Microbial and nutrient pollution of coastal bathing waters in Mauritius

Environment International 27 (2002) 555 – 566 www.elsevier.com/locate/envint

Microbial and nutrient pollution of coastal bathing waters in Mauritius D. Dabya,*, J. Turnerb, C. Jagob a

b

Faculty of Science, University of Mauritius, Reduit, Mauritius School of Ocean Sciences, University of Wales, Menai Bridge, Gwynedd LL59 5EY, UK Received 1 March 2001; accepted 4 September 2001

Abstract The coastal pollution problem in Mauritius is exacerbated by the hydrogeology of the volcanic substratum. Bacterial contamination of bathing waters and nutrients, water temperature, salinity, and dissolved oxygen (DO) were monitored at three different spatial and temporal scales along the coastline of Mauritius during 1997 – 1998. Standard techniques for water sample collection and analysis set by the American Public Health Association [APHA. Standard methods for the examination of water and wastewater. 19th ed. Washington, DC: APHA, 1995.] were used at: (a) 16 sites around the island over a period of 7 months; (b) 12 stations along a recreational beach over an 18-month period; and (c) at an underground freshwater seepage point over 1 day. Total coliform (TC), faecal coliform (FC), and faecal streptococci (FS) contamination reported during all surveys varied randomly (e.g., with maximum densities in the ranges of 346 – 2020 TC, 130 – 2000 FC, and 180 – 1040 FS at one site) and at times exceeded the established EEC and Environment Protection Agency (EPA) standards for bathing water (e.g., in > 90% of samples) to qualify for beach closure. Computed FC:FS ratios were used to pinpoint human faecal matter as the main source of contamination. Nitrate, phosphate, and silicate concentrations in seepage water were high (3600 – 9485, 38 – 105, and 9950 – 24,775 mg l 1, respectively) and a cause for concern when compared with levels (5 – 845, 5 – 72, and 35 – 6570 mg l 1, respectively) in cleaner lagoon water samples. Statistical analysis showed significant correlations (for TC and NO3: r = .75, P < .02; for TC and PO4: r = .779, P < .02; for TC and SiO4: r = .731, P < .05; for FC and NO3: r = .773, P < .02; for FC and SiO4: r = .727, P < .05; for FS and SiO4: r = .801 P < .01) between microbial densities and nutrients recorded, confirming the pathogen-contaminated water to be highly eutrophic. There is an urgency for Mauritius to properly address the issue of sewage treatment and wastewater discharge to safeguard its coastal environment, public health, and tourism expansion. D 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hydrogeology; Coastal pollution; Pathogens; Eutrophication; Public health; Environment vulnerability

1. Introduction Sewage-polluted water may contain different types of disease-causing microorganisms or pathogens. Numerous reports (e.g. Waldichuck, 1986; Ward and Singh, 1987; O’Caroll, 1987a,b,c; Wu, 1988; Phillips and Tanabe, 1989; Zoffman et al., 1989; Meynard et al., 1989; Morinigo et al., 1990) in studies in USA, Europe, Canada, Southeast Asia, the Mediterranean and Caribbean regions, and other coastal sites provide data on the epidemiological effects of sewage pollution in the marine environment. A potential risk of contracting a mild disease or an infection (e.g., stomachache, vomiting, diarrhoea, typhoid fever, bacterial or amoebic dysentery, guiardiasis, infective hep-

* Corresponding author. Fax: +230-465-6928. E-mail address: [email protected] (D. Daby).

atitis, poliomyelitis) by pathogenic organisms exists from bathing in sewage-polluted waters. Reports on outbreaks of food poisoning from consumption of faecally contaminated seafood are also common. Salmonella, Vibrio, Cholerae, and enteroviruses have been isolated from coastal waters worldwide, which imply a potential health problem of considerable significance (Ward and Singh, 1987), resulting from intensification of pathogen occurrences due to increased sewage contamination. Pathogenic bacterial indicators such as Escherichia coli are not killed by exposure to seawater (O’Caroll, 1987c), and if ingested, constitute a hazard to human health. International (e.g. EEC, US, Canada, UNEP/WHO) standards have been set to regulate the levels of total coliforms (TC), faecal coliforms (FC), and faecal streptococci (FS) as common indices of the recreational suitability of bathing waters. Noncompliance with these standards generally results in the closure of the recreational area.

0160-4120/02/$ – see front matter D 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 0 - 4 1 2 0 ( 0 1 ) 0 0 1 0 9 - X

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Serious effort needs to be directed towards meeting standards by introducing sewage and wastewater treatment in coastal resorts and industries or by extending submarine outfalls. This study demonstrates how microbial contamination of the shoreline by untreated marine outfall discharges as well as natural underground seepages represents a potential hazard to human health in Mauritius.

2. The problem of sewage pollution in Mauritius The Republic of Mauritius is an island state (1860 km2 in area) of volcanic origin in the Southwest Indian Ocean some 800 km east of Madagascar. With a population of 1.2 million and agriculture, industry, and tourism (mainly coastal area based) as principal economic bases, the country has enjoyed a decade and a half of rapid development, but further economic growth in the absence of appropriate control measures may outstrip the regenerative and assimilative capacities of the environment. The natural resource base of the coastal zone is increasingly being degraded by terrestrial inputs from intensified urbanisation. Corals in lagoons bounded by coral reefs are especially sensitive to hypernutrification from sewage, fertiliser, and other sources of organic pollution. Stressed corals are outcompeted by filamentous algae, causing death and disease in vulnerable colonies and loss of habitats for fish and other species. Degraded lagoons exhibit poor diversity and become unsightly to visitors. Oscore (1983) reported degradation of the reefs surrounding Port Louis harbour due to eutrophication caused by sewage pollution. Extensive siltation and subsequent death of coral communities was reported by UNEP/IUCN (1988) to be produced by suspended solids from sewage outfalls on the western coastline at Pointe Moyenne, Pointe aux Sables, and Roche Bois. Untreated sewage from 60% of the population is deposited via outfall only 500 m offshore (Hartnoll, 1994). Nutrient enrichment causes a phase shift from corals toward macroalgae and algal turfs, as observed by Lapointe (1997) in the Caribbean. Macroalgae outgrow and eventually replace slow growing corals (Pastorok and Bilyard, 1985) and excessive sedimentation may lead to altered growth forms and severe decreases in cover (Rogers, 1990). Organic matter stimulates the proliferation of oxygen consuming microorganisms, which may kill reef organisms by anoxia, hydrogen sulphide production, or directly by infection and disease. Soft-bottom macroinfauna show a shift to dominance of nematodes and polychaete worms (Fishelson, 1995). The report of Thomassin et al. (1998) on the impact of industrial and urban sewage on the fringing reef at Pointe aux Sables suggests a similar transformation, with the occurrence predominantly of a dense green carpet of Ulva reticulata as the indicator of eutrophication, and high densities (>100 m 2) of sea urchins potentially producing greater bioerosion on the reef. Generally, the site has very repulsive air and water quality conditions.

Problems are exacerbated by natural seepage and percolation of underground fresh water often contaminated with untreated domestic sewage and effluents through the coastal substrate into the lagoons. This feature, common around Mauritius, is due to the geological nature of the volcanic substratum of the island. The more recent basaltic coulees contain numerous caves, tunnels, cracks, and fissures, which form normal seaward sloping passages for underground water (Perroud, 1982; Muller, 1991). Such characteristics are typical of volcanic islands and confer a special significance to the problem of eutrophication and contamination of coastal waters by pathogenic bacteria. The mode of pollution is peculiar and somewhat different to what is generally encountered in urbanised coastal zones with engineered marine outfall discharges. Over 600,000 tourists visit Mauritius annually, and sea bathing occurs all round the coast, which also supports intensive recreational activities throughout much of the year. The epidemiological effects of sewage pollution in Mauritius have not been documented, so the sanitary fitness of bathing waters is not known. However, the importance of regularly assessing the bacterial quality of these waters to achieve an adequate prevention policy cannot be overemphasised for the protection of public health in general and expansion of the tourism industry in particular. The problem is especially acute in Mauritius due to the large population, rapid development, and strong tourism sector. Comparable problems can be expected to occur on other similar volcanic islands in the future.

3. Study aims and methodology The objective of this study was to undertake a bacteriological contamination survey of the bathing water fitness of intensively used recreational beaches around Mauritius. The study was designed to demonstrate whether (i) the urbanised shoreline of Mon Choisy-Trou aux Biches (MC-TAB) lagoon on the north west coastline is contaminated with human pathogens from sewage, (ii) there is temporal variation in the magnitude of contamination, (iii) localised zones of severe contamination occur along the shoreline due to tidal influence, and (iv) contamination is an islandwide problem. 3.1. Medium-term spatial survey around Mauritius Bathing water quality at recreational beaches around the island (Fig. 1) was evaluated during the period June – December 1998. Each of the four groups of sites (16 sites A – P) was sampled on a particular day and revisited on a monthly basis. Analysis was carried out each week on water samples collected from three stations established at each site (one middle and one at each end) from each group, except for Bain Des Dames (Site A: a positive control site south of

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Fig. 1. Microbial sampling stations around Mauritius coast used in the medium-term spatial survey (June – December 1998).

Port Louis Harbour receiving untreated sewage via an outfall) that had a single station. The standard membrane filter method (APHA, 1995) was used to quantify the density of FC, TC, and FS in the water samples. Serial dilutions of these samples were prepared to obtain the ideal number of 20 – 80 colonies on the 47-mm Millipore membrane filters (pore size 0.45 mm) and the numbers of colony forming units were calculated per 100 ml. A positive and negative control was included for each test, the rationale being to ensure that the positive control did produce microbial growth on the culture media used and that there was no such growth with the negative control. The positive control consisted of incubating a sample of seawater from a raw sewage discharge point on the coast, and was prepared by dilution of the raw sample in the ratio of 1:1000 using sterile Ringers solution and filtering

it through the membrane. The negative control sample was prepared by filtering 100 ml of only sterile Ringers solution. 3.2. Long-term lagoon shoreline survey There is no inflow of fresh river water but underground seepages occur in the southern part of MC-TAB lagoon (Fig. 2), which has a heavily urbanised, touristic waterfront. Wastewater discharges from tourist complexes and three submarine pipes discharge effluents directly into the lagoon in front of the aquarium and hotels. Two underwater springs of 3-m diameter also occur in the south of TAB. The water current flow around Mauritius reverses direction with each ebb and flood of the semidiurnal tide and so the dispersion of any seepage- or sewage-contaminated wastewater may contaminate the shoreline to the north and south of the

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Fig. 2. Microbial sampling stations along recreational beach of MC-TAB (Northwest Mauritius) used in the long-term survey (17 May 1997 to 5 November 1998).

lagoon. Microbial contamination along the waterfront was monitored at fortnightly intervals during 18 months (27 sampling occasions during 17 May 1997 to 5 November 1998) in duplicate samples of water collected from 11 established stations. A reference station, Station 12 ORE, located outside of the reef was sampled on every occasion.

3.3. Diel survey at seepage point Tidal influence was investigated on one occasion of full moon with low-water conditions occurring early in the morning of 1 June 1998 at Station 3, where freshwater percolates through the sandy beach and whose

Table 1 Time-series stationwise microbial density data recorded from MC-TAB beach during long-term survey (17 May 1997 to 5 November 1998) Station 1

2

3

4

5

6

7

8

9

10

11

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

TC

FC

FS

17 May 1997 7 June 1997 21 June 1997 5 July 1997 26 July 1997 9 August 1997 17 August 1997 31 August 1997 14 September 1997 27 September 1997 11 October 1997 25 October 1997 15 November 1997 29 November 1997 28 February 1998 13 March 1998 3 April 1998 24 April 1998 8 May 1998 21 May 1998 7 June 1998 20 June 1998 3 July 1998 31 July 1998 21 August 1998 23 October 1998 5 November 1998 Minimum Maximum

60 5 12 4 4 8 11 8 47 76 2 5 4 7 3 7 22 2 7 4 3 14 2 14 5 5 2 2 76

4 3 2 1 2 4 4 6 0 10 0 3 3 7 0 0 60 0 1 3 3 2 1 11 1 3 1 0 60

6 0 1 0 2 1 3 1 0 3 1 1 18 1 1 0 1 0 2 1 0 0 0 7 1 0 0 0 18

1 2 100 2 0 0 2 3 2 55 31 43 10 20 206 31 12 27 19 12 21 11 2 3 10 5 16 0 206

0 0 10 1 0 0 1 2 0 4 0 2 0 0 3 0 0 0 16 2 2 0 1 1 0 0 1 0 16

0 0 0 0 0 0 0 0 0 0 10 0 10 0 12 12 10 1 2 0 0 0 0 0 0 0 0 0 12

142 130 252 620 283 108 500 570 670 78 320 113 500 132 500 330 166 500 412 530 360 560 610 226 107 150 280 78 670

72 80 50 98 100 48 135 100 87 116 100 205 100 160 230 138 125 138 209 114 135 100 174 143 128 100 100 48 230

10 20 10 31 21 14 20 50 43 52 10 30 20 11 14 20 24 10 11 10 11 10 17 13 28 17 15 10 52

6 0 1 3 4 0 4 0 4 29 0 1 3 1 4 1 5 1 1 1 3 1 2 0 3 3 2 0 29

1 0 0 0 1 0 1 0 1 0 0 0 1 1 4 0 0 0 1 0 0 0 1 0 0 1 0 0 4

0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 1

9 2 1 0 1 0 2 1 0 7 1 6 34 1 10 0 1 2 2 25 37 2 0 1 0 0 6 0 37

1 0 0 0 0 0 0 1 0 0 1 3 18 1 1 0 0 0 1 7 18 0 0 0 0 0 3 0 18

0 0 0 0 1 0 0 0 0 0 0 0 2 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 2

0 0 0 0 3 1 0 2 0 1 1 0 0 1 2 0 9 1 1 1 1 1 3 12 0 3 0 0 12

0 0 0 0 0 1 0 1 0 0 0 0 0 0 2 0 0 1 0 0 0 0 1 11 0 0 0 0 11

0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 0 1 0 1 0 0 0 2 0 0 0 0 2

2 1 4 5 2 1 1 2 1 18 1 3 105 0 0 1 4 12 1 17 5 1 2 0 0 1 1 0 105

0 0 0 0 1 0 0 2 1 5 1 0 60 0 0 0 0 1 1 6 1 0 0 0 0 0 1 0 60

0 0 0 0 1 0 0 0 0 2 0 0 1 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 2

2 1 4 1 3 0 0 2 2 16 5 3 3 1 2 1 1 5 7 1 2 0 0 2 1 1 2 0 16

0 1 0 0 1 0 0 2 0 2 2 1 3 1 0 0 0 2 0 0 0 0 0 0 1 0 1 0 3

0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 1 0 0 0 0 0 0 0 0 1 0 1

1 1 1 0 0 0 1 0 3 6 1 1 4 3 7 2 22 1 0 0 1 4 1 3 1 5 0 0 22

0 1 0 0 0 0 0 0 0 0 1 0 3 1 5 0 1 0 0 0 0 1 0 0 0 0 0 0 5

0 0 1 0 0 0 0 0 0 0 0 0 2 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 2

5 0 1 2 2 1 28 3 0 4 5 0 5 14 1 2 2 11 2 2 0 3 3 1 0 1 0 0 28

0 0 0 0 0 0 7 2 0 1 1 0 5 8 0 0 0 0 0 0 0 2 0 0 0 0 0 0 8

2 0 0 0 1 0 1 0 0 0 2 0 2 0 1 0 0 0 0 1 0 1 0 0 0 0 0 0 2

5 0 2 2 2 1 4 19 0 7 0 1 4 0 0 1 1 11 2 5 1 0 1 0 1 0 0 0 19

0 0 0 0 0 1 2 7 0 1 0 1 3 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 7

2 0 1 0 1 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 2

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initial analysis had revealed the highest level of contamination by pathogenic bacteria. The station was sampled for 8 h from 07:30 h (first high water was at 00:30 h). Bacteria and nutrient analyses (nitrate (NO3), phosphate (PO4), and silicate (SiO4)) were recorded in duplicate water samples collected at half-hour intervals directly at the point where the seepage water met with the lagoon water. In situ measurements of water temperature, salinity, and dissolved oxygen (DO) were made simultaneously using portable instruments from a small glass fibre boat near the seepage point. 3.4. Data analysis methods The results were compared with (a) the recommended limits of coliform bacteria for recreational water established by the Environment Protection Agency (EPA) in USA (TC < 1000/100 ml and FC < 200/100 ml, for 80% of samples) and (b) the EEC guidelines for bathing beaches (TC < 500/100 ml and FC < 100/100 ml, for 80% of samples and FS < 100/100 ml, for 90% of samples). The highest microbial density values recorded were used to compute the FC:FS ratio for Stations 1– 12 of MC-TAB lagoon (long-term survey) and Sites A – P around Mauritius (medium-term survey) as a useful indicator for pinpointing the origin (human or animal) of contamination (Wyer and Kay, 1995). Correlation using the Pearson coefficient was examined by performing pairwise tests between the environmental and microbial density variables measured in the two surveys (long term and diel) of MC-TAB lagoon.

4. Results 4.1. Long-term MC-TAB shoreline survey The 18-month time-series of pathogen density data from MC-TAB long-term shoreline survey stations (Table 1) indicated the following: (a) generally, contamination by TC ranked highest, followed by FC and then FS, (b) pathogen contamination (for Station 3 in the ranges of: 78– 670 TC, 48 –230 FC, and 10– 52 FS; for other stations: 0– 206 TC, 0 – 60 FC, and 0 – 18 FS) varied temporally but did not show a clear pattern, (c) the magnitude and frequency of maximum contamination varied randomly between the stations, (d) the highest contamination was recorded at Station 3, with TC counts at and above 500 in 37% of the samples, and FC counts at and exceeding the permissible level of 100 in over 75% of the samples, (e) the next highest contaminations were recorded further south at Stations 2 (with 0 –206 TC, 0 – 16 FC, and 0– 12 FS) and 1 (with 2 –76 TC, 0– 60 FC, and 0– 18 FS), (f) none of the three pathogens was ever recorded from the reference station (Station 12) outside the reef (ORE), (g) the densities of all three bacterial types exhibited similar random fluctuation patterns temporally, as illustrated for Station 3 in Fig. 3, and (h) the fact that at times FC exceeded TC is difficult to explain. 4.2. Medium-term spatial survey around Mauritius Neither did the 7-month time-series of data from the island-wide (medium term) survey (Table 2) show a clear

Fig. 3. Fluctuation of microbial densities near seepage point (Station 3) on southern MC-TAB beach during long-term monitoring (17 May 1997 to 5 November 1998).

Table 2 Range of microbial densities recorded from sites around Mauritius during the medium-term survey (June – December 1998) Type

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

June

TC FC FS TC FC FS TC FC FS TC FC FS TC FC FS TC FC FS TC FC FS

129 – 320 15 – 22 0 – 10 142 – 257 7 – 39 15 – 19 205 – 296 13 – 18 0 – 23 218 – 302 130 – 500 180 – 1040 346 – 2020 120 – 2000 75 – 180

3–5 0–2 0–1 0 0 0–1 1–4 0–1 0–1 200 – 1000 1–4 0–4 9 – 13 5–8 0–4

0–6 0 – 12 1 – 10 2–3 0–6 0 0–1 0 0 0–7 0 – 124 0 – 42 10 – 12 9 – 13 1–4

7 – 30 0–1 0–1 0–1 0–1 0 24 – 36 4 – 21 5–6 5 – 157 1–3 0 10 – 23 12 – 18 1–5

0 0 0–1 0–1 0 0–1 0 0–1 0

0–3 0 0–1 1–2 0–1 0–1 1–6 0–1 0

0–3 0 0 0 0 0 0–1 0 0

1 – 154 3 – 123 1–2 0 – 14 0–5 0–3 12 – 13 1–2 0–1

2–8 0–1 0 0–4 0 0–1

0 0 0–1 0–2 0 0–3

. 0 0 0 1 – 11 0–6 0–1

0–3 0 0–1 9 0–1 0–2

119 – 260 13 – 14 0–4

14 – 16 1–4 0–4

6 – 16 4 – 14 1–4

42 – 50 9 – 31 3 – 44

1–2 1–2 0 0–1 1 0 0–1 1 0 0 0 0 0 0–1 0 1–8 0–1 0–3 0 – 113 0 – 120 0 – 56

2 – 44 1 – 155 0–1 0–1 0 0 0–1 0 0–2

0–9 0 0–1 2 – 14 0 – 10 0–3 3 – 16 6 – 19 0–3

0–3 0 0 1–2 0 0 1 0 0–1 1 0–9 0 1 0 0 0 – 46 1 – 36 0 – 34 0–1 1 0

0–7 0–3 0–1 2–3 0 0 1–3 0–1 0

7–9 0 0 7–9 1–4 0–2 0–1 0 0–3

0 – 13 0 – 14 0–1 0–1 0 0 0 0–9 0–6 0–1 0 0–1 0 0 0–2 0–6 0 0–3 3–5 0–1 0–3

1–2 1–2 0–5 0–8 1–8 0 2–6 0–2 0–1

0–1 0 0 0–3 2–5 0–2 0 0–2 0–2

115 – 242 24 – 56 13 – 15 17 – 151 21 – 121 0 124 – 226 26 – 115 0 – 13 23 – 179 13 – 19 0–5 38 – 114 21 – 101 13 – 30 68 – 558 24 – 320 0 – 18 3 – 19 0 – 11 0–1

July

August

September

October

November

December

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Month (1998)

Site A: positive control site (Bain Des Dames).

561

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variation pattern temporally. Site B (Pointe aux Sables: Fig. 1) was identified as being the most highly contaminated of all, with maximum bacterial densities varying in the ranges of 346 – 2020 TC, 130– 2000 FC, and 180– 1040 FS. TC counts above 200 and FC and FS counts above 100 were recorded in more than 90% and 50%, respectively, of the samples. Trou d’Eau Douce (Site I on east coast) was the second most contaminated, with maximum densities in the ranges of 68– 558 TC, 24 – 320 FC, and 13 –30 FS. The permissible limits (EEC Guidelines) for TC and FC were exceeded in 21% and 25% of the samples, respectively. As the name of the site implies, the shoreline has very brackish water, due to continuous seepage of underground fresh water into the lagoon. Contamination by the three pathogens was always present at all other sites around the coast but their densities did not exceed the specified limits. 4.3. Diel survey at seepage point During the diel survey at the seepage point bacterial density varied in the ranges of 24 –594 TC, 16 –220 FC, and 14– 138 FS. Temporal variation (Fig. 4a– c) was similar for the three pathogens. All concentrations remained low until 10:00 h and a small peak (278 TC, 136 FC, and 110 FS) occurred at 10:30 h, which lasted until 11:30 h. A second increase attained the highest peak (594 TC, 220 FC, and 138 FS) by 13:00 h, followed afterwards by a slow decline until 15:00 h. A second small peak (176 TC, 121 FC, and 48 FS) occurred by 16:00 h. The TC limit of 500 was exceeded only once at 13:00 h during the survey, but the FC limit of 100 was exceeded in 78% of the samples collected between 10:30 and 14:30 h, and the FS limit of 100 was exceeded in 28% of the total survey samples. Unusually very high levels of nutrients (i.e., 3600 – 9485 mg l 1 for NO3, 38 – 105 mg l 1 for PO4, and 9950– 24,775 mg l 1 for SiO4) were measured in comparison to the concentrations (i.e., 5– 845 mg l 1 for NO3, 5 – 72 mg l 1 for PO4 and 35– 6570 mg l 1 for SiO4) recorded routinely in cleaner water samples collected inside the lagoon nearer to the reef. All three nutrients showed increases in concentration, the timing of the initial increase corresponded to the morning peak in microbial levels and thereafter remained high until the end of the survey. For example, nitrate, phosphate, and silicate levels increased from 3600, 38, and 9950 mg l 1 at 07:30 h to 9485, 59, and 21,330 mg l 1 at 12:30 h and remained at 8563, 72, and 18,900 mg l 1 by 16:00 h, respectively. Salinity did not change, but temperature increased slowly but consistently until 15:00 h and declined slowly afterwards (23.49 C at 07:30 h, 24.17 C at 10:00 h, 24.69 C at 13:00 h, 25.0 C at 15:00 h, 24.6 C at 16:00 h). DO was very low (1.9 – 4 mg l 1) in the morning until 10:30 h, but it increased consistently to reach 9.8 mg l 1 by 14:30 h and declined slowly after (8.8 mg l 1 at 16:00 h). These results indicate that during spring tide low-water conditions (as on 1 June 1998), microbial contamination

Fig. 4. Variation of (a) microbial density, (b) nitrate, phosphate, and silicate concentration, and (c) temperature, salinity, and DO during diel survey (1 June 1998) at seepage point on MC-TAB beach.

was severe, with the highest densities of pathogens occurring around midday (a very pleasant sea-bathing time even in winter) when permissible limits were exceeded by about

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20% for TC (EEC Guidelines) and by over 100% for TC and 40% for FS (both EEC and EPA norms). Although these norms were not exceeded in 80% of the samples, during the wet season there can be more frequent and several-fold increases in bacterial density. The seepage water was highly eutrophic although the physical characteristics of the receiving water were not altered. The seepage flow rate (low under dry conditions) was not measured but the microbial concentrations prevailing during the low-water survey conditions do indicate potential risks to public health. However, the fact that this study was made on only one occasion means that the trends observed cannot be generalised.

source of contamination at all the coastal sites around Mauritius as indicated by ratio values above 2 at some stage during the survey. Ratio values were highest for the northern sites, Sites P (61.5) and O (55), in June 1998, and the only site with a ratio value consistently >2 throughout the study was Site B, suggesting its contamination predominantly by human faecal matter on a continuous basis. Ratio values for Site I (Trou d’Eau Douce) were above 10 for four out of seven surveys, also indicating contamination of a similar type.

4.4. FC:FS ratios

Pairwise tests were carried out on data from the MC-TAB long-term and diel surveys to determine correlations between the variables (Table 4). Pearson’s correlation coefficients were computed by performing linear regression, which quantifies the degree of linear association between two variables. The aim of this exercise was mainly to demonstrate the close association existing between the densities of the pathogens and concentrations of the nutrients recorded. The results showed 27 significant correlations (23 above P < .05) from the low-water seepage point

4.5. Correlation analyses

FC:FS ratios between 2 and 4 are indicative of contamination by human waste and those >4 indicate faecal matter of human origin (Wyer and Kay, 1995). The highest ratios obtained for MC-TAB Stations 1– 10 were in the range of 3– 60, indicative of contamination by human faecal matter (Table 3). Station 11 with a ratio value of 1.0 was possibly contaminated by livestock or poultry wastes (ratio values of 0.7 – 1.0). Human faecal matter was also the major

Table 3 FC:FS ratios for survey stations along MC-TAB shoreline and survey sites around Mauritius MC-TAB lagoon survey stations Sampling stations

Highest FC:FS ratio

Date of sampling

Origin of contamination

1 2 3 4 5 6 7 8 9 10 11 HFM: human faecal matter.

60 8 19 4 9 5.5 60 3 5 7 1

3 April 1998 8 May 1998 8 May 1998 28 February 1998 15 November 1998 31 July 1998 15 November 1997 15 November 1997 28 February 1998 17 August 1998 25 October 1998

HFM HFM HFM HFM HFM HFM HFM HFM HFM HFM livestock waste

Survey sites around Mauritius (1998) Sites

June

July

B C D E F G H I J K L M N O P Maximum

22 2 1.2 1 0 0 0 10.3 14 0 0 0.4 3 55 61.5 61.5

2.1 6 0 0 0 1 0 0 0 0 0 0 0 0 1.7 2.1

August

September

October

2 1 0 3.5 0 0 0 15 0 0 0 2 0 0 2 15

0.5 1 3 0

11 2 3.3 3.6 0 0 0 13 0 0 0 0 0 0 0 13

0 0 0 0

3

November

2.5 2 0.3 1.8 0 1.1 0.3 0 0 6 0.5 2.5

December 3.5 1 3.5 0.7 1 0 6.3 11 0.3 0 2.1

6.3

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Table 4 Results of correlation analyses on data sets generated during the MC-TAB long-term and low-water diel surveys Temperature

Salinity

DO

MC-TAB long-term survey: 13 significant correlations Salinity 1 .337 DO 2 .514 .238 pH 3 .022 .164 .083 Current speed 4 .329 .345 .248 NO3 5 .36 r = .908, .259 P < .01 PO4 6 .464 r = .692, r = .523, P < .02 P < .1 SiO4 7 .113 r = .845, .172 P < .01 TC 8 r = .615, .453 .44 P < .05 FC 9 r = .568, .209 .234 P < .1 FS 10 .459 .367 .497

MC-TAB low-water diel survey: 27 significant correlations Salinity 1 r = .617, P < .1 DO 2 r = .968, .581 P < .01 pH 3 r = .66, .313 r = .758, P < .05 P < .02 Current speed 4 .448 .127 .327 NO3 5 r = .868, .532 r = .805, P < .01 P < .01 PO4 6 r = .733, .312 r = .828, P < .05 P < .01 SiO4 7 r = .738, .092 r = .644, P < .05 P < .05 TC 8 r = .645, .343 r = .645, P < .1 P < .05 FC 9 r = .6, .452 .519 P < .1 FS 10 .415 .028 .345

pH

Current speed

.068 .225

.255

.366

.252

.024

.37

.033

.368

.036

.361

.033

.25

.32 .321 r = .669, P < .05 .255 .279 .007 .05

diel survey data and 13 (8 above P < .05) from the long-term MC-TAB shoreline survey data. Strong positive correlations between the densities of the three pathogens were apparent in both data sets. The densities also correlated positively with the nutrient levels but the results of the long-term survey showed weaker correlations (e.g. between TC and NO3: r = .586, P < .1; between FS and NO3: r = .575, P < .1).

5. Discussion The two sets of results differed widely with respect to the nature of correlations between the environmental variables. The long-term MC-TAB survey results showed very strong negative correlations between salinity and nutrients (for NO3: r = .908, P < .01; for PO4: r = .692, P < .02; for SiO4: r = .845, P < .01), indicating higher eutrophication in lagoon water of lowered salinity resulting from underground seepage, pipe discharge, and surface run-off inputs. This

NO3

r = .566, P < .1 r = .768, P < .01 r = .586, P < .1 .287 r = .575, P < .1

PO4

SiO4

TC

FC

r = .884, P < .01 r = .876, P < .01

r = .61, P < .02

r = .902, P < .01 r = .827, P < .01

r = .81, P < .01

FS

.398 .078 .123 .004

.243 .139 .133

.312 .224 r = .672, P < .05 .117 .146 .429

r = .722, P < .05 r = .771, P < .02 r = .75, P < .02 r = .773, P < .02 .48

r = .584, P < .1 r = .779, P < .02 .545 .481

r = .731, P < .05 r = .727, P < .05 r = .801, P < .01

observation was not apparent in the data set from the seepage point diel survey. Strong positive correlations were obtained between DO and nutrients (for NO3: r = .805, P < .01; for PO4: r = .828, P < .01; for SiO4: r = .666, P < .05) from the diel survey but not from the longer-term survey. Such differences may be because the diel survey was done only once and at a single station whereas the longer-term monitoring was undertaken over 18 months at 11 stations along the entire shoreline. The various parameters recorded during the diel survey exhibited a similar pattern of variation, hence a higher number (27) of positive correlations was generated. The longer-term monitoring data reflected the picture over a wide range of conditions, both spatial and temporal. The diel survey data were obtained from the most contaminated spot on the MC-TAB shoreline and also under conditions when the worst effects would be expected. The results of this study demonstrate that localised pollution by pathogenic bacteria from human waste and eutrophication effects can be severe along the MC-TAB

D. Daby et al. / Environment International 27 (2002) 555–566

shoreline, and such conditions may prevail throughout the coastline of Mauritius. However, all the shoreline survey stations were not always contaminated, except Station 3 at the seepage point that always exhibited the highest contamination when compared to the outside reef station ORE that was always clean. The degree of contamination varied randomly but in a similar pattern for all three bacterial indicator types. The MC-TAB shoreline contaminations were found to exceed the specified standards at certain times but never reached the threshold of 80% as required by the EEC and EPA norms for beach closure. During spring tide low-water conditions, microbial contamination can become very severe as indicated by the results of the diel survey on MC-TAB beach, and permissible limits can be exceeded during the worst conditions with high risks to public health. During this survey very strong positive correlations were found between concentrations of all three pathogens and between these and the nutrient concentrations, indicating the seepage water was both pathogen-loaded and eutrophic. Pathogen density variation resembled closely the frequency and pattern of toilet flushing. The medium-term island-wide survey data revealed no beach around Mauritius to be free from contamination by bacterial pathogens. Site B (Pointe aux Sables) located on the western sheltered coastline was most contaminated, with pathogen levels almost always exceeding the specified standards, hence qualifying for permanent beach closure. Such conditions are due to the discharge of untreated wastewater (through an old outfall only midway in the lagoon between the shore and reef) from industry and domestic sources from the highly urbanised zones of the districts of Port Louis and Plaine Wilhems (Fig. 1). The presence of dead corals, low fish catches, and occurrence of thick extensive beds of green algae (e.g. Ulva sp. and Enteromorpha sp.) that get exposed at low tide in the lagoon are clear indications of the impact of raw sewage effluent on the marine environment (SOGETI, 1995; Black and Veatch International, 1997). The very brackish shoreline water resulting from underground seepage at Site I (Trou d’Eau Douce) was also found to have much higher contamination than other sites around the island. However, this contamination was lower than at Site B, most probably because of greater dilution of the seepage water and mixing with seawater that occurs in the more exposed windward side of the island.

6. Conclusion The Government of Mauritius plans for island-wide sewerage networks and treatment of all wastewater before discharge or reuse, although major public investment is required to achieve this development. Unless the problem is addressed properly, pathogen contamination of bathing waters will continue to pose risks to public health, thus hampering further economic development. Seepage of

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contaminated coastal underground water into the lagoons is a common feature due to the geological nature of the volcanic substratum. Currently, a proposal to use the existing coastal underground lava tunnels as natural outlets into the sea for treated sewage and industrial effluents through borehole injection is undergoing intensive investigation and Environmental Impact Assessment. This novel proposal may be adopted for island-wide implementation if it is proven to be economically feasible and environmentally sound. Current apprehension exists because of lack of knowledge of lava tunnel hydrobiology and the possibility of transformation of the tunnels into real underground bacteriological reactors with unknown consequences for the coastal marine environment. The findings of this study suggest that the natural environment of Mauritius is much more vulnerable than the general belief. Other developing volcanic islands can be expected to face similar coastal pollution problems. Unless development is carefully planned, and the need for an increased awareness of such problems is stressed with appropriate environmental control and management measures, the small island developing states of the world will continue to face disadvantages to their sustainable development. Already, their small size, remoteness, dispersion, limited natural resources, and economic constraints constitute major handicaps.

Acknowledgments We thank the University of Mauritius for financially supporting the field and laboratory work. The support of the British Council and the University of Wales, Bangor, are duly acknowledged.

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