002) was used. The general characteristics of the six habitat types investigated are as follows: Site 1, Dobrohošt (D) - the main river channel/old river bed at Dobrohošt Village (r. km 1840.5); after damming the decrease in water level was significant. Site 2, Gabčíkovo (G) - the main river channel/old river bed at Gabčíkovo Village (r. km 1819.5). The mean depth in the km 1820 profile was 4.5-5.0 at a discharge equal to the long-term mean discharge; after damming, it decreased to 2.0-2.5 m (Vranovský, 1997). Site 3, Bodiky Fig. 1. Location of sampling sites in the Danube delta downstream from Bratislava (river km 1841-1804). (1) Dobrohošs main channel/old river bed et? Danube; (2) Gabčíkovo, main channel/old river bed; (3) Bodiky, side arm; (4) Istragov, side arm; (5) Kráľovská lúka, side arm; (6) Sporná sihoť, side arm. 276 M. Illyová, D. Némethová / Limnologica 35 (2005) 274-282 (B) - the Bodícke side arm at the village of Bodiky (r. km 1830). The side arm of a parapotamal type is situated in the upper part of the by-passed region. Due to artificial feeding after damming, the arm is permanently flowing, but current velocities are rather low (Vranovský, 1997). From 1995 onwards, macrophytes have increased significantly in its littoral zone. The bottom sediment is formed of gravel. Site 4, Istragov (I) - the Istragovské side arm is situated at Gabčíkovo Village (r. km 1815.5). A parapotamal type of the side arm; it is non-permanently flowing at present. The bottom sediment is formed of gravel and sand. Poor aquatic littoral macrophytes have been developing since 2000. Site 5, Kráľovská lúka (K) - the side arm near Trstená na Ostrove Village (r. km 1825). This plesiopo-tamal type of a water body is a remainder of an originally flowing arm. At present it is mostly a stagnant river arm, which is acquiring a paleopotamal character. About 60% of the side arm area is overgrown with macrophytes. The bottom sediment is formed of gravel, mud and clay. Site 6, Sporná sihoť (S) - a side arm near Kľúčovec Village (r. km 1804) is the mostly stagnant river arm. Prior to the damming it was a side arm of a plesiopotamal type, at present it is filled with shallow water. About 80% of its water level is overgrown with macrovegetation. It is not directly affected by the Gabčíkovo hydropower plant structures. It has a muddy bottom and rich macrophytic vegetation. Methods From 1991 to 2004 (except of 1998) samples of cladoceran assemblages from six sampling sites were collected and analyzed three times a year (spring, summer and autumn). The samples of cladocerans were collected from the medial, open-water zone of the side-arms and of the main channel. Samples were taken either from a boat by vertical tows or from the bank by tows from the bottom to surface. The samples were filtered through a 90-um mesh net. Samples of littoral plankton were also obtained. The samples were preserved in 4% formaldehyde. A total of 107 samples was collected and analyzed. Qualitative analyses and dominance determination were performed on preserved material. Data from field observations on the six sampling sites that lasted for 13 years were divided into three periods: first period (1991 and 1992) before damming of the Danube River (Dl, Gl, Bl, II, Kl and SI); second period (1993-1997) includes the first 5 years after damming (D2, G2, B2,12, K2 and S2); and third period (1999-2004) includes the last 6 years after damming (D3, G3, B3, 13, K3 and S3). The average values of these three periods are marked in Table 1. The relative abundance of Cladocera species from the six sampling sites and three periods were transformed to six categories based on proportion of total fauna collected (1: < 1%; 2: 1-3%; 3: 3-10%; 5: 10-20%; 7: 20-40%; 9: 40-100%; by Vranovský, 1997). Linear regression was used to confirm the increase of species richness (number of species) during the 14-year period on each of the six sampling sites. Species richness was logarithmically transformed before entering linear regression to ensure the normal distribution. The principal component analysis (PCA) was performed to assess the relationship among cladocerans and sampling sites using the program Canoco (Ter Braak & Smilauer, 1998). The taxa by sites matrix included 64 cladoceran species and 18 objects. The analysis was based on the transposed relative abundances of individual taxa (Table 1). Results Cladocerans taxa and their habitat requirements A total of 64 taxa of cladocerans were encountered during this study, of which 17 were euplanktonic and 47 were littoral species (Table 1). The increase in number of Cladocera species from 1991 to 2004 was significant (Table 2). Disparalona hamata was recorded for the first time in the Danube area. Eight invaders: Daphnia ambigua, Daphnia parvula, Bosmina coregoni, Bosmina longispina, Diaphanosoma mongolianum, Moina weismanni, Disparalona hamata and Pleuroxus denticulatus were recorded after 1995. The species Alona guttata var. tuberculata, Alonella exiqua, Anchistropus emarginatus, Camptocer-cus rectirostris, Ceriodaphnia laticaudata, C. rotunda, C. setosa, Daphnia pulicaria, Chydorus ovalis and Mono-spilus dispar were found only once and were proved to be rare species in the Slovak Danube floodplain area. On the contrary, Chydorus sphaericus was found to be the most widely distributed species in the study area, with high occurrence frequency (92%). Bosmina long-irostris and Simocephalus vetulus were present in more than 60% of samples followed by chydorids Alona affinis, A. rectangula, Disparalona rostrata and Pleuroxus aduncus (in more than 40%). In the first 2 years Moina brachiata was widely distributed as well, but after damming it has disappeared. The ordination diagram of PCA confirmed the presence of different cladoceran community groups corresponding to 3 habitats types (Fig. 2). Group I includes the main channel and parapotamal side arms before damming (Dl, Gl, Bl and II), habitats without macrovegetation. The most typical cladocerans were Bosmina longirostris, Diaphanosoma brachyurum, Table 1. Species composition and average dominance of Cladocera in the monitored sites of the Danube floodplain area in 1991-2004 Taxon/sampling sites Dominance Sampling sites Site 1 (D) Site 2 (G) Site 3 (B) Site 4 (I) Site 5 (K) Site 6 (S) Code/period Dl D2 D3 Gl G2 G3 Bl B2 B3 11 12 13 Kl K2 K3 SI S2 S3 Acroparus harpae (Baird)1 + 2 1 1 1 1 1 2 2 Acroperus neglectus (Lilljeborg)1 1 + + 3 1 3 Alona affinis (Leydig)1 ALOAFF 3 5 7 5 2 3 2 2 3 + 3 1 + + 1 1 Alona costata Sars1 + 2 2 1 Alona guttata Sars1 ALOGUTT 1 1 3 1 + 2 2 2 1 1 Alona guttata var. tuberculata Kurz1 1 + Alona protzi Hartwig1 1 3 + Alona quadrangularis (O.F.M.)1 ALOQUA 7 3 1 3 1 2 2 + + 1 1 Alona rectangula Sars1 ALOREC 2 3 3 3 1 3 3 3 2 1 3 3 3 2 1 1 Alonella excisa (Fischer)1 ALOEXS 2 1 1 1 2 1 Alonella exiqua (Lilljeborg)1 1 Alonella nana (Baird)1 ALONAN 2 1 1 3 1 Anchistropus emarginatus Sars1 1 Bosmina longispina Leydig BOSLNS 1 1 Bosmina coregoni Baird BOSCOR 2 1 3 1 3 + Bosmina longirostris (O.F.M.) BOSLON 9 5 5 9 5 7 9 5 3 7 7 9 5 5 9 9 3 5 Camptocercus rectirostris Schoedler1 1 Ceriodaphnia laticaudata (P.E.M)1 + 1 Ceriodaphnia megops Sars1 CERMEG 1 + 1 + 2 1 Ceriodaphnia pulchella Sars1 CERPULL 2 + 2 2 2 2 3 Ceriodaphnia quadrangula (O.F.M)1 + 2 Ceriodaphnia reticulata Sars1 CERRET + + 2 3 Ceriodaphnia rotunda Sars1 1 Ceriodaphnia setosa Matile1 + Daphnia ambigua Scourfield 1 1 1 Daphnia cucullata Sars DAPCUC 3 1 3 3 3 5 2 5 3 1 1 Daphnia galeata Sars DAPGAL 3 5 3 1 1 1 1 1 Daphnia longispina (O.F.M) DAPLON 5 2 3 2 1 3 3 2 1 3 1 Daphnia parvula Fordyce, 1901 1 Daphnia pulicaria (Forbes) 1 Diaphanosoma brachyurum (Lievin) DIABRA 3 2 2 7 7 2 1 1 1 1 Diaphanosoma mongolianum (Ueno) + 1 Diaphanosoma orghidani (Negrea) DIAORG 2 2 5 5 3 1 3 3 2 1 1 Disparalona hamata Birge1 1 Disparalona rostrata (Koch)1 DISROS 2 3 1 3 2 3 3 2 2 2 1 Eurycercus lamellatus (O.F.M.)1 EURLAM + 1 1 + 1 + 3 1 Graptoleberis testudinaria (Fischer)1 GRATES + 2 + 3 1 3 1 1 1 Chydorus ovalis Kurz1 + Table 1. (continued) Taxon/sampling sites Dominance Sampling sites Site 1 (D) Site 2 (G) Site 3 (B) Site 4 (I) Site 5 (K) Site 6 (S) Code/period Dl D2 D3 Cil G2 G3 Bl B2 B3 11 12 13 Kl K2 K3 SI S2 S3 Chydorus sphaericus (O.F.M.)' CHYDSPH 3 3 3 3 2 7 7 + 3 5 3 7 5 5 7 7 Ilyocryptus agilis Kurz' 1 1 1 + 1 Ilyocryptus sordidus (Lievin)' ILYSOR 2 + 2 1 2 1 + 1 Lathonura rectirostris (O.F.M.)' 1 Leptodora kindtii (Focke) 2 1 1 1 2 Leydigia leydigii (Schoedler)' LEYLEY 2 2 1 3 Macrothrix hirsuticornis N.et Brady' MACHIR 7 5 3 2 1 5 2 1 1 Macrothrix laticomis (Fischer)' MACLAT 1 1 1 5 1 2 1 1 Moina brachiata (Jurine) MOIBRA 2 2 5 2 2 Moina micrura Kurz 3 2 2 3 1 1 1 2 2 5 3 3 1 3 2 1 Moina weismanni Ishikawa 2 Monospilus dispar Sars' 1 Pleuroxus aduncus (Jurine)' PLEADU 3 + 2 3 + 1 4- 3 1 2 2 3 Pleuroxus denticulatus Birge' PLEDEN + 2 2 2 1 + 3 1 1 Pleuroxus laevis Sars' IM 1 I \l + + 1 Pleuroxus truncatus (O.F.M.)' PLETRU 2 3 + 1 1 1 Pleuroxus uncinatus Baird' PLFUNC 2 2 2 1 + 1 Polyphemus pediculus (Linne) + 1 1 Pseudochydorus globosus (Baird)' PSEGLO + + 1 + 1 Scapholeberis mucronata (O.F.M.)' SCAMUC 1 + 2 2 2 1 + 2 3 1 5 2 2 Scapholeberis rammneri D et P' 1 1 Sida crystallina (O.F.M)' SIDCRY 2 1 2 2 3 + 3 2 3 1 1 2 1 Simocephalus congener Schoedler' SIMCON 1 + 1 1 Simocephalus exspinosus (Koch)' 1 2 Simocephalus serrulatus (Koch)' SIMSER + + + 1 1 3 3 1 1 Simocephalus vetulus (O.F.M.)' SIMVET 3 2 1 2 1 2 2 + 2 1 3 3 3 7 5 Number of species Taxa: Taxa marked with ' are tychoplanktonic taxa. Code: Abbreviation of corresponding taxa (for Fig. 2 of PCA analysis) Sites: 1 = Dobrohošť, 2 = Gabčíkovo, 3 = Bodíky, 4 = Istragov, 5 = Kráľovská lúka, 6 = Sporná sihoť Dl, Gl, Bl, II, Kl, SI = 1st - period of year 1991-1992; D2, G2, B2, 12, K2, S2 = 2nd - period of year 1993-1997; D3, G3, B3, 13, K3, S3 = 3rd - period of year 1999-2004. Dominance (occurence): 1 means <1%; 2 = 1-3%; 3 = 4-10%; 5 = 11-20%; 7 = 21^10%: 9 = 41-100% + Species only in qualitative samples. M. Illyová, D. Némethová / Limnologica 35 (2005) 274-282 279 Table 2. Results of the linear regression analysis. Independent variable (x-year) was coded as follows: 1991-1, 1992-2,..., 2004-14 Sampling Regression equation Significance r2 p sites level Site 1 y = 0.807+ 0.023x P = 0.032 0.355 * Site 2 y = 0.771 +0.022x P = 0.023 0.388 * Site 3 y = 0.960 + 0.014x P = 0.153 0.177 ns Site 4 y = 0.744+ 0.033x P = 0.009 0.480 ** Site 5 y = 0.820+ 0.033x P <0.001 0.705 *** Site 6 y = 0.944 + 0.025x P = 0.004 0.540 ** Dependent variable (y) in all equations species richness. */)<0.05, **/)<0.01, ***/)<0.001, ns - not significant. Fig. 2. First two axes of PCA as a biplot of Cladoceran species and sampling sites. The first two ordination axes (k\ = 0.298 and X2 = 0.190) accounted for 48.8% of the total variance of the species data. From the 64 taxa in the analysis included (see Table 1 for codes of species) only taxa with best fit shown. Moina brachiata and Daphnia cucullata (Fig. 2) that are regular members of plankton in backwater and slow watercourses. Group II includes the main channel (D2, G2, D3, G3) and a parapotamal side arm without macrovegetation (12) during periods after damming. At these sites littoral species Alona affinis, Macrothrix hirsuticornis and the "pelagic" one Diaphanosoma orghidani were dominant. Also typical species for these habitats were Bosmina coregoni, Bosmina longispina, Daphnia galeata and from chydorids Disparalona rostrata, Pleuroxus uncinatus and Leydigia leydigii (Fig. 2). Group III includes parapotamal and plesiopotamal side arms with rich littoral macrovegetation during the period after damming. For all these habitats the number of littoral (phytophilous and benthic) species was the highest one. This group was divided into two subcategories: Ilia group - former parapotamal side arms (B2, B3 and 13) with a gravel bottom and with vegetation only in the littoral, and Illb group - former plesiopotamal side arms (K2, K3, S2 and S3) with a gravel and mud-clay bottom and with rich cover of macrovegetation both in the littoral and medial zone. Phytophilous species Ceriodaphnia reticulata, C. pulchella, C. megops, Simocephalus congener, Alonella exscisa and A. nana were associated with rich macrovegetation in former plesiopotamal side arms; whereas Sida crystallina, Eurycercus lamellatus, Graptoleberis testudinaria and Pleuroxus sp. were found mainly in former parapotamal side arms. Two plesiopotamal arms Kl and SI before damming are situated between Group I and II (Fig. 2). At these sites euplanktonic species Bosmina longirostris (Kl) or Diaphanosoma brachyurum dominated. The samplings from the first time period (before damming) are separated from the samplings taken after damming. The samplings from the second and third period are more similar when considering the sample site than regarding the time period (Fig. 2). Cladoceran community changes in several sampling sites Site 1 (Dobrohošť) - 31 species, 27 in an open water zone and 20 in a littoral zone were recorded. Before damming, Bosmina longirostris, Alona quadrangularis and Daphnia longispina were dominant species in an open water zone (Table 1). In 1993-1997 their relative abundance decreased. Littoral species appeared in plankton and their relative abundance increased significantly. The dominant species M. hirsuticornis usually occurred in March, often as the only representative of Cladocera (100%) being represented in the sample by ca. 9-30 individuals. In the third period (1999-2004) usually Alona affinis and species in the genus Daphnia and genus Bosmina are the major component of the cladoceran community (Table 1, Fig. 2). Site 2 (Gabčíkovo) - 28 cladoceran taxa were recorded on this sampling site. In the first period (1991-1992) Bosmina longirostris predominated in the main channel. After damming, the increase of relative abundance of tychoplanktonic species in potamoplank-ton was observed. Among the pelagic species, D. orghidani appeared and maintained a higher relative abundance since 1994. In the third period, Bosmina longirostris predominated again; its high relative abundance (72-87%) was recorded particularly in spring of 02 D3 AL0AFF DIS ROS LEYLEY MACHIR : G3 DIAORG 12 BOSCOR ILYSOR MACLATPLEUNC MAOLAI BOSLNS DAPGAL_ \Group II G2 ALOQUA Group III B2 SIDCRY ALOREC B3 EURLAM Group D1 jG1 GRATE ST PSEGLO |3 PLEDEN PLELAE PLETRU PLEADUS|MSERRSIMVET ALOGUTT ALONAN CHYDSPH SCAMUC K^ SIMCON CERMEGCERPULLCERRET B1 BOSLON S3 S2 K3 MOIBRA DIABRA ALOEXS S1 K1 280 M. Illyová, D. Némethová / Limnologica 35 (2005) 274-282 2002 and 2003. After 1999, number of littoral species increased in plankton but they showed a low relative abundance (