persistence and growth of faecal culturable bacterial indicators in water column and sediments of...
TRANSCRIPT
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: [email protected]
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) in
the surface sediments and the water column of Vidy Bay (Lake Geneva, City of Lausanne, Switzerland). The study was carried out for
60 d using microcosms containing Sewage Treatment Plant (STP) effluent and nonsterile water without CBI, as well as contaminated
and non-contaminated sediments. The effects of water temperature and of organic matter associated with sediments on the survival of
CBI in the sediments and the water column were observed. The number of CBI colonies in the contaminated sediments of Vidy Bay and
in 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 water
for TC, E. coli, ENT, and AMB respectively. A degradation of CBI was observed in the sediments where organic mater content was
low 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 the
sediment which is rich in organic matter at 20°C. The results of this study indicate: (1) the higher concentrations of the CBI observed
in different points in the water column of Vidy Bay may not be explained only by the recent contribution of the three potential sources
of the Bay contamination including STP and the Chamberonne and Flon Rivers, but also by the persistence, removal from sediment
and multiplication of CBI in the sediment and water column; (2) the sediment of Vidy Bay constitute a reservoir of CBI and can even
support 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 rainwater
drainage systems constitute the principal sources of con-
tamination of Swiss rivers, reservoirs, and lake sediments
by different kinds of pollutants (Wildi et al., 2004).
There is still a paucity of information concerning the
contamination of sediments by microorganisms and there
is little information to be found regarding the persistence
of culturable bacterial indicators (CBI) in the sediments of
Vidy Bay and their release in the water column.
In the aquatic environment, sediment may constitute a
reservoir of different pollutants including heavy metals and
microorganisms. Some studies have assessed the presence
of bacterial indicators in the sediments and the water
column (Anderson et al., 2005; Baghel et al., 2005; Davis
et al., 2005). The influence of environmental conditions
on the persistence and accumulation of sediment hosted
bacteria was at levels 100–1 000 times higher than that
in the water column, with a potential risk of pollution
for coastal recreational water have been demonstrated
* Corresponding author. E-mail: [email protected]
(LaLiberte and Grimes, 1982; Davies et al., 1995; Evason
and Ambrose, 2006). Sedimentation is one of the mecha-
nisms involved in the accumulation and immobilization of
pathogens (Karim et al., 2004). Accumulation of CBI and
pathogenic organisms in sediments has been attributed to
the sorption of the microorganisms to particles suspended
in water. Therefore, sediments can constitute an important
reservoir of CBI in freshwater (Burton et al., 1987; Crabill
et al., 1999; An et al., 2002; Alm et al., 2003).
The presence of Escherichia coli in lake water indicates
the water contaminated by faecal material of humans
or other warm-blooded animals, and also indicates the
potential 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 a
variety of factors including solar radiation, temperature,
pH, salinity, heavy metals, predation, and competition by
native microflora, sediment grain size and organic matter
content (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.
No. 1 Persistence and growth of faecal culturable bacterial indicators in water column and sediments······ 63
The surface recreation water generally contains indige-
nous microorganisms, pathogenic, and non-pathogenic
microbes. The choice of bacterial indicators is thus very
important for the management of aquatic environmental
quality. 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, exposure
to high concentrations of the bacterial indicators TC, E.coli and ENT in coastal waters may increase the risk of
illness 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 not
necessarily pathogenic, but their high concentration can
be taken as an indication of faecal water contamination
and consequently as a signal of the presence of human
pathogenic microorganisms (Noble et al., 2003; Pearce etal., 2006).
Several studies regarding the fate of CBI in the aquatic
environment relied on laboratory microcosm experiments.
Various microcosms have been used to examine the per-
sistence and survival of bacterial indicators in the water
column and the sediments (Gerba and McLeod, 1976;
Laliberte and Grimes, 1982; Craig et al., 2004; Anderson
et al., 2005; Mar Lleo et al., 2005). The majority of
these studies were performed using bacterial indicator
strains inoculated in microcosms. In the present study,
the microcosms, with contaminated surface water lying
over contaminated sediments were designed to examine
the persistence of CBI in the sediment and in the water
column. The survival of CBI can be controlled by biotic
and abiotic water and sediment parameters. These parame-
ters may vary substantially according to the type of coastal
waters. The Vidy Bay constitutes a special case and the
research on this site proved to be of particular interest. The
objective of this study was to examine: (1) the incidence
and occurrence of CBI in sediments of Vidy Bay, (2) the
persistence of the CBI in sediments and water column,
and (3) the remobilization of the CBI from the natural
contaminated sediments and their growth capacity in the
water column.
1 Materials and methods
1.1 Study area
Lausanne is the most important city discharging treated
domestic and industrial wastewater into Lake Geneva. The
STP is located at Vidy and treats mostly 1 to 3 m3/s and
exceptionally up to 5 or 6 m3/s of raw water. Treated
wastewater is then discharged directly into the bay. The
Chamberonne River presently drains surface water from
its natural drainage basin and some untreated wastewater.
The Flon River is currently a collector of surface and
wastewater in the western part of Lausanne. The water is
generally treated at the Vidy STP but flows directly into
the lake during storms and floods if the discharge exceeds
5 to 6 m3/s (Fig. 1). As a result, the Vidy Bay is the most
contaminated 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 bottom
sediments (layer of 0 to 6 cm thickness) from Creux-
de-Genthod (CDG) and Vidy Bay using a “Buchi grab
sampler”. The sediments from Versoix were collected
manually. The STP effluent water was sampled directly
from the STP outlet pipe. Water from the lake was sampled
in the area of Versoix at 5 m depth with a centrifugal
pump and filtered at 1.2 μm (CUNO filter). The regions
of Versoix and Creux-de-Genthod of Lake Geneva are
known to be free of STP pollution. Sampled sediments and
water were kept at 4°C and transported immediately to the
laboratory. Sample analysis was undertaken within 24 h.
The GPS locations of the sites of sediments sampled are
presented in Fig. 1.
Fig. 1 Schematic location of sampling area: Vidy Bay, sewage treatment plant, Chamberonne River and Flon River.
64 POTE John et al. Vol. 21
1.3 Sediment and water characterization
The physicochemical parameters of sediments and water
are given in Tables 1 and 2. The sediments were dried
at 60°C for 48 h and the water content was calculated
from a weight difference. The dried sediments were then
heated at 560°C for 1 h to determine the organic matter
content. The particle size distribution was measured with
a laser Coulter� 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 was
measured with a Multi 350i and a titrimetric field kit
(Merck N◦ 1.11107). Dissolved organic carbon (DOC) was
measured on acidified samples (200 μL of 2 mol/L HCl in
30 mL sample) using the Shimadzu TOC 5000 (GmbH,
Switzerland).
1.4 Microcosms
The microcosms consisted of plastic aquaria of size 46.5
cm × 22 cm × 26 cm (L × W × H) with an overflow at
21 cm. The sediments were homogenized with a spatula,
sieved at 2 mm and filled into the microcosms to a height of
3–5 cm. Pre-filtered lake water or STP effluent was added
carefully and the suspension in the water column was left
for deposition. Water renewal was made with peristaltic
pumps and Tygon R-3607 (Milian SA, Switzerland) and
silicone tubing. Renewal rate of water was about 18 L/d
and water was introduced at mid-height close to the wall
opposite 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 mL
of water onto membrane filters (47 mm in diameter with
Table 1 Sediment characteristics in Lake Geneva in Swiss coordinates.
Parameter Sediment
Vidy Bay Creux-de- Versoix
Genthod
Depth (m) 35 50–55 < 1
Distance to coast (m) 300 1 000
Water content (%) 68–72 80–82 29
Organic matter content (%) 32–38 7–11 2–3
pH 7.2–7.8 7–8 7.4–8.2
Particle size d (μm)
Mean 63.66 25.92 23.79
Median 73.87 27.7 23.81
Mode 80.66 30.73 24.80
95% Confidence limit 4.49–902 6.02–112 1.57–360
Table 2 Water characteristics
STP effluent Filtered
waters lake water
(Vidy Bay) (versoix area)
Conductivity (μS/cm) 700–900 244
pH 6.8–8.1 8.2 ± 0.2
DOC (mg/L) 67–89 0.9–1.2
Dissolved 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 Agar
medium, incubated at 37°C for 4 h and transferred into
Agar Mug medium at 44°C for 24 h. ENT: Slanetz Bartley
Agar medium, incubated at 37°C for 48 h and transferred
into Bile Aesculin Agar medium at 37°C for 4 h. The
AMB number was determined by inoculating aliquots of
water samples into Plate Count Agar (PCA) medium and
incubated at 30°C for 72 h. The CBI in sediment were
resuspended by adding 100 g (wet weight) of sediment to
500 mL of 0.2% Na6(PO3)6 (Bruni et al., 1997) in 1 L
sterile plastic bottles and mixed for 1 h using the agitator
rotary printing-press Watson-Marlow 601 controller (Skan,
Switzerland). The mixture was then centrifuged at 2300
r/min for 15 min at 15°C. The supernatant was used for
the CBI counting. The results were expressed as colony
forming units (CFU) per 100 mL of water or 100 g of
sediment.
1.6 Mobilization and survival studies
Four microcosms (M1, M2, M3 and M4) were per-
formed to simulate natural conditions. M1 contained Vidy
Bay sediments and STP effluent; M2 contained Vidy Bay
sediments and filtered non-sterile lake water; M3 contained
sediments from Versoix and STP effluent water; and M4
contained sediments from Creux-de-Genthod and STP
effluent. The control microcosms M5 and M6 contained
sediments from Creux-de-Genthod and Versoix, respec-
tively, with filtered lake water from the Versoix region.
The microcosms were thermostabilized and kept in
dark room at different temperatures: 10±1, 20±2, and
25±1.5°C. Both water and sediments from all microcosms
were sampled on day 1, 5, 10, 20, 30, 40, and 60 for CBI
analysis. The sediments were sampled and homogenised
after removing overlying water from microcosms using a
peristaltic pump. After sampling, both water and sediments
were immediately analysed for CBI quantification. All
microcosm experiments were conducted in triplicate. For
each sample, about 500 mL of water and/or 100 g of
sediment were sampled from microcosms.
2 Results
2.1 Concentration of CBI in sediments and lake water
Before assessing the survival and the remobilization of
bacterial indicators in the microcosm sediments and water
column, the CBI number was quantified in the sediments
from three different sites of the lake (Vidy Bay, Creux, and
Versoix), 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 were
measured in the STP effluent and Vidy Bay sediments.
The concentration of CBI in the STP effluent ranged from
106–107, 105–106, 104–105, and 104–107 CFU/100 mL
for TC, E. coli, ENT, and AMB respectively. In the Vidy
Bay sediments, CBI concentration ranged from 106–107,
No. 1 Persistence and growth of faecal culturable bacterial indicators in water column and sediments······ 65
105–106, 103–105, and 105–107 CFU/100 g sediment for
TC, E. coli, ENT, and AMB respectively. In the sediments
from Creux-de-Genthod and Versoix and the filtered water
from the Versoix region, the number of CBI was very low
or not detected (Table 3).
2.2 Physicochemical parameters in the microcosms
The physicochemical parameters in microcosms were
monitored during the experiments. The microcosm tem-
peratures were stabilised. Compared with in situ values
(measured in the lake), the physicochemical parameters
in microcosms did not vary significantly during experi-
mentation. The average pH of the water column and the
sediments for all microcosms were in the range of 7.2
to 8, the average conductivity values were about 750 and
400 μS/cm for microcosms M1 and M3 respectively, and
about 230 μS/cm for others. The average value of dissolved
oxygen 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 in
both the water column and the sediments in microcosms
(Fig. 2). The concentrations of the CBI remained stable
in microcosms M1, M3, and M4. This concentration is
in the same order in the water column and sediments.
The remobilization of CBI from sediments was observed
in the water column of microcosm M2 on the first day
of experimentation. However, the decay of all CBI in
the water column and sediment of this microcosm was
observed after 5 d of experiments. E. coli, ENT, and AMB
concentrations decayed (concentration < 20 CFU/100 mL
water or 100 g sediment) and were comparatively less than
the initial concentration in the natural sediments of Vidy
Bay within 5 d (Fig. 2d). However, all CBI persisted in
both water column and sediments and maintained their
culturability 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 water
column and the sediments of microcosms incubated at 20
and 25°C presented almost the same pattern of results. In
general, persistence and growth of CBI were observed in
the water column and the sediments of all microcosms. No
variation of bacterial indicator concentration was observed
between the water column and the sediments of control
microcosms (M5 and M6).
M1, M2, and M4 presented a higher survival rate of CBI
in sediments than in the water column. In contrast, no great
difference in the CBI numbers was observed between the
sediments and the water column of microcosm M3 during
experimentation. Compared with others microcosms, no
growth of CBI was observed in the sediments of micro-
cosm M3. The persistence of the CBI observed in these
sediments was attributed to the continuous flow of STP
effluent into microcosms. As found in the water column
of microcosms M2 incubated at 10°C, the remobilization
of CBI from sediments was observed in the water column
of microcosm M2 incubated at 20°C during the first day
of the experiment, followed by an increase in the CBI
concentration during the next days.
The correlation of survival between different bacterial
indicators was observed. The growth of TC and E. coli in
sediments 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 and
Versoix 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 ± 4
2005/07/04 22/20 (9.2 ± 3.6) × 106 120 ± 13 142 ± 16
2006/02/22 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 ± 24
E. coli 2005/05/24 15/13 (2.6 ± 0.5) × 106 (6.4 ± 1.6) × 106
2005/05/26 15/12 ND
2005/07/04 22/20 (6.9 ± 2.3) × 106 ND 4 ± 0.5
2006/02/22 11/10 (7.1 ± 4.2) × 105 (3.9 ± 1.7) × 105
2006/03/16 8/9 (5.9 ± 1.1) × 105 ND
ENT 2005/05/24 15/13 (3.8 ± 0.5) × 104 (9.5 ± 4.8) × 103
2005/05/26 15/12 ND
2005/07/04 22/20 (3.5 ± 1.3) × 105 ND ND
2006/02/22 11/10 (4.7 ± 2.9) × 104 (6.5 ± 0.7) × 105
2006/03/16 8/9 (8.1 ± 2.3) × 104 ND
AMB 2005/05/24 15/13 (5.1 ± 0.4) × 107 (2.2 ± 0.7) × 105
2005/05/26 15/12 720 ± 80
2005/07/04 22/20 (4.8 ± 0.2) × 106 590 ± 15.3 410 ± 53
2006/02/22 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 CBI
analysis 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.
66 POTE John et al. Vol. 21
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 sediments
from Versoix region and the sewage treatment plant effluent water; (c) microcosm M4 containing the sediments from Creux de Genthod region and the
sewage 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 their
initial concentration measured in the sediment of Vidy Bay
before their introduction into the microcosm. However, the
concentration of E. coli was greater in sediments than in
the water column (Fig. 3).
Even though the degradation of ENT and AMB is
greater in the sediments, no major difference was observed
in the survival of TC, E. coli, ENT, and AMB in the water
column of all microcosms incubated at 20 and 25°C. A
decay of ENT and AMB was observed in the sediments
of microcosm M2 after 20 d of experimentation. The
degradation of ENT was more important than degradation
of 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.
No. 1 Persistence and growth of faecal culturable bacterial indicators in water column and sediments······ 67
Fig. 4 Survival of enterococcus and aerobic mesophilic bacteria in the
water 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 the
concentration of TC, FC, and ENT in the water column and
sediments ranges from 102 to 107 CFU/100 mL of water
or 100 g sediment depending on the source of pollution
such as STP and urban runoff (Bruni et al., 1997; Crabill
et al., 1999; Noble et al., 2004). Our results showed the
highest concentrations of CBI in the sediments of Vidy
Bay 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 by
flow from the STP, Chamberonne River, and Flon River.
Runoff of Chamberonne River and Flon River during the
sampling 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 natural
water (Arvanitidou et al., 2005). The results of bacteri-
ological analysis of water sampled from rivers revealed
concentrations of CBI in the same order as those found
in the municipal STP effluent (Table 4). Also, the high
concentrations of the CBI are observed in the sediments
close 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 sediments
to the water column due to recreational activities or natural
turbulence, such as lake water currents, may contribute to
potential human health risk (An et al., 2002; Craig et al.,2004). The water current speeds and current directions on a
vertical profile between the bottom and surface of the lake,
with a vertical step of 2 m in the Vidy Bay, were measured
by Goldscheider et al. (2007).
Several parameters control the survival and growth of
bacterial indicators in aquatic environments, including
solar 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) controls
the growth of bacteria (Piriou et al., 1998). This fraction is
parameterized by BOD values. The BOD and COD tests
are usually performed to determine the relative oxygen
requirements of wastewater effluents and polluted water
(Reynolds and Ahmad, 1997; Liu and Mattiasson, 2002).
The influence of organic matter and temperature has been
selected for investigation in this study because of their
strong influence on the growth of faecal bacteria (Hughes,
2003). The results of this study show that the composition
of 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 for
CBI analysis after dilution (103 to 107 times)).
– : analysis not performed.
68 POTE John et al. Vol. 21
has the strongest effect on the survival and growth of
bacterial indicators. The sediments from Vidy Bay and
Creux de Genthod have the highest organic matter content
(Table 1). The high concentrations of CBI observed in
sediments of microcosms M1, M2, and M3 incubated at
20±2°C and 25±1.5°C indicate that the temperature of
approximately 20°C and the organic matter content in
sediments from Vidy Bay and Creux de Genthod are the
favorable conditions to support of the multiplication of
CBI in sediments. The types of organic matter content in
the sediments are different than those present in the water
column and are more easily utilized by faecal coliforms
(Gerba and McLeod, 1976; LaLiberte and Grimes, 1982).
Faecal bacterial indicators are able to take profit from
nutrients associated with the sediment particles; therefore,
sediments may contain 100 to 1000 times as much faecal
indicator bacteria as the overlying water (Davies et al.,1995). The results of this study showed higher survival and
growth of the CBI in sediments than in water column of the
microcosms.
The bacteria can activate survival strategies which allow
them to persist even in adverse environmental conditions
where cell division is restricted (Mar Lleo et al., 2005).
At 24°C, the bacteria E. coli is able to utilise nutrients
adsorbed to sediments from sewage effluent and to multi-
ply from concentrations of 102 to more than 107 CFU/100
g of sediment within 5 to 6 d (Gerba and McLeod,
1976). However, Figs. 2a, 2b, and 2c clearly show the
persistence of the same proportion of CBI in the water
column and sediments at the temperature of 10°C during
experimentation. These results suggest that the sediments
and the water column may constitute reservoirs of faecal
pollution even at low temperatures. We suppose that other
system parameters, such as nutrients or salinity, may have
a responsibility in this fact. Some studies (Ghoul et al.,1986; Davies et al., 1995; Hughes, 2003) demonstrated
that the bacterial indicators and virus adsorbed in sediment
particles may be protected from the influence of many
factors such as predators, heavy metal toxicity, salinity
and UV radiation. Therefore, the sediment populations of
faecal coliforms can be on average 2 200 times greater than
that in the water counts (Crabill et al., 1999). The presence
of CBI in the water column of microcosm M2 indicates the
remobilization of CBI from sediments and the transfer into
the water column (Fig. 2d and Fig. 4). However, the natural
turbulence of the system and the influence of the variation
of the physicochemical parameters on this remobilization
process remain unknown.
Growth and degradation of TC, E. coli and ENT in the
aquatic environment may compromise the use of a single
indicator to evaluate the faecal contamination (Noble etal., 2003; Anderson et al., 2005). The results of this study
demonstrate some variation on the survival and decay
between CBI in different sets of experiments (Fig. 3a
and Fig. 4). Therefore, as recommended by the European
Union (EU, 2006), E. coli and ENT should both be used
to assess the hygienic safety of recreational waters in Vidy
Bay.
4 Conclusions
The results of this study demonstrate that the sediments
of Vidy Bay are a reservoir and a substrate for multipli-
cation of faecal CBI provided by the STP, Chamberonne
River, and Flon River. Moreover, these microorganisms
can be remobilized and transferred to the water column
as well as the permanent bacteriological pollution in Vidy
Bay and greater risk human health during recreational
activities. The key parameters of these processes are the
water temperature and the organic matter content. The high
levels of CBI observed in the sediments of microcosms
incubated at 20°C with continuous flow of STP effluent
indicate that the sediment high organic matter content
and the accumulation of CBI constitute the conditions
to support the multiplication of CBI in sediments. The
remobilization of the CBI from the sediments and the
transfer to the water column indicate that the higher con-
centrations of the CBI observed in different points in the
water column of Vidy Bay may not be explained only by
the recent contribution of the three sources, but also by the
persistence and multiplication of microorganisms in these
particular conditions. The results of this study will help to
understand the problematic of microbial pollution in Vidy
Bay and to guide future decisions on the improvement of
the bacterial quality of lake water.
Acknowledgments
This work was supported in part by Errst & Lucie
Schmidheing foundation and by the Municipality of Lau-
sanne, Switzerland. We would like to thank Dr. Benoıt
Ferrari, F. A. Forel Institute for precious help for the setting
up and handling of the microcosms.
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