spatial and temporal distribution of the zooplankton

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296 Splaiul Independenţei, C.P. 56-53, District 6, 59651. Bucharest, Romania e-mails: larisa.florescu@ibiol.ro laura.parpala@ibiol.ro alina.dumitrache@ibiol.ro.
Travaux du Muséum National d’Histoire Naturelle «Grigore Antipa»

Vol. LVI (1)

pp. 109–124

© 31 août 2013

DOI: 10.2478/travmu-2013-0009

SPATIAL AND TEMPORAL DISTRIBUTION OF THE ZOOPLANKTON BIOMASS IN SFÂNTU GHEORGHE BRANCH (THE DANUBE DELTA, ROMANIA) IN RELATION TO ENVIRONMENTAL FACTORS LARISA FLORESCU, LAURA PARPALĂ, ALINA DUMITRACHE, MIRELA MOLDOVEANU Abstract. Sfântu Gheorghe fluvial branch belongs to the macroregional hydrological system of the Danube Delta. In order to improve the navigation on the river branches, between 1983 and 1989 various technical works have been initiated, which later proved to be one of the most aggressive factors leading to changes over time in the Danube Delta. The influence of anthropogenic impact on the zooplankton biomass distribution patterns in Sfântu Gheorghe was investigated in 2008 – 2010 interval. From this perspective, the aim of this paper was to emphasize the structure and distribution of zooplankton in the new conditions of the river system. Seasonal sampling shows that Copepoda and Cladocera were the dominant groups while the protozoans (Ciliata and Testacea) presented the smallest values of biomass and also emphasized distinct differences among the three sampling periods: summer, spring and autumn. Résumé. Sfântu Gheorghe est la branche fluviale qui appartient au système hydrologique macrorégionale de Delta du Danube. Afin d’améliorer la navigation sur les bras du fleuve, entre 1983-1989 divers travaux techniques ont été lancés, lesquels plus tard se sont avérés être l’un des facteurs les plus agressifs conduisant à des changements au fil du temps dans les Delta du Danube. L’influence de l’impact anthropique sur la distribution de la biomasse du peuplement de zooplancton a été étudiée dans Sfântu Gheorghe entre 2008 et 2010. Dans cette perspective, l’objectif de cette étude était de mettre l’accent sur la structure et le rôle du zooplancton dans les conditions nouvelles du ce système fluvial. L’échantillonnage saisonnier montre que les Copepoda et les Cladocera étaient les groupes dominants tandis que les protozoaires (Ciliata et Testacea) présentent des valeurs plus faibles de la biomasse et met également l’accent sur les différences marquées entre les trois périodes de prélèvement: été, printemps et automne. Key words: zooplankton, biomass, Sfântu Gheorghe branch, spatial and temporal distribution, anthropogenic impact.

INTRODUCTION

The Danube flows and builds its delta in to the Black Sea, a very complex of ecosystems whose hydrographical network ranged among three branches: Chilia, Sulina and Sfântu Gheorghe. In the last four decades, on the Danube River and the Danube Delta there was a period of strong anthropogenic changes. The most important changes on the Romanian Danube River basin were determined by the building of the Iron Gates dams in 1970 (Iron Gates I, at km. 942.95 from the Black Sea), in 1983 (Iron Gates II at Ostrovul Mare, km. 864) and the navigation improvement of the Danube distributaries (Sulina, Sfântu Gheorghe) (Petrescu, 2009; Zinevici & Parpală, 2007). The intensive development of economy was based on policies and management plans which have not considered the main functions of the ecosystems. The increase of nutrients associated with hydraulic works on the branches, the variations in the hydrological regime and temperature, had a negative cumulative effect over ecosystems leading to hypertrophy (Vădineanu, 1998). Sulina

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and Sfântu Gheorghe were the most affected branches by anthropogenic changes (Zinevici et al., 2006). The Sfântu Gheorghe branch begins at the Ceatal with the same name and it has a length of 110 km. The main structural changes in Sf. Gheorghe branch consisted in cutting-off the seven meanders between 1983 and 1989 and construction of new canals in order to facilitate the navigation (Tudorancea & Tudorancea, 2006). Thus, after these hydro-geomorphological changes, a complex of ecosystems with different features consisting in natural sectors, meanders and new canals was born. The navigation improvements changed the water flow between the three main Danube branches. Some major differences appeared after those events, thus, in the situation of the high flood recorded in 1997, Chilia branch took 57% from water discharge of the Danube measured at Ceatal Ismail, 24% went in the Sf. Gheorghe branch, and 19% in the Sulina branch. These values differ significantly from the average repartition of the 65% for Chilia, 21% for Sf. Gheorghe, and 14% for Sulina (Popa, 1997). This fact can be explained either by the rectification influence of the Sfântu Gheorghe branch on deltaic system, or by the homogenization tendency of high flood propagation waves. Following the hydro geomorphological changes, the accumulation and erosion processes along the branch were modified. Thus, at the bifurcation of the canal with the cut off meanders there are accumulation processes while in the newly created canals intense erosion occurred. The observed morphology clearly records the impact of the canal on the whole alluvial system. The canal adapts to the flow decrease, but its response varies stream wise because of sediment infilling. On the contrary the canal is characterized by an increase in depth that creates a well calibrated shape suitable for maintaining high energy (Jugaru Tiron et al., 2009). In this paper we present spatial and temporal distribution patterns of the zooplankton biomass, in these different areas. There are known very few studies addressing these issues, so it was considered necessary to cover the scientific results of these identified gaps in this area. MATERIAL AND METHODS

Study area Sfântu Gheorghe branch carries 24% of the Danube water discharge and 21% of the Danube sediment discharge. The width of the distributaries varies from 150 to 550 m, and the water depth varies between 3 and 27 m under the local low water mark (Jugaru Tiron et al., 2009). For sampling, the three investigated areas were divided into seven transects with two sampling points (medial and littoral), starting from Sf. Gheorghe settlement. The stations were positioned along the water stream; the medial points were established in the middle of the branch and the littoral ones parallel with the medial, but close to the shore (Fig. 1). The seven sampling points were set as follows: in natural sectors 3 sampling points (1, 4 and 7), in meanders (2 and 5) and in canals (3 and 6). The sampling points were chosen in order to reflect the heterogeneity of river branch resulting from the emergence of three types of areas: new canals, former meanders, natural sectors. The samples were collected seasonally, between 2008 and 2010, throughout the water column, resulting 126 samples in a total.

DISTRIBUTION OF ZOOPLANKTON IN SFÂNTU GHEORGHE BRANCH (ROMANIA)

111

Sfântu Gheorghe Ceatal

Mahmudia meander

7 canal

6

5

littoral

meanders

4

medial

Black Sea

natural sector

2 3

1 Fig. 1 - Geographical position of the study area.

Sampling Water samples have been taken for the physical-chemical variables determinations. The transparency was established with Secchi disk and the depth with Humimbird 260 sonar. The temperature, pH, dissolved oxygen content were measured in the field with a multiparameter WTW 340i (Germany). Samples for chemical analyses were frozen for further analyses in the lab. Nutrients were determined spectrophotometrically (CECIL 1100, UK): NH4+ - as yellow compound with Nessler reagent, NO2- - as red compound with sulphanilic acid and α-naphthylamine, NO3- - as yellow compound with sodium salicylate (Tartari & Mosello, 1997), total reactive phosphorus (TRP) - as blue phosphomolybdate, reduced by ascorbic acid, total phosphorus (TP) - by oxidation with potassium peroxodisulphate (Tartari & Mosello, 1997); the organic matter content was estimated from the chemical oxygen demand determined by oxidation with K 2Cr2O7 (COD-Cr). The zooplankton samples were collected by filtering 50 litters of water using a Patalas-Schindler device (5 l) on water column through a 65 µm Ø mesh network, and preserved with 4% formaldehyde solution. The zooplankton species was identified by following keys: for Ciliata (Foissner et al., 1991, 1992, 1994, 1995), Testacea (Bartoš, 1954; Grospietsch, 1972), Lamellibranchia (Marsden, 1992), Rotifera (Voigt, 1956; Rudescu, 1960), Cladocera (Negrea, 1983; Brooks, 1959), Copepoda (Damian-Georgescu, 1963, 1966, 1970). The abundance (ind L-1) was assessed by microscopic methods, using a Zeiss inverted microscope type, by direct counting into a Kolkwitz chamber (Utermöhl, 1958). For zooplankton biomass calculations, the wet weight of the organism (µg wet weight L-l) was used according to: Winberg (1971) for Ciliata and Testacea, Stanczykowska (1976) for Lamellibranchia, Dumont et al. (1975) for Rotifera and Odermatt (1970) and Sebestyen (1955, 1958 a, b) for crustacean biomass estimation. The biomass was evaluated by volumetric and gravimetric measurements, taking into account the organism volume of each identified species.

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Data processing Data were grouped in years, seasons and sampling stations (littoral and medial). For getting an overview of the distribution of the zooplankton biomass and to assess the differences between seasons and sampling sites (medial and littoral) multidimensional scaling (MDS) analysis it was used. To confirm the results, oneway ANOVA was used. The relationship between measured environmental variables and zooplankton assemblages was explored using the canonical correspondence analysis (CCA). CCA is useful for identifying which environmental parameters are important in the determination of community composition as well as spatial variation in the communities. CCA identifies environmental variables that explain directions of variance in the species data along one or more axes (Albania et al., 2009). Further, simple correlations aimed to identify the independent variables that affect the biomass dynamics of six zooplankton groups were used. For statistical processing, PAST (Hammer, 2001) and XLSTAT (trial version) software was used. RESULTS

Environmental characteristics The variability of physical-chemical parameters shows strong interactions with biological processes at the population level, which is transmitted throughout food web. So, the mechanisms bottom-up and top down are connected to the key physical-chemical parameters (Hoover et al., 2006; Hillebrand, 2002). The variations of these factors synergistically and separately affect the zooplankton living. During the study period the seasonal values of the temperature fluctuated from 10-29.2°C in the littoral side and 9-29.2°C in the medial one. These values did not present significant differences among the sites, into a sampling campaign. The depth ranged from 0.8 to 26.0 m and transparency 0.16-0.9 m. Dissolved oxygen concentration varied between 4.22 and 11.59 (mg O2/l) while the average of oxygen saturation was 79.78%. The seasonal distribution of nutrients shows that the total organic carbon (TOC) concentration has the same trend for both sides, with values ranged between 0.008 and 38.25 mg C/l in medial and 0.03 - 21.75 mg C/l for the littoral ones. The PO4 concentrations variability was between 21-128 µg/l and the DIN concentration marked the following highest values 19.7 N/l in medial and 21.6 N/l in littoral and the lowest were 0.57 mg N/l in medial and 0.439 mg N/l. in littoral. The average values of the physical-chemical parameters of the three studied areas showed that no significant differences were found (Tab. 1). In order to evaluate the differences in the physical-chemical parameters of the littoral and the medial areas, an ANOVA analysis it was conducted. Thus, only two parameters showed significant differences between the two sampling sites: depth (F = 90.906; F crit. = 3.919, p = 1.9E-16; significance level ****) and the T/D index (F = 42.72; F crit. = 3.919, p = 1.5E-09, significance level ****). Spatial and temporal distribution of zooplankton biomass Many deltas around the world have been experiencing a drastic change in their evolution due to anthropogenic alteration (Giosan et al., 1996).

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Table 1 The values of physical and chemical parameters in the three studied areas of Sfantu Gheorghe branch. Natural sectors littoral

medial

Meanders

Canals

littoral

medial

littoral

medial 18.5

Depth (m)

2.4

10

3.23

4.82

4.44

Transparency (m)

0.5

0.5

0.46

0.51

0.44

0.45

T/D

0.27

0.05

0.2

0.12

0.12

0.02

Temperature (Cº)

18.4

18.24

18.22

17.95

17.91

17.91

pH

8.46

8.33

8.37

8.22

8.52

8.44

O2 (mg O/l)

7.53

7.88

7.72

7.87

7.66

7.65

TOC (mgC/l)

7.91

6.06

6.03

7.85

6.62

4.31

DIN (mgN/l)

4.39

4.6

5.03

4.43

4.68

4.87

P tot (ug/l)

83.07

87.37

87.44

94.56

81.33

92.17

The hydrotechnical engineering works carried out along the Danube River and its tributaries as well as within the Danube Delta have significantly influenced the river water and sediment discharge and consequently the particle flux. Also, it was expected that this work should lead to some major changes in the water and sediments flowing system on Sfântu Gheorghe branch (Popa, 1997). Effects of these changes were reflected directly on the biocoenosis. The meanders correction affects both the hydraulic and morphodynamical profiles of the modified branches that sedimentation occurs in time, in former meanders, while the canals usually experience degradation (Jugaru Tiron et al., 2009). A comparative analysis of zooplankton biomass among the three areas showed that canals had the highest value (134.39 µg L-1 wet weight), followed by meanders (118.50 µg L-1 wet weight) and natural areas (113.17 µg L-1 wet weight) (average values). This could due to an overtaken of water flow by canals from natural sectors, comparing with former meander, and simultaneously many zooplankton organisms are involved. Also, unfavourable conditions in this area determine lower species diversity but compensates with large crustaceans presence. According to Jugaru Tiron et al. (2009), the analysis of the velocity data provides evidence as to the highly heterogeneous hydrodynamical conditions in the different reaches of the Mahmudia alluvial system (former meander and canal). The upstream bifurcation (29505 m3s-1, 0.9 m s-1) splits the main flow to the artificial canal (2436 m3s-1, and 1.4 m s-1) while, the former meander receives only 18% of the upstream flow (510 m3s-1, 0.8 m s-1), being also characterized by a progressive decrease in average velocity (0.7 to 0.1 m s-1). High values of the mean water velocity (m s-1) comparing with adjacent areas, was found also in other canals of the branch (Popa, 1997). The annual averages of biomass of the study period show that the littoral of natural sectors (124.14 µg wet weight L-l) was more favourable for the development of zooplankton community compared to the medial area (102.20 µg wet weight L-1). In meanders, the annual averages in the littoral area were 96.25 µg wet weight L-l and 140.74 µg wet weight L- 1 in the medial. The higher value of total biomass in the medial than in littoral area may be due to slower flow rates in the meander than the main course of the river. Comparing with natural sectors and meanders, in

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canals, there were no major differences between the two areas. The values of annual averages of biomass were 135.55 µg wet weight L- 1 in the littoral areas and 133.22 µg wet weight L- 1 in the medial ones (Tab. 2). Table 2 Seasonal distribution of total zooplankton biomass (µg wet weight L- 1) during the survey period. medial

Areas

littoral

Spring

Summer

Autumn

Spring

Summer

Autumn

Natural sectors

69.724

221.929

14.952

97.786

261.459

13.178

Meanders

104.299

294.381

23.560

100.617

170.280

17.877

Canals

93.315

297.392

8.962

72.169

319.036

15.474

The analysis of seasonal variations of total biomass in Sfântu Gheorghe branch shows a resemblance among the three studied areas, the spring presents at least half (14.98 µg wet weight L-1) of the summer values (44.48 µg wet weight L-1). These two seasons are periods of intense development of the communities, the peak being reached in summer, while autumn can be regarded as a period of decline (3.972 µg wet weight L-1). In the spring, in natural sectors, the seasonal distribution of zooplankton assemblage is dominated by Copepoda (46.73% littoral and 40.29% medial) followed by Rotifera (33.65% littoral and 25.16% medial) and Lamellibranchia (11.65% littoral, 26.57% medial) while the other groups had low percents in total biomass (Fig. 2). In summer, there was a major change in total biomass composition: Copepods (14.08% littoral, 18.12% medial) lose percentage in favour of cladocerans (71.44% littoral and 65.91% medial). 100% 80% 60% 40% 20% 0%

Spring

Summer

Autumn

littoral

Ciliata Rotifera

Spring

Summer

Autumn

medial

Testacea Cladocera

Lamellibranchia Copepoda

Fig. 2 - Spatial and temporal distribution of biomass in natural sectors of the Sfântu Gheorghe branch.

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115

The meanders keep the same characters which were found in natural sectors (Fig. 3). The spring presents high values of copepods biomass (littoral 50.54%, medial 30%) and rotifers (22.78% littoral, 23.40% medial). In summer, also an increase of cladocerans (60.09% littoral, 69.82% medial) was noted, followed by copepods (31.85% littoral, 23.67% medial). The autumn was marked by some differences between littoral and medial biomass. In the medial areas, a change in groups distribution found in summer was noted, copepods represent 54.64% of total zooplankton biomass and cladocerans followed them with 20.51%, like in the same season in natural sectors. In the littoral side, cladocerans (57.64%) remained the major group in biomass composition and copepods the seconds (20.51%). The seasonal distribution of zooplankton biomass in canals presents, for both medial and littoral, in spring and autumn Copepoda were the dominant group while the summer was represented by Cladocera followed by Lamellibranchia and Rotatoria (Fig. 4). In the study period, the seasonally highest value of biomass were found in spring and autumn in meanders (littoral and medial) and in summer, in canals (coastal and medial) (Tab. 2). DISCUSSIONS

The hydrotechnical changes are very common in lotic ecosystems. They are of several types with different degrees of deterioration. Man has altered aquatic ecosystems by building canals and dams to ensure the water supply and and to facilitate the shipping. These changes have had negative effects by changing the quantity and quality of biocenosis but also have led to changes in the geomorphology of surrounding areas (Dudgeon et al., 2006). Previous studies (1981-1985), carried out on the branches of the Danube River indicated maximum levels of algal blooms in 1984 and 1985. The triggering

100% 80% 60% 40% 20% 0%

Spring

Summer

Autumn

Spring

littoral Ciliata Rotifera

Summer

Autumn

medial Testacea Cladocera

Lamellibranchia Copepoda

Fig. 3 - Spatial and temporal distribution of biomass in meanders of the Sfântu Gheorghe branch.

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100% 80% 60% 40% 20% 0%

Spring

Summer

Autumn

Spring

littoral Ciliata

Testacea

Summer

Autumn

medial

Lamellibranchia

Rotifera

Cladocera

Copepoda

Fig. 4 - Spatial and temporal distribution of biomass in canals of the Sfântu Gheorghe branch.

factors were chemical pollution of industrial origin, domestic or agriculture. Thus, due to the increased nutrient concentration, an increase in phytoplankton biomass at 9.5 mg L-1 in Sfântu Gheorghe branch has been reported (Popescu Marinescu et al., 1990). The zooplankton biomass showed (Zinevici & Parpală, 2007) much higher values during 1981-1985 and 1991-1992 than the existing ones in our recent study (Tab. 3). These decreases may be due to the diminishing pressure of the above mentioned factors and less to the hydrogeomorphological changes. Therefore the comparison is indicative only. Table 3 The annual averages of zooplankton biomass in different study periods in Sf Gheorghe branch 1981-1992 (Zinevici & Parpală, 2007) and 2008-2010. Period

Sf. Gheorghe Ceatal

Sf. Gheorghe branch

Total zooplankton biomass (µg wet weight L-1 )

1981

269.8

1982

223.6

1983

698.6

1984

503.3

1985

266.8

1991

2696.2

1992

3636.8

2008

186.41

2009

32.87

2010

161.095

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117

Between 2008–2010 the seasonal distribution of zooplankton assemblages presented different values of biomass among the three areas, which demonstrates the existence of hydro-geomorphological changes on biocoenosis. Using the Multidimensional Scaling (MDS) spatial and temporal distributions of the zooplankton assemblages was obtained. The plot (Fig. 5) describes especially a temporal grouping of the zooplankton biomass. It can be noticed a clear difference among summer and the other two seasons. In terms of spatial distribution, MDS shows a slight resemblance between meanders and natural areas. The single factor ANOVA analysis could confirm the existence of differences between seasons in both littoral medial environments (Tab. 4), while for the three types of ecosystems, no significant differences were found (Tab. 5). This approximately the same patterns in seasonal distribution of dominant groups in terms of total biomass structure were found during the study period. This may be due to nutritional resources available in those periods. The trophic structure of the zooplankton revealed the existence of a complex organization, which are distinguished levels, types and groups (Zinevici & Parpală, 2007). By the ingested feed sizes, the zooplankton consumers are divided in two trophic types: microand macroconsumers. The food of the first group is represented by nanoplankton, bacterioplankton and detritus particles (Ø1-20 µm), and for the second one, is mainly by nano and microplankton (Ø>50 µm) and complementary by detritus-bacterial aggregates (Karabin, 1985). Due the phytoplankton summer composition was mostly represented by nanoplankton and no algae blooming was recorded, we may conclude that the development of cladocerans microconsumers took advantaged. Configuration (Kruskal’s stress (1) = 0.142) 0.6

0.4

N.S sm 1 M sm m C sm 1 1 sm 1 N.S sm m M au M C sm m

0.2

C au 1

-0.6

-0.4

-0.2

0 C au m 0.2M au m -1E-15

0.4 au 1 N.S N.S au m

M spr m -0.2

-0.4

N.S sp L

0.6

0.8

M spr 1 C sp m N.S sp m C sp 1

Fig. 5 - MDS ordinations of zooplankton community from different sampling times and sites (N.S = natural sectors, M = meanders, C = canals, sp = spring, sm = summer, au = autumn, l = littoral, m = medial).

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Table 4

Anova single factor analysis of seasons in the two studied areas. Source of Variation

SS

df

MS

F

P-value

F crit.

littoral

1478.825

2

739.412

11.601

9.39E-06

2.997

medial

4384.397

2

2192.199

18.696

8.08E-09

2.997

Table 5 Anova single factor analysis of the three types of ecosystems in the two studied areas. Source of Variation

SS

df

MS

F

P-value

F crit

littoral

4.149

2

2.074

0.204

0.815

3.011

medial

9.363

2

4.681

0.409

0.663

3.011

The taxonomic phytoplankton composition in the studied period showed the dominance of Bacillariophyceae (80%) followed by Chlorophyceae and Cyanobacteria with equal proportions (10%). The role of diatoms in lotic aquatic ecosystems is widely recognized Bere & Tundisi (2010). The diatoms (silicate species) and cyanobacteria (species with potentially toxic) are characterised by a low edibility degree, the zooplankton choose to eat green algae belonging to Chlorophyceae instead. This seasonal succession of taxonomic groups in terms of biomass is not due entirely to the existing diversity and density at a time in those areas. The crustaceans that dominated in biomass showed no relevant species richness compared to the other groups. The ciliates, testate amoebas, Lamellibranchia and rotifers, showed a much higher species number than crustaceans, but due to their much smaller size, their biomass contribution is much lower (Tab. 6). Table 6

The biomass variations in different periods in Sfântu Gheorghe branch. Study areas

Natural sectors

Meanders

Canals

Period

Other groups

Crustaceans

n.o of species

biomass

n.o of species

biomass

2008

56

33.790

13

159.408

2009

53

12.636

6

13.739

2010

56

44.655

9

110.205

2008

61

25.598

16

131.385

2009

60

16.205

9

24.864

2010

46

25.908

12

140.644

2008

51

29.985

5

179.084

2009

44

14.711

8

16.462

2010

37

47.098

9

114.777

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During the survey, the highest values of biomass abundance were found for two cladoceran species: Moina micrura Kurz, 1874 and Diaphanosoma orghidani Negrea, 1983. Both species were mentioned in literature being able to graze diatoms (González, 2000; Fulton, 1988). Additionally, the development of cladoceran microconsumers was done on behalf of detritus particles and detrito-bacterial aggregates through filtering action of large amounts of water (Kim et al., 2000; Agasild & Nõges, 2005). In Sfântu Gheorghe branch, in summer, cladoceran microconsumers were more favoured comparing with copepod macroconsumers. Autumn was similar to spring in terms of dominant groups in biomass composition. Thus, Copepoda become again the main group (74.49% littoral, 47.95% medial) and then, Rotifera, Cladocera and Lamelibranchia follow them with close values. The contribution of other groups as Ciliata and Testacea was negligible. The physical and chemical features and zooplankton biomass was correlated using canonical correspondence analysis (CCA) in order to establish the factors that modulate the zooplankton assemblages in the studied areas. The CCA ordination diagram reveals that, in general, zooplankton groups are correlated to the same physical and chemical factors, both coastal and in the medial arm. Thus, in the littoral area, cladocerans have positively correlated with NH4, PO4, TP, depth, pH and negative with forms of nitrogen (NO2, NO3) and organic phosphorus. Copepods and ciliates are determined mainly by the positive variation of NO3, NO2 and organic phosphorus and negative of NH4, PO4, TP, depth and pH. What is noteworthy is the fact that the two groups Cladocera and Copepoda are in an inverse report, regarding the influence of physical and chemical factors, which correlates positively with cladocerans biomass, influence negatively the development of copepods (Fig. 6). Testacea, Rotifera and Lamellibranchia are positive correlated with the level O2 and TOC and of and negative with T/D, transparency and temperature. It was found some notable differences regarding the two studied sites. Thus, compared to medial area, in the littoral Rotifera, Testacea, Lamellibranchia are positively correlated with some physical and chemical factors: TOC, O2, TP, NH4, organic P and negative with T/D, transparency, DIN, depth and temperature. The structure of biocenosis is determined both by physical - chemical factors and the biological ones, those influencing both the specific composition and diversity, interspecific relationships and their succession (Makarewicz & Likens, 1979; Balcer et al., 1984). The physical and chemical factors are responsible for structuring of the communities both spatially and temporally (Shurin, 2000). In order to know the degree of influence of the environment and its significance level on zooplankton biomass of the main groups simple correlations were performed (Tab. 7). Conclusions During the study period, the highest values of biomass were found in meanders (both, medial and littoral) for spring and autumn and canals in summer. Seasonal distribution of zooplankton biomass presented different values among the three areas, which demonstrates the existence of hydro-geomorphological changes effects on biocoenosis. For both studied areas, littoral and medial, the biomass is represented mainly by Cladocera and Copepoda. In spring and autumn, Copepoda is the dominant group in biomass fallowed by Cladocera. This is due to the larger body size of crustaceans compared to other taxonomic groups that forms the structure of zooplankton

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3.6 Ciliata

3

Transp.(m)

T/D

2.4 NO2_(mgN/l)

Axis 2

1.8

DIN_(mgN/l)

Copepoda

1.2 0.6

Temp.(C) NO3_(mgN/l)

Porg_(ug/l) TP_(ug/l)

0 -0.6

Cladocera pH NH4_(mgN/l) TRP_(ug/l)

TOC_(mgC/l)

-1.2 -1.8 O2_(mg_O/l) -3

-2.4

-1.8

Testacea Rotifera Depth.(m) Lamellibranchia -1.2

-0.6

0 0.6 Axis 1

1.2

1.8

2.4

a) Littoral

4 DIN_(mgN/l)

3.2 2.4

NO2_(mgN/l)

Temp.(C)

1.6 Transp.(m) Axis 2

Ciliata

Copepoda

Depth(m)

0.8

NO3_(mgN/l)

T/D

0 pH

-0.8

Cladocera TRP(ug/l)

Rotifera TOC_(mgC/l) Porg_(ug/l) Testacea TP_(ug/l) NH4_(mgN/l)

-1.6 -2.4

Lamellibranchia -4.8

-4

-3.2 -2.4 -1.6 -0.8 Axis 1

0

0.8

1.6

O2_(mg_O/l) 2.4

b) Medial Fig. 6 - Canonical correspondence analysis (CCA) ordination plots for zooplankton community and environmental variables in Sfântu Gheorghe branch (a-littoral, b-medial).

Copepoda

Cladocera

Rotifera

Lamellibranchia

Testacea

Ciliata

Groups

medial

littoral

medial

littoral

medial

littoral

medial

littoral

medial

littoral

medial

littoral

R2 P sig. R2 P sig. R2 P sig. R2 P sig R2 P sig. R2 P sig. R2 P sig. R2 P sig. R2 P sig. R2 P sig. R2 P sig. R2 P sig.

0.066 0.040 *

Depth (m)

0.277 9.058E-06 **** 0.117 0.006 **

0.174 0.0006 ***

Transparency (m)

0.069 0.03 *

T/D

0.342 4.693E-07 **** 0.210 0.0001 *** 0.199 0.0002 *** 0.103 0.010 *

0.077 0.026 * 0.083 0.021 *

Temperature (Cº)

0.355 2.488E-07 **** 0.319 1.365E-06 ****

0.094 0.014 *

pH

0.304 4.09E-06 **** 0.190 0.0004 ***

O2 (mg O/l)

TOC (mgC/l) 0.193 0.0003 *** 0.075 0.029 *

0.086 0.019 *

0.061 0.04943 * 0.114 0.006 ** 0.084 0.020 *

NO2 (mgN/l)

0.158 0.001 ** 0.174 0.0006 ***

0.176 0.0006 *** 0.165 0.0009 ***

NO3 (mgN/l)

0.170 0.065 0.0007 0.042 *** * 0.236 5.391E-05 ****

0.102 0.010 * 0.074 0.030 *

0.078 0.025 *

0.078 0.025 *

DIN Ptot (mgN/l) (ug/l) 0.455 1.296E-09 **** 0.501 8.511E-11 ****

Table 7 The coefficient of determination and the significance of simple correlations established between physico-chemical parameters and the biomass of the main zooplankton groups. DISTRIBUTION OF ZOOPLANKTON IN SFÂNTU GHEORGHE BRANCH (ROMANIA) 121

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community. Lamellibranchia and Rotifera found in following positions are much smaller than cladocerans and copepods and their biomass values reflect high density in the studied sites. The lowest values of biomass in entire study period belong to the ciliates and testate amoebas because the number and size of these groups were very small. The approximately the same seasonal distribution pattern of dominant groups of zooplankton assemblage, may due to nutritional resources, available in those periods. In spring and autumn, copepod macroconsumers took advantage and in summer cladoceran microcosumers. The changes in dominant groups from Copepoda in spring to Cladocera in summer and again Copepoda in autumn may be also the ground of so obviously difference in contribution of biomass among the seasons. A similar seasonal distribution of zooplankton biomass could be evidenced by MDS. This analysis describes especially a temporal distribution of biomass and less a spatial one. CCA ordination diagram showed that all physical and chemical parameters influenced in different measures the zooplankton development. ACKNOWLEDGEMENTS Our study was part of the project named: “The impact of hydraulic works on the ecological systems of Sfântu Gheorghe branch, in the context of sustainable development”. The study was funded by project no. RO1567-IBB02/2010 from the Institute of Biology Bucharest of Romanian Academy. We thanks to dr Victor Zinevici for taxonomical revision and to Stela Sofa for technical support.

DISTRIBUŢIA SPAŢIALĂ ŞI TEMPORALĂ A BIOMASEI ZOOPLANCTONULUI ÎN BRAŢUL SFÂNTU GHEORGHE (DELTA DUNĂRII, ROMÂNIA) ÎN RELAŢIE CU FACTORII DE MEDIU REZUMAT Braţul fluvial Sfântu Gheorghe aparţine sistemului macroregional Delta Dunării. În perioada 1983-1989 au fost iniţiate numeroase lucrări hidrotehnice cu scopul de a îmbunătăţi transportul naval pe braţele Dunării. Aceste lucrări care au modificat trăsăturile geomorfologice ale braţelor, s-au dovedit a fi printre cele mai agresive influenţe antropice, de-a lungul timpului în Delta Dunării. În perioada 2008-2010 au fost investigate efectele lucrărilor hidrotehnice asupra distribuţiei spaţiale şi temporale a comunităţilor zooplanctonice. Scopul acestei lucrări este de a evidenţia structura biomasei zooplanctonului în noile condiţii ecologice. Analiza distribuţiei sezoniere a zooplanctonului evidenţiază dominanţa în biomasă a grupelor Cladocera şi Copepoda, în timp ce Ciliata şi Testacea prezintă cele mai mici valori. Rezultatele studiului indică o distribuţie semnificativ diferită a biomasei atât în dinamica sezonieră cât şi în cea spaţială. LITERATURE CITED AGASILD, H., T. NÕGES, 2005 - Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes: in situ measurement with fluorescent microspheres. Journal of Plankton Research, 27 (11): 1155-1174. ALBANIA, A., F. VILLANTE, I. URIARTE, 2009 - Zooplankton communities in two contrasting Basque estuaries (1999-2001): reporting changes associated with ecosystem health. Journal of Plankton Research, 31 (7): 739-752. BALCER, M. D., N. L. KORDA, S. I. DODSON, 1984 - Zooplankton of the Great Lakes: a guide to the identification and ecology of the common crustacean species. University of Wisconsin Pres. Madison, Wisconsin: 62-64. BARTOŠ, E., 1954 - Koreòono ce Radu Testacea. Vydavatel’stvo Slovenskej Akadémie Vied Bratislava, 180 pp. (in Slovak) BERE, T., J. G. TUNDISI, 2010 - Biological monitoring of lotic ecosystems: the role of diatoms. Brazilian Journal of Biology, 70 (3): 493-502. BROOKS, J. L., 1959 - Cladocera. Pp. 587-656. In: W. T. Edmonson (ed.), Freshwater Biology, Second edition. John Wiley and Sons, New York. xx + 1248 pp.

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