Elżbieta Szeląg-Wasielewska1*, Tomasz Joniak1, Michał Michałkiewicz2, Tomasz Dysarz3,4, Beata Mądrecka2

1 Department of Water Protection, Faculty of Biology, Adam Mickiewicz University, Umultowska 89, 61-614 Poznań,

*e-mail: eszelag@amu.edu.pl2 Institute of Environmental Engineering, Poznań University of Technology,

Piotrowo 5, 60-965 Poznań3 Department of Hydraulic Engineering, Faculty of Reclamation and Environmental Engineering, Poznań University of Life Sciences, Piątkowska 94 A, 60-649 Poznań

4 Research Center for Agricultural and Forest Environment, Polish Academy of Sciences, Bukowska 19, 60-809 Poznań

AbstractAbundance and biomass of bacterioplankton were studied in the middle section of the lowland Warta River (Wielkopolska Lake District, Poland) from May to December 2009 in relation to environmental parameters, nutrient concentrations, and fl ow rate. The abundance of planktonic non-phototrophic bacteria ranged from 2.1 106

to 10.6 106 cells cm-3, while phototrophic from 0.8 103 to 5.1 105 cells cm-3, and both showed maximum in July. Phototrophic bacteria were dominated by cells >2 μm and showed greater variability among sampling dates than non-phototrophic bacteria, which were dominated by cells <2 μm. Total abundance of bacterioplankton was signifi cantly correlated with 9 parameters. Two of the correlations were signifi cant at p≤0.001 (total organic carbon, total suspended solids), and two at 0.01≥p>0.001 (temperature, particulate organic carbon). Five correlations were signifi cant at 0.05≥ p>0.01 (dissolved oxygen, biochemical oxygen demand, total phosphorus, electric conductivity, chlorophyll a concentration). No correlation was found between abundance of bacterioplankton and water fl ow. Bacterial dynamics in the Warta River is infl uenced by a cyanobacterial bloom in a reservoir located upstream from the studied section of the river.Key words: phototrophic bacteria, non-phototrophic bacteria, water quality, lowland river, water level.

Bacterioplankton of the Warta River in relation to physicochemical parameters and flow rate

1. Introduction

Bacteria constitute most of the plankton com-munity of various water bodies, and their distri-

bution and diversity are highly variable. They are very heterogeneous, as they include hetero-trophic, mixotrophic, and autotrophic (i.e. pho-toautotrophic or chemoautotrophic) prokaryotes.

DOI: 10.2478/v10104-010-0008-x

Vol. 9No 2-4, 225-2362009

E. Szeląg-Wasielewska et al.

In this study, a simplifi ed division of bacteria into two groups was applied: (1) Ph = photototrophic, mostly cyanobacteria, and (2) NPh = non-photo-trophic, mostly heterotrophic bacteria, but in fact including also chemoautotrophs. All of them af-fect signifi cantly the functioning of aquatic eco-systems, their ecological stability, and the process of eutrophication (Freese et al. 2006). It should be notifi ed that according to the ecohydrology con-cept (Zalewski 2000) the understanding of the re-lations between biota dynamics, including bacte-ria as decomposers, and hydrology are crucial for the water quality.

Bacterioplankton play a fundamental role in decomposition and mineralization of organic mat-ter, as well as regeneration of mineral substances, and production of biomass that is necessary for functioning of other groups of organisms (Azam et al. 1983). Particularly in polluted waters rich in nutrients, bacterial biomass and production are high (Chróst, Siuda 2006) and bacterioplankton are then numerically dominated by heterotrophic bacteria (Szeląg-Wasielewska, Stachnik 2010). Interaction between phototrophic and hetero-trophic bacteria is very important for the control of productivity in plankton, but these two parts of the community remain poorly characterized, espe-cially in lowland eutrophic rivers. Few studies are concerned with both the major groups of bacterio-plankton at the same time, as most studies focus on one component: either phototrophic or hetero-trophic bacteria (Sommaruga, Robarts 1997).

In the present paper the abundance, biomass, and dynamics of bacterial communities in the middle section of the Warta River (Wielkopolska Lakeland, Poland) are described. In this section of the river, water is eutrophic and polluted, and fl ow rate varies greatly between seasons. The Warta has an economic importance as the drinking water so-urce for the city of Poznań. It is crucial to know the dynamic of bacterioplankton communities which signifi cantly infl uence the chemistry of aquatic ecosystems (are a major biological force in nu-trient cycles and biocenosis structure). The major objective of these studies was to evaluate the re-lationships between the bacterioplankton and the environmental factors: selected physicochemical parameters of water and water fl ow.

2. Materials and methods

Study area

The Warta is a medium-sized river, the ma-jor right tributary of the Odra River, and third largest river in Poland. It is 808 km long, and its catchment area accounts for about 1/6 of the area of Poland. This study was concerned with the mid-dle section of the Warta River (Wielkopolska Lake

District, mid-western Poland), and more precisely with the Poznań segment of the river, which has been regulated. In Poznań city, the annual avera-ge discharge of the Warta River is 111 m3 s-1, but the mean discharge at its mouth is almost twice as high: 210 m3 s-1. Upstream from Poznań, the Warta River fl ows through the man-made reservoir Je-ziorsko created in 1986, with an area of 42 km2 and volume of 203 million m3.

Sampling

This study was carried out in 2009, from May to December. Water samples were collected from the current, 0.2 m below the water surface (14 ti-mes, usually every 2 or 4 weeks) at one sampling site located in Poznań city centre, near Królowej Ja-dwigi Bridge (244 km downstream from the sour-ce). Water temperature, pH, dissolved oxygen (DO), and electric conductivity (EC) were measured at the site. Water samples for chemical analysis were col-lected in polyethylene fl asks washed in 10% HCl and rinsed several times in distilled water. The samples for picophototrophic and non-phototrophic bacteria were preserved with buffered formaldehy-de to a fi nal concentration of 1% (Caron 1983) and stored at 4ºC in the dark, and for larger cyanobac-teria with Lugol solution. River water level at the Poznań Water-Level Indicator was read every time immediately before sampling, while data on daily discharge were obtained from the Institute of Me-teorology and Water Management, Poznań Branch.

Laboratory methods

For microscopic analyses in the laboratory, water samples were concentrated on black-coloured Nuclepore fi lters (pore size 0.2 μm) and stained with DAPI. The fi lters were next examined by using au-tofl uorescence of pigments for phototrophs, and secondary fl uorescence for non-phototrophic cells in the epifl uorescence microscope under a magni-fi cation of 1500×, by using green, blue, and ultra-violet light excitation (Porter, Feig 1980; MacIsaac, Stockner 1993). Within bacteria, four morphotypes were distinguished: cocci, rods, curved, and fi la-mentous organisms. In addition, two size groups were distinguished: picoplankton (<2 μm) and nano- with microplankton (>2 μm). Cyanobacterial cells >2 μm were analysed with an inverted micro-scope after sedimentation in settling chambers of 9 cm3 in volume, according to the method of Wetzel and Likens (2000). Cell abundance was expressed per 1 cm3, and cell volume was estimated on the ba-sis of measurements of their size with the Olympus image analysis system, and then their biomass was expressed as wet weight, assuming that the volume of 106 μm3 is equivalent to 1 μg.

Laboratory physicochemical analyses inclu-ded measurements of turbidity Tn (nephelometri-

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Riverine bacterioplankton in relation to physicochemical parameters and flow rate 227

cally, using Eutech Instr. TN-100; unit: NTU), to-tal suspended solids TSS (after fi ltration through cellulose nitrate fi lter 0.45 μm and desiccation at 105ºC; unit: mg dm-3), inherent water colour WC (spectrophotometrically, using Cadas 200 UV--VIS, Dr Lange spectrophotometer, on fi ltrate at 436 nm; unit: mg Pt dm3), total hardness TH (ti-tration method with EDTA; unit: mg CaCO3 dm3), BOD5 (without dilution; unit: mg O2 dm3), total phosphorus TP (after mineralization with H2SO4 in 420°C, molybdate method with ascorbic acid; unit: mg P dm3), total reactive phosphorus TRP (molybdate method with ascorbic acid; unit: mg P dm3), soluble reactive phosphorus SRF (molybdate method with ascorbic acid, on fi ltrate GF/F; unit: mg P dm3), nitrate NNO3 (method with sodium sa-licylate; unit: mg NNO3 dm3), nitrite NNO2 (method with sulfanilic acid; unit: mg NNO2 dm3), ammo-nium NNH4 (Nessler method; unit: mg NNH4 dm3), organic nitrogen Norg (after determination of nitro-gen by a Kjeldahl-type method, using UDK-142 Velp Scientifi ca; unit: mg Norg dm3), total nitrogen TN (sum of mineral forms and Norg; unit: mg N dm3). These analyses were carried out following methods reported in Hermanowicz et al. (1999). Organic carbon was analysed as total (TOC), par-ticulate (POC), and dissolved (DOC) form on fi l-trate (after fi ltration through cellulose nitrate fi lter 0.45 μm), using a Shimadzu organic carbon analy-zer (TOC 5050A).

Chlorophyll-a (Chl-a) concentration was de-termined spectrophotometrically, based on 0.5-l samples fi ltered through Whatman GF/F glass fi -bre fi lters. Pigments were extracted with 90% ace-tone, and the calculations were carried out using Lorenzen’s formula (1967).

3. Results

Water chemistry

During the sampling, air temperature in May and June was low (about 10°C), and it was often

raining, with overcast sky. Consequently, water temperature in early June reached up to 16°C, and it increased as late as in July. Thus the wa-ter temperature curve indicates that the summer season lasted only 2 months (July-August, mean 21.6°C). In early September, water temperature declined (16.1°C), and next it further decreased. A positive correlation was observed between the size of the phytoplankton community, expressed as Chl-a concentration, and water temperature (r=0.73, p<0.003).

Water chemistry did not show any consi-derable seasonal variation, indicating that wa-ter quality was relatively stable. Water pH was slightly alkaline, conductivity relatively low, and total hardness medium (Table I). Conductivity was strongly negatively correlated with water temperature (r=-0.84, p<0.000), very much like oxygen content (r=-0.81, p<0.000). Dissolved oxygen content in the period when water tempe-rature was <18°C, reached approximately 10 mg O2 dm3, while in summer 8 mg O2 dm3 (at least 7 mg O2 dm3). BOD5 values (although moderate-ly high) were variable, but with no clear trend. Statistical analysis did not reveal any signifi cant relationship between BOD5 and dissolved oxy-gen level. BOD5 was also positively correlated with POC (r=0.66, p<0.009) and Chl-a (r=0.65, p<0.012), i.e. various kinds of organic suspended matter.

Among physical properties of water, the least variable was water colour (Table I), whose values were correlated with fl ow rate (r=0.67, p<0.008) and DOC level (r=0.61, p<0.02).

Variation in turbidity and TSS was much higher and dissimilar, although both parameters theoretically refl ect concentrations of the same components (Table I). Temporarily, from May till late July, Tn level was high (mean 15.7 NTU), and next it decreased (9.0 NTU). The only ex-ception was a very high value in early September (40.7 NTU). Tn was not correlated with any phy-sicochemical parameter. In contrast, correlations of TSS enabled identifi cation of the composition

Table I. Physicochemical properties of water in the Warta River.

Parameter Mean Min-Max Parametr Mean Min-Max T (°C) 15.7 3.0-23.7 TP (mg P dm3) 0.303 0.071-1.32 pH* 8.2 7.9-8.5 SRP (mg P dm3) 0.081 0.030-0.148 EC (μS cm1) 575 521-624 PRP (mg P dm3) 0.042 0.015-0.070 TH (mg CaCO3 dm3) 165 121-214 NNH4 (mg dm3) 0.29 0.198-0.38 DO (mg O2 dm3) 9.1 7.0-11.6 NNO3 (mg dm3) 1.15 0.45-2.3 BOD5 (mg O2 dm-3) 4.1 2.0-7.0 N org (mg dm3) 2.26 1.1-3.4 TSS (mg dm3) 12.0 3.2-20.4 TN (mg N dm3) 3.74 2.75-4.4 Tn (NTU) 14.2 3.9-40.7 TOC (mg C dm3) 8.2 6.4-11.0 WC (mg Pt dm3) 30 24-35 DOC (mg C dm3) 6.9 5.1-8.8 Chl-a (μg dm3) 43.6 4.5-133.0 POC (mg C dm3) 1.3 0.1-2.4

E. Szeląg-Wasielewska et al.

of the suspension (Fig. 1 C-D). Organic carbon concentrations were average, but POC values were low (Table I). In spite of seasonal variation, DOC did not show any clear trend, while POC gradually decreased starting from August. DOC accounted for over 60% of TOC, and was partly correlated with fl ow rate (r2=0.37, p<0.022).

Levels of mineral forms of nitrogen did not exceed the norms of water quality class I. Nitrates and nitrites did not show any correlation with fl ow

rate, but a strong positive correlation was recor-ded for ammonium nitrogen (Fig. 1C). The major form of biogenic nitrogen pollution was organic nitrogen, whose level was generally high (Tab-le I). Periodical decline in Norg resulted from an increase in the effi ciency of fl ow rate (Fig. 1D).

Phosphorus level was low for most of the stu-dy period (TP ≤0.2 mg P dm3), but higher in July, associated with an increase in water level (Fig. 2). The role of phosphorus as a factor stimulating

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Fig. 1. Relationships between TSS vs. T (A), TSS vs. Chl-a (B), water level vs. NNH4 (C) and water level vs. Norg (D).

Fig. 2. Seasonal variability of phosphorus compounds and water level of the Warta River.

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Riverine bacterioplankton in relation to physicochemical parameters and flow rate 229

primary productivity was limited, as refl ected by a moderate correla-tion with Chl-a (r=0.55, p<0.042). However, a slightly stronger corre-lation with TOC (r=0.62, p<0.022) confi rmed the association of a part of P with seston. Phosphates were highly variable (Fig. 2). The mean contribution of SRP to the TP pool in summer (July-August) reached only 23%, while in other periods it was about 50%. In the case of parti-culate reactive phosphorus (on ave-rage 22% of TP), no similar seasonal trend was observed.

Microorganisms

The total abundance of bacte-rioplankton ranged from 2.09 106

to 10.72 106 cells cm-3 (mean 5.1 106

cells cm-3) and biomass between 0.18 and 34.61 mg dm-3 (mean 5.9 mg dm-3). Seasonal changes were more clearly visible in biomass than in total abun-dance of bacteria. The maximum va-lue of abundance occurred on 20 July, but the maximum biomass a week la-ter (Fig. 3, 4). This disparity did not result from a lower abundance of NPh but mostly from an increase in abun-dance of phototrophic cells, which are generally larger than cells of NPh.

The abundance of Ph was al-ways considerably smaller than the abundance of NPh, so the maxi-mum contribution of Ph to the total abundance of bacterioplankton did not exceed even 10% (mean 2.2%). However, the mean biomass of Ph was about 24-fold higher than that of NPh, and the contribution of Ph to total bacterioplankton biomass varied widely, but the mean reached over 60% in total bacterioplankton biomass (Table II).

Fig. 3. Seasonal variation in abundance of phototrophic (Ph) and non-phototrophic bacteria (NPh) in the Warta River in 2009.

Fig. 4. Seasonal variation in biomass of phototrophic (Ph) and non-phototrophic bacteria (NPh) in the Warta River in 2009.

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Group Range Mean Range Mean Abundance cells cm-3 Contribution (%) Ph 808 – 507455 113466 0.03 – 9.7 2.2 NPh 2080713 – 10566930 4985219 90.3 – 99.7 97.8 Biomass mg dm-3 Contribution (%) Ph 0.007 – 34.296 5.576 4.0 – 99.2 63.4 NPh 0.087 – 0.527 0.235 0.8 – 96.0 36.6

Table II. Abundance and biomass of phototrophic (Ph) and non-phototrophic bacteria (NPh) and their contribution to the total abundance and biomass of bacterioplankton in the Warta River in 2009.

E. Szeląg-Wasielewska et al.

The abundance but not biomass of bacterio-plankton were signifi cantly and positively correla-ted with water temperature, BOD5, TP, TOC, POC, Chl-a, and TSS, and negatively correlated with dis-solved oxygen and conductivity only (Table III).

Phototrophic bacteria (Ph)Among Ph, larger phototrophic (LPh) cells

were often more numerous than picophoto-trophic bacteria (PPh), particularly from August to November. LPh abundance fl uctuated from

0.3 103 to 4.9 105 cells cm-3 (mean 9.45 104 cells cm-3) and was higher than the abundance of PPh in 9 out of 14 sampling sessions (Fig. 5). Mean abundance of LPh was 5 ti-mes higher than that of PPh. The contribution of LPh cells to the total abundance of Ph often exceed 60% (mean 56%), but their contribution to the total biomass of Ph was obvio-usly higher, up to 99.9% (mean 93%) (Table IV).

Within phototrophic bacteria, the highest abundance was reached by Aphanizomenon and Microcy-stis species. Picoplanktonic cells of phototrophic bacteria were not very numerous, ranging from 0.5 103 to 60 103 cells cm-3, with a peak in mid-August (Fig. 5). PPh were single-cel-led or formed colonies, which were morphologically identifi ed as Apha-nocapsa and Cyanogranis species.

230

Table III. Linear correlation coeffi cients between the abundance and biomass of total bacterioplankton, phototrophic, non-phototrophic bacteria with the investigated parameters.

Bacterioplankton Phototrophic Non-phototrophic Parameters Abundance Biomass Abundance Biomass Abundance Biomass

Temperature **0.75 - - - **0.75 **0.75 Turbitity - - - - - - Colour - - - - - - PH - - - - - - Dissolved oxygen *-0.56 - - - *-0.54 *-0.61 Oxygen saturation - - - - - - BOD5 *0.60 - - - *0.60 - Ammonia nitrogen - - - - - - Nitrite nitrogen - - - - - - Nitrate nitrogen - - - - - - Mineral nitrogen - - - - - - Organic nitrogen - - - - - - Total nitrogen - - - - - - Ortophosphates - - - - - - Total phosphorus *0.57 - - - *0.57 **0.74 TOC ***0.79 - - - ***0.79 ***0.87 DOC - - - - *0.58 - POC **0.77 - - - **0.76 **0.62 Total hardness - - - - - - Electric conductivity *-0.64 - - - *-0.63 **-0.68 Chlorophyll-a *0.64 - - - *0.63 **0.69 Total suspended solids ***0.78 - - - ***0.78 **0.76 Water level - - - - - - Discharges - - - - - -

Significance level: * 0.05��p����������0.01�p�0.001; *** p�0.001; - non-significant.

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Riverine bacterioplankton in relation to physicochemical parameters and flow rate 231

Ph biomass varied from 0.007 mg dm-3 to 34.3 mg dm-3 (mean 5.7 mg dm-3). It was the highest at the end of July and the lowest in December, but se-veral peaks were recorded, for example at the beginning of September and Octo-ber. The peaks in July were due to Apha-nizomenon fl os-aquae, Microcystis and Anabaena species, whereas those in Oc-tober were caused mostly by Microcystis aeruginosa and Pseudanabaena mucico-la. Biomass was mostly affected by LPh. The biomass of LPh varied widely: from 0.007 in December to 34 mg dm-3 in late July, and the mean reached nearly 5.7 mg dm-3. The biomass of PPh was very small (below 0.03 mg dm-3), so that its maximum and mean contributions to Ph biomass were only 39% and 6.8%, re-spectively (Figs. 6 and 7, Table IV).

The abundance and biomass of Ph was not signifi cantly correlated with the investigated parameters of water quality and fl ow (Table II).

Non-phototrophic bacteria (NPh)Throughout the study period, the

abundance of NPh fl uctuated from 2.08 106 to 10.57 106 cells cm-3 (mean 4.99 106 cells cml-3), and NPh biomass from 0.087 to 0.527 mg dm-3. From May to July, both abundance and bio-mass distinctly increased, and on 20 July a maximum was noted. Later on, abundance and biomass generally gra-dually decreased but on the last sam-pling date again increased values were observed (Fig. 8).

NPh abundance and biomass were positively and signifi cantly correlated with water temperature, BOD5 (only abundance), Chl-a, TSS, TP, TOC, DOC (only abundance), and POC, and negatively with con-ductivity and dissolved oxygen (Table III).

Among the NPh morphotypes observed, coc-ci dominated in all water samples. Their mean con-

tribution to NPh abundance was 57% (43−70%). The mean contributions of rods and curved cells were 27% (16−46%) and 16% (4.6−23%), respec-tively. In NPh biomass, cocci again were the most important morphotype, as their mean contribution

Table IV. Abundance and biomass of picophototrophic (PPh) and larger phototrophic bacteria (LPh) and their contri-butions to the total abundance and biomass of phototrophic bacteria in the Warta River in 2009.

Group Range Mean Range Mean Abundance cells cm-3 Contribution (%) PPh 548 – 59968 18918 0.8 – 98.7 44.0 LPh 260 – 493280 94548 1.3 – 99.2 56.0 Biomass mg dm-3 Contribution (%) PPh 0.0003– 0.025 0.009 0.002 – 39.0 6.8 LPh 0.007 – 34.286 5.666 1.3 – 99.9 93.2

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Fig. 7. Contributions of picophototrophic (PPh) and larger phototrophic bacteria (LPh) to phototrophic biomass in the Warta River in 2009.

Fig. 6. Seasonal variation in biomass of picophototrophic (PPh) and larger phototrophic bacteria (LPh) in the Warta River in 2009.

E. Szeląg-Wasielewska et al.

to NPh biomass was 49% (35−61%). The mean contributions of rods and curved cells were similar: 18.0% (7.5−37.3%) and 18.6% (9.8−31.9%), respectively (Table V, Fig. 8).

Larger non-phototrophic bacteria (LNPh, >2 μm) were less abundant than picoplanktonic non-phototrophic bacteria (PNPh, <2 μm), and fi lamentous bacte-ria occurred within LNPh (Fig. 9). This morphological form had a considerable effect on NPh biomass on several sam-pling dates, i.e., at the end of July (23%), mid September (29%) and October (27%), but on average its contribution reached only 14.5% (Fig. 10, Table V).

4. Discussion

In the study period, the level of pollu-tion in the Warta River in Poznań was rela-tively stable. According to the trophic state classifi cation by Wetzel (1983 after Schulz, Bischoff 2008), the river water was usually eutrophic. An unexpected increase in fer-tility, due to phosphorus, was recorded in July. It was probably caused by an increa-sed water discharge from the dam reservoir Jeziorsko, which is rich in phytoplankton, as refl ected by very high concentrations of Chl-a. The Warta River is a regulated watercourse. In the dam reservoir Jezior-sko, located upstream from Poznań, water management is realised within an annual cycle. Melt waters are accumulated in the

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Fig. 8. Biomass of morphotypes of non-phototrophic bacteria in the Warta River in 2009.

Fig. 9. Seasonal variation in abundance (upper panel) and biomass (lower panel) of non-photo-trophic cells smaller and larger than 2 μm in the Warta River in 2009.

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cells smaller than 2 μm

Group Range Mean Range Mean Abundance 103cells cm-3 Contribution (%) Cocci 1.364 – 5.545 2.829 42.5 – 70.0 56.8 Rods 0.516 – 2.535 1.334 16.0 – 45.8 26.8 Curved 0.095 – 2.422 0.794 4.6 – 22.9 15.9 Filamentous 0.007 – 0.061 0.023 0.1 – 1.4 0.5 Others 0.000 – 0.038 0.005 0.0 – 1.2 0.1 Biomass mg dm-3 Contribution (%) Cocci 0.046 – 0.264 0.114 35.4 – 61.2 48.6 Rods 0.019 – 0.063 0.042 7.5 – 37.3 18.0 Curved 0.008 – 0.069 0.044 9.8 – 31.9 18.6 Filamentous 0.004 – 0.099 0.034 4.8 – 28.6 14.5 Others 0.000 – 0.005 0.001 0.0 – 1.4 0.3

Table V. Abundance and biomass of non-phototrophic bacteria morphotypes and their contributions to the total abun-dance and biomass of non-phototrophic bacteria in the Warta River in 2009.

Riverine bacterioplankton in relation to physicochemical parameters and flow rate 233

reservoir in spring, from April till June the dam water level is almost constant, whereas later the water reserves are partly released, depending on needs.

In summer, as reported by Kajak (2001), the major factor affecting wa-ter chemistry is temperature. This was confi rmed especially in the case of TSS, which is strongly and positively corre-lated with Chl-a. It is noteworthy that TSS is not correlated with Tn, although both parameters refl ect concentrations of similar components. Moreover, turbi-dity defi ned as the ability of substances suspended in water to absorb or diffuse a stream of light passing through water, was not signifi cantly correlated with any studied property of water. This suggests that turbidity, particularly high in summer, could result from the presence of amorphous mixtures of orga-nic colloids in water. Due to water movements, the colloids did not precipitate and thus did not increase the values of TSS.

In relation to the development of organisms, large amounts of mineral nutrients are taken up from water, especially the easily assimilated so-luble reactive phosphorus and NNO3 (Grobbela-ar, House 1995). Thus the contribution of these compounds to the total pool of elements was low and not signifi cant. In the case of phosphorus, positive but moderately strong correlations with Chl-a and TOC indicated an association of a part of P with seston (Schulz et al. 2003). Changes in fl ow rate were strongly and negatively correlated with Norg. This may be an effect of dilution or an unfavourable infl uence on phytoplankton (Goł-dyn 2000).

River water was well oxygenated also in summer. This could be due to a low oxygen de-mand during biological decomposition of organic matter, or intensive replenishment of its reserves in the process of photosynthesis and diffusion of atmospheric oxygen. Statistical analysis did not reveal any signifi cant relationship between BOD5 and dissolved oxygen level, although such a rela-tionship is often recorded in rivers (Moczulska et al. 2006). This indicated that easily oxygenated organic compounds did not play any important role. The lack of associations between BOD5 and both DOC and Norg meant that part of dissolved organic matter is constituted by coloured organic compounds with high molecular weight, which are humic-mineral complexes of soil origin (Szpa-kowska 1999). Their importance for water quality was confi rmed by a positive correlation between fl ow rate and both water colour and DOC. In ri-vers, DOC concentrations depend on the intens-ity and character of water supply: autochthonous production and export from catchment (Ivarson, Jansson 1994; Zieliński 2004). Chemical comple-

xes in freshwater habitats are decomposed (pho-todegradation, biodegradation), which results in a release of ions from the complexes, mostly of NH4

+ (Wetzel et al. 1995), which could also be the case in the Warta.

Within bacterioplankton, both phototrophic (Ph) and non-phototrophic bacteria (NPh) cells were observed. Throughout the study period in the Warta River, NPh prevailed numerically over Ph. Mean NPh abundance was about 44-fold higher than that of Ph. In contrast, mean NPh biomass was about 24-fold lower than that of Ph. The maximum densities of the two groups were recorded on different sampling dates. The appea-rance of maximum abundance of NPh and Ph in different periods may attest to differences in their environmental requirements, the ability to use food resources, and/or sensitivity to the pressure of consumers.

Bacterioplankton in nature are preyed upon by a wide range of consumers, and the larger cells get eaten preferentially over the smaller cells (Ko-ton-Czarnecka, Chróst 2003). With such a high density recorded in the Warta River, the smaller -sized bacterioplankton might serve as a single source of nutrition for most aquatic animals, in-cluding ciliates. In turn, for large fi lamentous and colonial bacterioplankton, other phenomena may be more important, e.g. sedimentation or viral infections. However, the small size of bacteria in nature gives them a great advantage when living in dilute nutrients. Because their surface area to volume ratio is much greater than that of larger organisms, such as phytoplankton, they can more easily take up nutrients from their environment (Boulton, Brock 2001). Thus cell size is an impor-tant parameter from the functional point of view and it is often analysed in relation to morpholo-gical forms. In the bacterioplankton of the Warta in 2009, picoplanktonic cocci accounted for 57% of total abundance. Their contribution to biomass was slightly lower (49%). Generally, this study confi rmed the results of research on bacterioplank-ton in the Warta in 2005 (Szeląg-Wasielewska,

Fig. 10. Biomass proportions of non-phototrophic cells smaller and larger than 2 μm in the Warta River in 2009.

0%

20%

40%

60%

80%

100%

25.0

5

8.06

22.0

6

6.07

20.0

7

28.0

7

11.0

8

24.0

8

7.09

21.0

9

5.10

19.1

0

16.1

1

14.1

2

Sampling date

cells smaller than 2 μm cells larger than 2 μm

E. Szeląg-Wasielewska et al.

Stachnik 2010), where cocci accounted for 58% of total abundance and 51% of total biomass of NPh. A similar phenomenon was also found in lake and marine waters. Levis et al. (1986) found that cocci dominated in Lake Valencia (2/3 of total bacterial cells), Robarts et al. (1996) in Mediterranean Sea (about 90%) and Jochem (2001) in Gulf of Mexico (40-60%). In contrast, others report on dominan-ce of rods in morphological structure, e.g. Żmuda (2005) in surface bacterioplankton along the Gdy-nia-Brest transect (82-96%) and Jankowska et al. (2005) in two Arctic glacial fjords (over 60%).

One of the environmental factors that may have affected the bacterioplankton community in the Warta was water temperature. Some investi-gators (Freese et al. 2006; Zieliński, Leśniewska 2006) observed a positive relationship between water temperature and the abundance of bacteria. The present study of the Warta confi rms that this correlation is signifi cant, although only with bac-terioplankton abundance.

The Ph abundance in the Warta during the sampling period in 2009 was generally high (mean 1.1 105 cells cm-3) and exceed the mean value of 3.3 104 cells cm-3, which was recorded in an earlier study of this river (Szeląg-Wasie-lewska et al. 2009). That study was carried out in 2005 and mean Ph abundance was over 3-fold lower then than in 2009. In the present study also the maximum biomass of Ph was almost 3-fold higher than in 2005. However, in both years, biomass of the same two species was the most important: large fi laments of Aphanizomenon fl os-aquae and globular colonies of Microcystis species.

The maximum Ph abundance was recorded on 20th July, and the maximum biomass a week later, when the abundance of LPh (>2 μm) increa-sed signifi cantly. Within Ph, the biomass of cells smaller than 2 μm, i.e. picophototrophic bacteria (PPh), was about as low as in 2003 (Szeląg-Wa-sielewska 2004) and 2005 (Szeląg-Wasielewska et al. 2009), which suggests that the smallest phototrophic cells do not fi nd suitable conditions for development in polluted aquatic ecosystems, like the Warta. This may be associated with the sensitivity of PPh to eutrophication and pollu-tion. Many earlier studies detected that the con-tribution of picoplankton to total phytoplankton biomass decreases with increasing trophic state. Especially, as reported by Burns and Stockner (1991), the abundance of picocyanobacteria dec-lines with increasing trophic state. Furthermore, as found by Weisse and Mindl (2002), picocya-nobacteria are highly sensitive bioindicators of pollution.

Regarding the relationships between the abundance and biomass of Ph with physicoche-mical parameters and fl ow, no signifi cant relation was found. Also in rivers in the province of Ma-

drid (Spain), no relation was found between inor-ganic dissolved nitrogen content and cyanobacte-rial Cy biomass, but soluble reactive phosphorus had a threshold effect on Cy biomass (Doutere-lo et al. 2004). Also a marginal effect of water temperature on Cy biomass was recorded in the study conducted in 2005 and the correlation was positive (Szeląg-Wasielewska et al. 2009). In con-trast, in 2009 this correlation was completely non-signifi cant. This was due to the fact that the high Ph biomass, caused by a massive bloom of A. fl os-aquae, was observed both at a high water tem-perature and at a much lower water temperature, in September and October, when M. aeruginosa appeared. This confi rms the earlier observations that in natural communities with a diversifi ed spe-cies composition, optimum temperatures prevail throughout the year, since species characteristic of warm waters are replaced by species characte-ristic of colder waters when water temperature is decreasing (Nyholm 1978).

In 2009, the mean NPh abundance in the Warta River was about 5.0 106 cells cm-3, and fell within the range of values reported for eu-trophic environments (Sorokin 1999). This va-lue was only slightly lower than the mean of 5.9 106 cells cm-3, recorded in the Warta in 2005 at the same site. In spite of the lower mean value, the range of variation in 2009 was larger, and the maximum value exceeded 10 106 cells cm-3, which was not the case in 2005 (Szeląg-Wasie-lewska, Stachnik 2010). Higher mean values of NPh abundance were recorded in the nearby Cybina River (a tributary of the Warta), at its infl ow into the Antoninek (10.9 106 cells cm-3) and Maltański (8.3 106 cells cm-3) reservoirs (Gołdyn, Szeląg-Wasielewska 2005). The lower abundance of bacteria in most Australian rivers is about 1-5 106 cells cm-3 (Boulton, Brock 2001), but in Poland a much lower value (1.59 106 cells cm-3) was reported for the Narew River at its infl ow into the Siemianówka Reservoir (Zieliń-ski, Leśniewska 2006). Bacterial abundance ex-ceeding 20 106 cells cm-3, as reported by Freese et al. (2006), is generally rare, and there are only few brackish or marine lagoons and ponds with higher abundances.

The Warta River carries large amounts of suspended solids and chlorophyll a, partly ori-ginating from the fertile Jeziorsko Reservoir, which is located upstream. Thus bacterioplank-ton growth in the river is infl uenced by the large amounts of substrates from that reservoir and from some other sources, as the Warta receives treated and untreated wastewater from extensive urban areas, as well as agricultural runoff. Consi-dering this, concentrations of organic substrates and inorganic nutrients (nitrogen, phosphorus) may be suffi cient and not limit bacterial growth, as is the case in meso- to oligotrophic waters.

234

Riverine bacterioplankton in relation to physicochemical parameters and flow rate 235

Conclusions

This study throws light on the importance of lowland river waters as habitats for prokaryo-tes and reservoirs of their biomass, and relations between phototrophic and non-phototrophic com-ponents of bacterioplankton. Non-phototrophic bacteria were dominated by picoplanktonic cocci, while phototrophic bacteria by large fi lamentous and colonial species of the genera Aphanizome-non, Microcystis, and Anabaena. Total abundance of bacterioplankton (but not biomass) was signi-fi cantly correlated with 9 parameters, i.e. 38% of all the studied parameters. Two of the correlations were signifi cant at p≤0.001 (total organic carbon, total suspended solids), and two at 0.01≥p>0.001 (temperature, particulate organic carbon). Five correlations were signifi cant at 0.05≥ p>0.01 (dis-solved oxygen, biochemical oxygen demand, to-tal phosphorus, electric conductivity, chlorophyll a concentration). In the river system of the Warta in its middle section, affected by a large reservoir located upstream from it, the dynamics of bacte-rioplankton abundance and biomass is high but not signifi cantly associated with fl ow rate and most parameters of water quality. It can be suppo-sed that their dynamics is also modifi ed by biotic factors, e.g. zooplankton foraging, including ci-liates. In this river system, in the future interdis-ciplinary research on bacterioplankton should be carried out, including also the infl uence of chosen biotic factors.

Acknowledgements

This study was supported by grant no 51200074 (interproject between three universities in Poznań: Adam Mickiewicz University, Poznań University of Technology, and Poznań Univers-ity of Life Sciences). Microscopy analyses were possible due to sponsorship of the Foundation for Polish Science, which bought the Olympus BX-60 microscope.

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