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Journal of Environmental Sciences 21(2009) 62–69

Persistence and growth of faecal culturable bacterial indicators in watercolumn and sediments of Vidy Bay, Lake Geneva, Switzerland

POTE John∗, HALLER Laurence, KOTTELAT Regis, SASTRE Vincent,ARPAGAUS Philippe, WILDI Walter

University of Geneva, F.-A. Forel Institute, 10 route de Suisse, 1290 Versoix, Switzerland. E-mail: john.pote@terre.unige.ch

Received 23 January 2008; revised 14 April 2008; accepted 29 April 2008

AbstractThe aims of this study was to investigate the persistence and the growth of culturable bacterial indicators (CBI) including total

coliforms (TC) and faecal coliforms represented by Escherichia coli, enterococcus (ENT), and aerobic mesophilic bacteria (AMB) inthe surface sediments and the water column of Vidy Bay (Lake Geneva, City of Lausanne, Switzerland). The study was carried out for60 d using microcosms containing Sewage Treatment Plant (STP) effluent and nonsterile water without CBI, as well as contaminatedand non-contaminated sediments. The effects of water temperature and of organic matter associated with sediments on the survival ofCBI in the sediments and the water column were observed. The number of CBI colonies in the contaminated sediments of Vidy Bay andin the STP effluent was almost identical in the order of 105–107, 104–106, 103–105, and 104–107 CFU/100 g sediment or /100 mL waterfor TC, E. coli, ENT, and AMB respectively. A degradation of CBI was observed in the sediments where organic mater content waslow and in the water column at a temperature of 10°C after 5 d of experimentation. In addition, a growth of CBI was observed in thesediment which is rich in organic matter at 20°C. The results of this study indicate: (1) the higher concentrations of the CBI observedin different points in the water column of Vidy Bay may not be explained only by the recent contribution of the three potential sourcesof the Bay contamination including STP and the Chamberonne and Flon Rivers, but also by the persistence, removal from sedimentand multiplication of CBI in the sediment and water column; (2) the sediment of Vidy Bay constitute a reservoir of CBI and can evensupport their growth; and (3) the CBI not only survive in sediments, but also can be remobilized and increased in the water column,therefore, it become a permanent microbiological pollution in Vidy Bay.

Key words: Lake Geneva; faecal pollution; persistence; multiplication; human health risk

Introduction

The sewage treatment plant (STP) and rainwaterdrainage systems constitute the principal sources of con-tamination of Swiss rivers, reservoirs, and lake sedimentsby different kinds of pollutants (Wildi et al., 2004).There is still a paucity of information concerning thecontamination of sediments by microorganisms and thereis little information to be found regarding the persistenceof culturable bacterial indicators (CBI) in the sediments ofVidy Bay and their release in the water column.

In the aquatic environment, sediment may constitute areservoir of different pollutants including heavy metals andmicroorganisms. Some studies have assessed the presenceof bacterial indicators in the sediments and the watercolumn (Anderson et al., 2005; Baghel et al., 2005; Daviset al., 2005). The influence of environmental conditionson the persistence and accumulation of sediment hostedbacteria was at levels 100–1 000 times higher than thatin the water column, with a potential risk of pollutionfor coastal recreational water have been demonstrated

* Corresponding author. E-mail: john.pote@terre.unige.ch

(LaLiberte and Grimes, 1982; Davies et al., 1995; Evasonand Ambrose, 2006). Sedimentation is one of the mecha-nisms involved in the accumulation and immobilization ofpathogens (Karim et al., 2004). Accumulation of CBI andpathogenic organisms in sediments has been attributed tothe sorption of the microorganisms to particles suspendedin water. Therefore, sediments can constitute an importantreservoir of CBI in freshwater (Burton et al., 1987; Crabillet al., 1999; An et al., 2002; Alm et al., 2003).

The presence of Escherichia coli in lake water indicatesthe water contaminated by faecal material of humansor other warm-blooded animals, and also indicates thepotential for the presence of pathogenic organisms (An etal., 2002). The survival of bacterial indicators in the soil,the water column, and the sediments is controlled by avariety of factors including solar radiation, temperature,pH, salinity, heavy metals, predation, and competition bynative microflora, sediment grain size and organic mattercontent (Gerba and McLeod, 1976; Noble et al., 2004;Muruleedhara et al., 2006; Yang et al., 2007). Therefore,the use of one single indicator organism to monitor bacte-riological water pollution seems inadequate.

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The surface recreation water generally contains indige-nous microorganisms, pathogenic, and non-pathogenicmicrobes. The choice of bacterial indicators is thus veryimportant for the management of aquatic environmentalquality. In this study, four CBI including TC, E. coli, en-terococcus (ENT) and aerobic mesophilic bacteria (AMB)were used as microbial indicators to monitor water micro-biological quality. During recreational activities, exposureto high concentrations of the bacterial indicators TC, E.coli and ENT in coastal waters may increase the risk ofillness and disease, including gastrointestinal and respira-tory illnesses and skin, ear and eye infections (Kay et al.,1994; Haile et al., 1999). TC and AMB in particular are notnecessarily pathogenic, but their high concentration canbe taken as an indication of faecal water contaminationand consequently as a signal of the presence of humanpathogenic microorganisms (Noble et al., 2003; Pearce etal., 2006).

Several studies regarding the fate of CBI in the aquaticenvironment relied on laboratory microcosm experiments.Various microcosms have been used to examine the per-sistence and survival of bacterial indicators in the watercolumn and the sediments (Gerba and McLeod, 1976;Laliberte and Grimes, 1982; Craig et al., 2004; Andersonet al., 2005; Mar Lleo et al., 2005). The majority ofthese studies were performed using bacterial indicatorstrains inoculated in microcosms. In the present study,the microcosms, with contaminated surface water lyingover contaminated sediments were designed to examinethe persistence of CBI in the sediment and in the watercolumn. The survival of CBI can be controlled by bioticand abiotic water and sediment parameters. These parame-ters may vary substantially according to the type of coastalwaters. The Vidy Bay constitutes a special case and theresearch on this site proved to be of particular interest. Theobjective of this study was to examine: (1) the incidenceand occurrence of CBI in sediments of Vidy Bay, (2) thepersistence of the CBI in sediments and water column,

and (3) the remobilization of the CBI from the naturalcontaminated sediments and their growth capacity in thewater column.

1 Materials and methods

1.1 Study area

Lausanne is the most important city discharging treateddomestic and industrial wastewater into Lake Geneva. TheSTP is located at Vidy and treats mostly 1 to 3 m3/s andexceptionally up to 5 or 6 m3/s of raw water. Treatedwastewater is then discharged directly into the bay. TheChamberonne River presently drains surface water fromits natural drainage basin and some untreated wastewater.The Flon River is currently a collector of surface andwastewater in the western part of Lausanne. The water isgenerally treated at the Vidy STP but flows directly intothe lake during storms and floods if the discharge exceeds5 to 6 m3/s (Fig. 1). As a result, the Vidy Bay is the mostcontaminated area of Lake Geneva (Loizeau et al., 2004).

1.2 Sediment and water sampling

The boat “La Licorne” of the Forel Institute (Univer-sity of Geneva, Switzerland) was used to collect bottomsediments (layer of 0 to 6 cm thickness) from Creux-de-Genthod (CDG) and Vidy Bay using a “Buchi grabsampler”. The sediments from Versoix were collectedmanually. The STP effluent water was sampled directlyfrom the STP outlet pipe. Water from the lake was sampledin the area of Versoix at 5 m depth with a centrifugalpump and filtered at 1.2 µm (CUNO filter). The regionsof Versoix and Creux-de-Genthod of Lake Geneva areknown to be free of STP pollution. Sampled sediments andwater were kept at 4°C and transported immediately to thelaboratory. Sample analysis was undertaken within 24 h.The GPS locations of the sites of sediments sampled arepresented in Fig. 1.

Fig. 1 Schematic location of sampling area: Vidy Bay, sewage treatment plant, Chamberonne River and Flon River.

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1.3 Sediment and water characterization

The physicochemical parameters of sediments and waterare given in Tables 1 and 2. The sediments were driedat 60°C for 48 h and the water content was calculatedfrom a weight difference. The dried sediments were thenheated at 560°C for 1 h to determine the organic mattercontent. The particle size distribution was measured witha laser Coulterr LS 100 diffractometer (Beckman Coulter,Fullerton, USA). In water, the conductivity, temperature,and the pH were measured using a Multi 350i (WTW,Germany). The concentration of dissolved oxygen wasmeasured with a Multi 350i and a titrimetric field kit(Merck N◦ 1.11107). Dissolved organic carbon (DOC) wasmeasured on acidified samples (200 µL of 2 mol/L HCl in30 mL sample) using the Shimadzu TOC 5000 (GmbH,Switzerland).

1.4 Microcosms

The microcosms consisted of plastic aquaria of size 46.5cm × 22 cm × 26 cm (L × W × H) with an overflow at21 cm. The sediments were homogenized with a spatula,sieved at 2 mm and filled into the microcosms to a height of3–5 cm. Pre-filtered lake water or STP effluent was addedcarefully and the suspension in the water column was leftfor deposition. Water renewal was made with peristalticpumps and Tygon R-3607 (Milian SA, Switzerland) andsilicone tubing. Renewal rate of water was about 18 L/dand water was introduced at mid-height close to the wallopposite to the overflow.

1.5 Bacteria quantification

The CBI number in water was carried out using mem-brane filter method. TC and FC were represented by E. coliand ENT. This method was performed by filtering 100 mLof water onto membrane filters (47 mm in diameter with

Table 1 Sediment characteristics in Lake Geneva in Swiss coordinates.

Parameter SedimentVidy Bay Creux-de- Versoix

Genthod

Depth (m) 35 50–55 < 1Distance to coast (m) 300 1 000Water content (%) 68–72 80–82 29Organic matter content (%) 32–38 7–11 2–3pH 7.2–7.8 7–8 7.4–8.2Particle size d (µm)Mean 63.66 25.92 23.79Median 73.87 27.7 23.81Mode 80.66 30.73 24.8095% Confidence limit 4.49–902 6.02–112 1.57–360

Table 2 Water characteristics

STP effluent Filteredwaters lake water(Vidy Bay) (versoix area)

Conductivity (µS/cm) 700–900 244pH 6.8–8.1 8.2 ± 0.2DOC (mg/L) 67–89 0.9–1.2Dissolved oxygen (O2 mg/L) 7.5–8.3 7–8.5

STP: sewage treatment plant.

0.45 µm pore size, Schleicher & Schuell MicroScience,Germany) using the following culture media (Oxoid, Ltd.,England) and incubation conditions. TC: Endo agar medi-um, incubated at 35°C for 24 h. E. coli: Tryptic Soy Agarmedium, incubated at 37°C for 4 h and transferred intoAgar Mug medium at 44°C for 24 h. ENT: Slanetz BartleyAgar medium, incubated at 37°C for 48 h and transferredinto Bile Aesculin Agar medium at 37°C for 4 h. TheAMB number was determined by inoculating aliquots ofwater samples into Plate Count Agar (PCA) medium andincubated at 30°C for 72 h. The CBI in sediment wereresuspended by adding 100 g (wet weight) of sediment to500 mL of 0.2% Na6(PO3)6 (Bruni et al., 1997) in 1 Lsterile plastic bottles and mixed for 1 h using the agitatorrotary printing-press Watson-Marlow 601 controller (Skan,Switzerland). The mixture was then centrifuged at 2300r/min for 15 min at 15°C. The supernatant was used forthe CBI counting. The results were expressed as colonyforming units (CFU) per 100 mL of water or 100 g ofsediment.

1.6 Mobilization and survival studies

Four microcosms (M1, M2, M3 and M4) were per-formed to simulate natural conditions. M1 contained VidyBay sediments and STP effluent; M2 contained Vidy Baysediments and filtered non-sterile lake water; M3 containedsediments from Versoix and STP effluent water; and M4contained sediments from Creux-de-Genthod and STPeffluent. The control microcosms M5 and M6 containedsediments from Creux-de-Genthod and Versoix, respec-tively, with filtered lake water from the Versoix region.

The microcosms were thermostabilized and kept indark room at different temperatures: 10±1, 20±2, and25±1.5°C. Both water and sediments from all microcosmswere sampled on day 1, 5, 10, 20, 30, 40, and 60 for CBIanalysis. The sediments were sampled and homogenisedafter removing overlying water from microcosms using aperistaltic pump. After sampling, both water and sedimentswere immediately analysed for CBI quantification. Allmicrocosm experiments were conducted in triplicate. Foreach sample, about 500 mL of water and/or 100 g ofsediment were sampled from microcosms.

2 Results

2.1 Concentration of CBI in sediments and lake water

Before assessing the survival and the remobilization ofbacterial indicators in the microcosm sediments and watercolumn, the CBI number was quantified in the sedimentsfrom three different sites of the lake (Vidy Bay, Creux, andVersoix), in filtered lake water (LW) and in STP effluent.The highest number of CBI was observed in the sedi-ments from Vidy and STP effluent. High CBI levels weremeasured in the STP effluent and Vidy Bay sediments.The concentration of CBI in the STP effluent ranged from106–107, 105–106, 104–105, and 104–107 CFU/100 mLfor TC, E. coli, ENT, and AMB respectively. In the VidyBay sediments, CBI concentration ranged from 106–107,

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105–106, 103–105, and 105–107 CFU/100 g sediment forTC, E. coli, ENT, and AMB respectively. In the sedimentsfrom Creux-de-Genthod and Versoix and the filtered waterfrom the Versoix region, the number of CBI was very lowor not detected (Table 3).

2.2 Physicochemical parameters in the microcosms

The physicochemical parameters in microcosms weremonitored during the experiments. The microcosm tem-peratures were stabilised. Compared with in situ values(measured in the lake), the physicochemical parametersin microcosms did not vary significantly during experi-mentation. The average pH of the water column and thesediments for all microcosms were in the range of 7.2to 8, the average conductivity values were about 750 and400 µS/cm for microcosms M1 and M3 respectively, andabout 230 µS/cm for others. The average value of dissolvedoxygen for all microcosms was 8 mg/L.

2.3 Survival of CBI in the sediments and water columnmicrocosms

2.3.1 Survival of CBI in microcosms maintained at10°C

The correlation between four CBI was observed inboth the water column and the sediments in microcosms(Fig. 2). The concentrations of the CBI remained stablein microcosms M1, M3, and M4. This concentration isin the same order in the water column and sediments.The remobilization of CBI from sediments was observedin the water column of microcosm M2 on the first dayof experimentation. However, the decay of all CBI inthe water column and sediment of this microcosm wasobserved after 5 d of experiments. E. coli, ENT, and AMB

concentrations decayed (concentration < 20 CFU/100 mLwater or 100 g sediment) and were comparatively less thanthe initial concentration in the natural sediments of VidyBay within 5 d (Fig. 2d). However, all CBI persisted inboth water column and sediments and maintained theirculturability during the 60 d of experimentation.

2.3.2 Survival of CBI in microcosms maintained at 20and 25°C

The analysis of these bacterial indicators in the watercolumn and the sediments of microcosms incubated at 20and 25°C presented almost the same pattern of results. Ingeneral, persistence and growth of CBI were observed inthe water column and the sediments of all microcosms. Novariation of bacterial indicator concentration was observedbetween the water column and the sediments of controlmicrocosms (M5 and M6).

M1, M2, and M4 presented a higher survival rate of CBIin sediments than in the water column. In contrast, no greatdifference in the CBI numbers was observed between thesediments and the water column of microcosm M3 duringexperimentation. Compared with others microcosms, nogrowth of CBI was observed in the sediments of micro-cosm M3. The persistence of the CBI observed in thesesediments was attributed to the continuous flow of STPeffluent into microcosms. As found in the water columnof microcosms M2 incubated at 10°C, the remobilizationof CBI from sediments was observed in the water columnof microcosm M2 incubated at 20°C during the first dayof the experiment, followed by an increase in the CBIconcentration during the next days.

The correlation of survival between different bacterialindicators was observed. The growth of TC and E. coli insediments was similar. After 30 d in M1, the concentrations

Table 3 Date and concentration of Culturable Bacterial Indicators (CBI) in sewage treatment plant effluent in sediments from Creux de Genthod andVersoix regions and in filtered lake water

CBI Sampling Temp.water/ Concentration of CBI (CFU/100 g sediment or 100 mL water)organisms date Temp. sediment STP effluent Vidy Bay Creux-de-Genthod Versoix Filtered

(°C) sediment sediment sediment lake water

TC 2005/05/24 15/13 (3.1 ± 0.2)× 106 (1.8 ± 0.4) × 106

2005/05/26 15/12 15 ± 42005/07/04 22/20 (9.2 ± 3.6) × 106 120 ± 13 142 ± 162006/02/22 11/10 (4.3 ± 1.2) × 106 (3.1 ± 0.5) × 107

2006/03/16 8/9 (6.8 ± 2.4) × 107 83 ± 9.6 112 ± 24E. coli 2005/05/24 15/13 (2.6 ± 0.5) × 106 (6.4 ± 1.6) × 106

2005/05/26 15/12 ND2005/07/04 22/20 (6.9 ± 2.3) × 106 ND 4 ± 0.52006/02/22 11/10 (7.1 ± 4.2) × 105 (3.9 ± 1.7) × 105

2006/03/16 8/9 (5.9 ± 1.1) × 105 NDENT 2005/05/24 15/13 (3.8 ± 0.5) × 104 (9.5 ± 4.8) × 103

2005/05/26 15/12 ND2005/07/04 22/20 (3.5 ± 1.3) × 105 ND ND2006/02/22 11/10 (4.7 ± 2.9) × 104 (6.5 ± 0.7) × 105

2006/03/16 8/9 (8.1 ± 2.3) × 104 NDAMB 2005/05/24 15/13 (5.1 ± 0.4) × 107 (2.2 ± 0.7) × 105

2005/05/26 15/12 720 ± 802005/07/04 22/20 (4.8 ± 0.2) × 106 590 ± 15.3 410 ± 532006/02/22 11/10 (2.3 ± 0.2) × 104 (1.3 ± 0.7) × 107

2006/03/16 8/9 (7.6 ± 2.8) × 105 430 ± 26

Data expressed using mean ± standard deviation of three replicates (100 mL of water/supernatant from STP/sediments of Vidy Bay were used for CBIanalysis after dilution (103 to 107 times). For sediments from Creux de Genthod and Versoix, 100 mL of supernatant were used without dilution).ND: not detected; TC: total coliforms; ENT: enterococcus; AMB: aerobic mesophilic bacteria.

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Fig. 2 Correlation of the survival of CBI in the water column and sediments of microcosms incubated at 10°C. CFU: colony forming units. (a)microcosm M1 containing the sediments from Vidy Bay and the sewage treatment plant effluent water; (b) microcosm M3 containing the sedimentsfrom Versoix region and the sewage treatment plant effluent water; (c) microcosm M4 containing the sediments from Creux de Genthod region and thesewage treatment plant effluent water; (d) microcosm M2 containing the sediments from Vidy Bay and filtered water from Versoix region.

of both TC and E. coli were about 5% higher than theirinitial concentration measured in the sediment of Vidy Baybefore their introduction into the microcosm. However, theconcentration of E. coli was greater in sediments than inthe water column (Fig. 3).

Even though the degradation of ENT and AMB isgreater in the sediments, no major difference was observed

in the survival of TC, E. coli, ENT, and AMB in the watercolumn of all microcosms incubated at 20 and 25°C. Adecay of ENT and AMB was observed in the sedimentsof microcosm M2 after 20 d of experimentation. Thedegradation of ENT was more important than degradationof AMB (Fig. 4).

Fig. 3 Survival of total coliforms and E. coli in water column and sediments of microcosms M1 (a) and M3 (b) incubated at 20°C.

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Fig. 4 Survival of enterococcus and aerobic mesophilic bacteria in thewater column and sediments of microcosm M2 incubated at 20°C.

3 Discussion

3.1 Occurrence of the CBI in sediments of Vidy Bay

The studies regarding the fate of faecal bacterial in-dicators in the aquatic environment indicate that theconcentration of TC, FC, and ENT in the water column andsediments ranges from 102 to 107 CFU/100 mL of wateror 100 g sediment depending on the source of pollutionsuch as STP and urban runoff (Bruni et al., 1997; Crabillet al., 1999; Noble et al., 2004). Our results showed thehighest concentrations of CBI in the sediments of VidyBay in the range of 103–107 CFU/100 g sediment for TC,E. coli, ENT and AMB. In the sediments of Vidy Bay,the accumulation of this bacterial biomass is caused byflow from the STP, Chamberonne River, and Flon River.Runoff of Chamberonne River and Flon River during thesampling periods was in the range of 1–4 and 4–6 m3/S,respectively. Municipal sewage, agriculture pollution, and

storm water runoff are main sources of pathogens in naturalwater (Arvanitidou et al., 2005). The results of bacteri-ological analysis of water sampled from rivers revealedconcentrations of CBI in the same order as those foundin the municipal STP effluent (Table 4). Also, the highconcentrations of the CBI are observed in the sedimentsclose to the mouths of these rivers.

3.2 Survival, mobilisation and multiplication of the CBIin the sediments and the water column

Resuspension of CBI and pathogens from the sedimentsto the water column due to recreational activities or naturalturbulence, such as lake water currents, may contribute topotential human health risk (An et al., 2002; Craig et al.,2004). The water current speeds and current directions on avertical profile between the bottom and surface of the lake,with a vertical step of 2 m in the Vidy Bay, were measuredby Goldscheider et al. (2007).

Several parameters control the survival and growth ofbacterial indicators in aquatic environments, includingsolar radiation, temperature, pH, salinity, heavy metals, or-ganic matter, predation, biological oxygen demand (BOD)and chemical oxygen demand (COD). It has been demon-strated that the biodegradable fraction of organic matter(measured either as assimilable organic carbon) controlsthe growth of bacteria (Piriou et al., 1998). This fraction isparameterized by BOD values. The BOD and COD testsare usually performed to determine the relative oxygenrequirements of wastewater effluents and polluted water(Reynolds and Ahmad, 1997; Liu and Mattiasson, 2002).The influence of organic matter and temperature has beenselected for investigation in this study because of theirstrong influence on the growth of faecal bacteria (Hughes,2003). The results of this study show that the compositionof sediments, in particular their organic matter content,

Table 4 Sampling date and concentration of CBI in STP effluents and in Chamberonne River and Flon River

CBI organisms Sampling date CBI concentration (CFU/100 mL water)STP effluent Chamberonne River Flon River

TC 2005/07/04 (9.2 ± 3.6) × 106 (9.2 ± 0.6) × 105 (1.7 ± 0.9) × 107

2005/11/07 (3.7 ± 0.6) × 107 (3.8 ± 1.3) × 105 (8.4 ± 2.4) × 106

2006/02/22 (4.7 ± 1.2) × 106 (7.2 ± 3.2) × 105 –2006/03/16 (6.8 ± 2.4) × 107 (4.3 ± 3.5) × 107 –2006/05/23 (5.3 ± 0.4) × 107 (5.8 ± 1.2) × 106 (6.8 ± 2.7) × 107

E. coli 2005/07/04 (6.9 ± 2.3) × 106 (9.2 ± 2.6) × 105 (3.9 ± 0.9) × 106

2005/11/07 (4.2 ± 1.5) × 105 (9.8 ± 3.5) × 104 (6.1 ± 2.4) × 105

2006/02/22 (7.1 ± 4.2) × 105 (2.9 ± 1.4) × 104 –2006/03/16 (5.9 ± 1.1) × 105 (8.5 ± 1.9) × 105 –2006/05/23 (7.1 ± 1.2) × 105 (3.1 ± 0.4) × 104 (2.5 ± 0.3) × 105

ENT 2005/07/04 (3.5 ± 1.3) × 105 (6.2 ± 1.9) × 105 (8.6 ± 0.9) × 104

2005/11/07 (9.7 ± 1.4) × 103 (9.4 ± 2.1) × 103 (3.4 ± 1.8) × 103

2006/02/22 (4.3 ± 2.9) × 105 (3.5 ± 0.7) × 103 –2006/03/16 (8.1 ± 2.3) × 104 (6.2 ± 0.3) × 105 –2006/05/23 (6.1 ± 1.4) × 105 (8.1 ± 0.8) × 104 (4.1 ± 0.3) × 105

AMB 2005/07/04 (4.8 ± 0.2) × 106 (4.4 ± 0.9) × 106 (7.3 ± 0.9) × 104

2005/11/07 (3.9 ± 1.1) × 106 (6.8 ± 0.4) × 104 (3.7 ± 0.8) × 106

2006/02/22 (2.3 ± 0.2) × 105 (5.2 ± 0.2) × 106 –2006/03/16 (7.6 ± 2.8) × 106 (1.9 ± 0.6) × 104 –2006/05/23 (4.1 ± 2.3) × 104 (6.1 ± 1.5) × 106 (9.2 ± 2.3) × 106

Data expressed as mean ± standard deviation (SD) of three replicates (100 mL of water from STP, Chamberonne River and Flon River were used forCBI analysis after dilution (103 to 107 times)).– : analysis not performed.

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has the strongest effect on the survival and growth ofbacterial indicators. The sediments from Vidy Bay andCreux de Genthod have the highest organic matter content(Table 1). The high concentrations of CBI observed insediments of microcosms M1, M2, and M3 incubated at20±2°C and 25±1.5°C indicate that the temperature ofapproximately 20°C and the organic matter content insediments from Vidy Bay and Creux de Genthod are thefavorable conditions to support of the multiplication ofCBI in sediments. The types of organic matter content inthe sediments are different than those present in the watercolumn and are more easily utilized by faecal coliforms(Gerba and McLeod, 1976; LaLiberte and Grimes, 1982).Faecal bacterial indicators are able to take profit fromnutrients associated with the sediment particles; therefore,sediments may contain 100 to 1000 times as much faecalindicator bacteria as the overlying water (Davies et al.,1995). The results of this study showed higher survival andgrowth of the CBI in sediments than in water column of themicrocosms.

The bacteria can activate survival strategies which allowthem to persist even in adverse environmental conditionswhere cell division is restricted (Mar Lleo et al., 2005).At 24°C, the bacteria E. coli is able to utilise nutrientsadsorbed to sediments from sewage effluent and to multi-ply from concentrations of 102 to more than 107 CFU/100g of sediment within 5 to 6 d (Gerba and McLeod,1976). However, Figs. 2a, 2b, and 2c clearly show thepersistence of the same proportion of CBI in the watercolumn and sediments at the temperature of 10°C duringexperimentation. These results suggest that the sedimentsand the water column may constitute reservoirs of faecalpollution even at low temperatures. We suppose that othersystem parameters, such as nutrients or salinity, may havea responsibility in this fact. Some studies (Ghoul et al.,1986; Davies et al., 1995; Hughes, 2003) demonstratedthat the bacterial indicators and virus adsorbed in sedimentparticles may be protected from the influence of manyfactors such as predators, heavy metal toxicity, salinityand UV radiation. Therefore, the sediment populations offaecal coliforms can be on average 2 200 times greater thanthat in the water counts (Crabill et al., 1999). The presenceof CBI in the water column of microcosm M2 indicates theremobilization of CBI from sediments and the transfer intothe water column (Fig. 2d and Fig. 4). However, the naturalturbulence of the system and the influence of the variationof the physicochemical parameters on this remobilizationprocess remain unknown.

Growth and degradation of TC, E. coli and ENT in theaquatic environment may compromise the use of a singleindicator to evaluate the faecal contamination (Noble etal., 2003; Anderson et al., 2005). The results of this studydemonstrate some variation on the survival and decaybetween CBI in different sets of experiments (Fig. 3aand Fig. 4). Therefore, as recommended by the EuropeanUnion (EU, 2006), E. coli and ENT should both be usedto assess the hygienic safety of recreational waters in VidyBay.

4 Conclusions

The results of this study demonstrate that the sedimentsof Vidy Bay are a reservoir and a substrate for multipli-cation of faecal CBI provided by the STP, ChamberonneRiver, and Flon River. Moreover, these microorganismscan be remobilized and transferred to the water columnas well as the permanent bacteriological pollution in VidyBay and greater risk human health during recreationalactivities. The key parameters of these processes are thewater temperature and the organic matter content. The highlevels of CBI observed in the sediments of microcosmsincubated at 20°C with continuous flow of STP effluentindicate that the sediment high organic matter contentand the accumulation of CBI constitute the conditionsto support the multiplication of CBI in sediments. Theremobilization of the CBI from the sediments and thetransfer to the water column indicate that the higher con-centrations of the CBI observed in different points in thewater column of Vidy Bay may not be explained only bythe recent contribution of the three sources, but also by thepersistence and multiplication of microorganisms in theseparticular conditions. The results of this study will help tounderstand the problematic of microbial pollution in VidyBay and to guide future decisions on the improvement ofthe bacterial quality of lake water.

Acknowledgments

This work was supported in part by Errst & LucieSchmidheing foundation and by the Municipality of Lau-sanne, Switzerland. We would like to thank Dr. BenoıtFerrari, F. A. Forel Institute for precious help for the settingup and handling of the microcosms.

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