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Spatial and temporal variations of heavy metal contamination in sediments of a mangrove swamp in Hong Kong
Pergamon
0025-326X(95)00141-7
Marine Pollution Bulletin, Vol. 31, Nos 4-12, pp. 254-261, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0025-326X/95 $9.50 + 0.00
Spatial and Temporal Variations of Heavy Metal Contamination in Sediments of a Mangrove Swamp in Hong Kong N. F. Y. TAM* and Y. S. WONGI"
*Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong "~Research Centre, The Hong Kong University of Science and Technology, Hong Kong
Spatial and temporal variations of heavy metal contamination in sediments of a small mangrove stand in Hong Kong were examined by laying two transects perpendicular across the shore. Surface sediment samples were taken along the two transects running landward to seaward at intervals of 5 or I0 m during December 1989, and March, July and September 1990. Total concentrations of Cu, Zn, Mn and Pb did not show any specific trend along each transect, although the maximum concentration of heavy metals tended to occur at the landward edge. There was a high level of variability among locations within each transect; for instance, the Cu concentrations fluctuated from 1 to 42 Ixg g-1. Certain sites contained exceptionally high levels of total metals. Total concentrations of Cu, Zn, Mn and Pb as high as 42, 150, 640 and 650 pg g - l , respectively, were recorded, implying contaminated sediment. A comparison of the two transects indicated that the sediments of Transect B seemed to contain higher total Zn but lower Cu and Mn concentrations than those of Transect A. Most of the heavy metals accumulated in the sediments were not extractable with ammonium acetate and no Cu or Pb was detected in these extracts. The concentrations of extractable Zn and Mn were low, less than 10% of the total metal concentration in the sediment, and appeared to decrease from the landward to seaward samples. For both total and extractable metals, there were significant seasonal fluctuations for both transects, but no specific trends could be identified. These spatial and temporal variations suggest that the scale and representativeness of sampling require careful planning, and a single sample might not give a satisfactory evaluation of the levels of heavy metal contamination in mangrove ecosystems.
The continual development of agricultural, industrial and urban activities has given rise to a number of environmental problems. Heavy metal contamination in aquatic environments is of critical concern, due to the toxicity of metals and their accumulation in aquatic habitats. Heavy metals, in contrast to most pollutants, 254
are not biodegradable, and they undergo a global ecological cycle in which natural waters are the main pathways. Salt marshes, particularly those located near or along estuaries such as mangrove forests, are often polluted by river-borne and marine-derived particles and pollutants. As in other coastal marshes, mangrove sediments are reduced and act either as sources or sinks of heavy metals (Harbinson, 1986; Silva et al., 1990). Knowledge of the levels of such contaminants in mangrove sediments is important for understanding the degree of heavy metal pollution in aquatic systems, throwing light on the degree and sources of metal pollution, the transport, fate and bioavailability of the metals concerned (Japenga et al., 1990). However, information on heavy metal concentrations in mangrove sediments, especially in sub-tropical regions, is scarce (Harbinson, 1986; Lacerda et al., 1988). No 'natural' or 'background' data on heavy metal levels are available, which makes it difficult to determine whether contamination has occurred. It is essential to assess contaminant levels, identify 'hot spots' and attempt to predict the behaviour of heavy metals in mangrove sediments. The mangrove ecosystem, being intertidal, has a very varied sediment composition. As the chemical behaviour and quantities of trace metals are highly dependent on the physico-chemical conditions of sediments and pore-water, sediment heavy metal concentrations fluctuate from place to place, even within an apparently homogenous habitat (Harbinson, 1986; Lacerda et al., 1988). Different plant cover also results in different sediment conditions, patterns of water movement and heavy metal concentrations. Significant spatial and temporal variability is often a major problem in providing an adequate description of heavy metal contamination in this type of ecosystem. Different degrees of human activity and anthropogenic input further aggravates the problems of spatial heterogeneity. Therefore, a single or even a few samples might give an unsatisfactory evaluation of contaminant levels in a mangrove ecosystem, even though the study site may be relatively small. The present study aims to 1.
Volume 31/Numbers 4-12/April-December 1995 investigate the heavy metal concentrations in sediments of a small sub-tropical mangrove in Hong Kong, 2. evaluate the spatial and temporal variation, and 3. identify any 'hot spots' of heavy metal contamination.
Materials and Methods Study site The study was carried out in a mangrove ecosystem located at Sai Keng, along the north-east facing coast of Three Fathoms Cove (22°25'N, 114°16'E), on the eastern side of Hong Kong. The study site was 0.6 km wide, covering an area of 0.03 km 2. The mangrove plants were located in an enclosed bay, inundated by the incoming tide twice a day, with the largest tidal difference during spring at 2.8 m. The effects of wave action were relatively small. The substrata of the mangrove floor varied from hard to soft, comprising large pebbles and clay particles. The dominant plant species found were Kandelia candel (L.) Druce, Aegiceras corniculatum (L.) Blanco and Avicennia marina (Forsk.) Vierh. The plants were dwarf, immature or marginal, with a height seldom greater than 2 m. Sampling and analyses Two transects, labelled as A and B, were laid perpendicular across the shore, stretching over the entire zone occupied by mangrove plants. Transect A was located in the northern part of the mangrove ecosystem and crossed a relatively wide band of mangrove vegetation (100 m), while Transect B was in the south-eastern region with a narrow coverage of mangrove vegetation (40 m) which was more disturbed by human activities. Transect B was selected specifically to explore the relationship between human activity and metal contamination as the transect was close to a residential area and a small recreational centre. Along each transect, duplicate samples (approximately 1 kg) of surface sediment (2-10 cm deep) were collected every 5 or 10 m intervals, from landward to seaward. Samples were collected on four occasions for Transect A: December 1989, and March, July and September 1990. For Transect B, the September sampling was omitted due to the presence of wasps, which made it dangerous to collect samples. Sediment samples were air-dried, sieved using a 2-mm mesh size and analysed for general chemical properties according to Page et al. (1982). For measurement of the total concentrations of heavy metals, samples (0.5 g air-dried weight) were digested using 10 ml concentrated nitric acid and 1 ml sulphuric acid at 160°C. The filtered digests were then analysed for Cu, Zn, Mn, Pb, Cd, Cr and Ni by flame atomic absorption spectrophotometry (AA-680, Schimadzu Corporation). The detection limits of this AAS for Cu, Zn, Mn, Pb, Cd, Cr and Ni were 0.15, 0.04, 0.10, 0.50, 0.05, 0.20 and 0.30 ~tg g - l dry wt, respectively. The less strongly bound heavy metal concentration was determined by extracting the samples with 1M ammonium acetate (pH 4) at a 1:5 sediment:extractingsolution ratio (w/v) followed by AAS measurements. The mean, standard deviation and range of the heavy metal concentrations were calculated. All data were
expressed in terms of 105°C oven-dried weight. The spatial and temporal variations in sediment heavy metal content of each transect were determined by two-way analysis of variance (ANOVA). Any difference between the two transects was assessed using a one-way ANOVA. Simple correlation coefficients between heavy metals and organic matter were also calculated. All statistical analyses were carried out using the statistical package SPSS on an IBM-compatible PC.
Results Total heavy metal concentrations The general chemical properties of sediments collected from the two transects are different (Table 1). Transect A, with more plant coverage, higher production and decomposition of litter, had higher levels of organic matter, conductivity and nutrients but a lower pH than Transect B. Among the heavy metals determined, the concentrations of Cr, Ni and Cd in sediments were below the detection limits of AAS. In general, Transect A had higher total Zn and Mn but lower Cu concentrations than Transect B (Table 2). There was no significant difference between the total Pb concentrations in samples from the two transects. The significant spatial variations of the total Cu, Zn, Mn and Pb concentrations along and between the two transects were found to be significantly different (Figs 1 and 2). Although no specific trend was identified, the heavy metal concentrations were higher in the landward sites, decreasing gradually seaward especially in Transect B. Heavy metal concentrations recorded in this study were generally below levels found in polluted estuaries (Everaarts & Fischer, 1992; MacKey et aL, 1992; Machiwa, 1992; Real et al., 1993). In both transects, however, there were sites with unusual and exceptionally high concentrations of total Cu, Zn, Pb and Mn, with values at 40, 150, 640 and 650 ~tg g-~, respectively (Figs 1 and 2). These particular sites could be considered as local contaminated 'hot spots'. The distribution of these 'hot spots' was patchy and their specific locations varied from transect to transect, sample to sample, and from one metal to another. Nevertheless, it seems that most of the contaminated sites were found closer to the landward region, implying that the contamination might be due to anthropogenic input. The Sai Keng mangrove ecosystem is located in an enclosed bay with no industry, mining or dredging TABLE 1 Chemicalproperties of mangrovesedimentsalong TransectsA and B (mean+ 1 SD). Properties
Transect A
TransectB
pH Conductivity(mmhoscm- ~) Organicmater (%) Total K (%) Extractable K (lagg- J) Total KjeldahlN (%) NI-I~-N (I.tgg-l) NO~--N (lagg- ~) Total P (%) Olsen P (lagg-l)
5.86+-0.44 90.6+-37.5 9.43+__3.48 2.44-t- 1.76 147.3_ 184.5 0.182+-0.048 31.7+-18.2 2.73+-1.47 0.039-1-0.019 26.2_+14.6
6.51 4-0.65 69.9___9.89 5.46+ 1.02 0.57_+0.28 299.9__.102.4 0.116_+0.021 9.27:1:1.95 4.64+-4.99 0.021+-0.005 19.9+-5.68 255
Marine Pollution Bulletin TABLE 2 Results of ANOVA tests showing spatial and temporal differences in sediment heavy metal concentrations of Sai Keng mangrove stand.*
Mean of Mean of Transect A Transect B (lag g-~) (lag g-~)
Heavy metals Total Cu
9.27
16.44
Total Zn
57.65
47.84
Total Mn
107.92
83.92
Total Pb
49.93
69.21
Extractable Zn
1.77
3.51
Extractable Mn
8.49
4.84
F value between Transects A and B
F value of Transect A Between time
52.5 (0.000) 7.6 (0.006) 9.3
26.1 (0.000) 78.4 (0.000) 80.3
2.7 (0.103) 26.4 (0.000) 9.6 (0.002)
3.3 (0.024) 31.2 (0.000) 82.1 (0.000)
(0.003)
Between sites
(0.000)
F value of Transect B
Interaction
Between time
Between sites
Interaction
2.7 (0.001) 2.4 (0.004) 10,1
3.4 (0.000) 1.2 (0.179) 5.7
2.2 (0.133) 79.4 (0.000) 94.4
2.6 (0.021) 28.3 (0.000) 40.8
1.4 (0.178) 6,2 (0.000) 21.4
4.7 (0.000) 10.1 (0.000) 44.3 (0.000)
5.0 (0.000) 5.5 (0.000) 17.6 (0.000)
26.6 (0.000) 53.6 (0.000) 4.4 (0.020)
3.7 (0.002) 16.9 (0.000) 1.6 (0.158)
0.6 (0.864) 3.6 (0.001) 1.1 (0.359)
(0.000)
(0.000)
(0.000)
(0.000)
(0.000)
*The bracketed values show the significance of the calculated F values from the ANOVA tests.
activities in the vicinity. The input might come from the construction industry, waste dumped at the back of the shore and/or from holiday/weekend visitors who might dump domestic and metal-containing waste. This would explain why the 'hot spots' appeared sporadically. Total Pb, Zn and Mn concentrations varied significantly with sampling time, but no specific seasonal trends could be identified, except that sediments of Transect A collected in June 1990 seemed to have lower concentration of total Cu and Mn (Fig. 3, Table 3).
Mn than Transect B (Table 2). Exceptionally high concentrations of extractable Zn and Mn were observed at certain sites of Transect A (Fig. 3). Significant correlations were found between total and extractable Zn and Mn concentrations in both transects (Table 5). Positive correlations were observed between concentrations of Cu, Zn and Mn and organic matter taken from the sediments of Transect B (Table 5).
Extractable Zn and M n content
In this study, the degree of heavy metal contamination in mangrove sediments was assessed by analysing the total and extractable metal concentrations in the bulk surface sediments, as recommended by previous researchers (Everaarts & Fischer, 1992). Except for a handful of 'hot spots', the concentrations of heavy metals in mangrove sediments were low, and most were not extractable and therefore assumed not bioavailable. Although there are very few data on sediment heavy metal concentrations in mangrove ecosystems available, the mean and maximum heavy metal concentrations recorded in this study are generally comparable to or lower than the reported values for surface sediments of estuarine habitats (Table 6). These findings suggest that
The NH4OAc-extractable Pb and Cu concentrations were below detection limits (<0.15 and <0.05 lxg g-~ dry wt, respectively). The mean values of extractable Zn and Mn were 2.51 and 6.95 ~tg g - l , respectively, accounting for less than 10% of the total metal concentrations (Table 4). Spatial and temporal fluctuations in the concentrations of extractable Zn and Mn were similar to those of the total metal concentrations (Fig. 3). In Transect B, the extractable Zn and Mn concentrations appeared to be higher at the landward end, decreasing towards the sea; any such trend was less obvious in Transect A. Transect A had lower extractable Zn but higher concentrations of extractable
Discussion
TABLE 3
Spatial and temporal variations of concentrations of total heavy metals (lag g-1 dry wt) in Sai Keng mangrove sediments.* Total Cu
Total Zn
Total Pb
Sampling month
Mean 4- SD
Range
Mean __.SD
Range
Transect A December March June September LSD
12.4 4- 5.6a 8.9 4- 2.2b 7.4+2.6 c 8.5_+3.0 ~ 1.33
5.6-28.6 4.6-12.7 1.1-11.3 2.8-13.7
30,1_9.9 d 70,74_14.l b 81.6+21.3 a 46.9_6.6 c 8.29
16.6-53.6 34.9-94.2 60.9-147.4 33.2-57.2
Transect B December March June LSD (p = 0.05)
16.4_ 5.7ab 12.1 -t-7.2b 20.8 ___8.7a 7.49
10.9-27.2 4.7-30.8 11.6-41.2
47.5+ 10.8c 31.1+13.1 b 64.9_34.5 a 6.53
33.5-72.9 21.3~8.7 30.3-143.8
Mean _+SD
Total Mn Range
Mean 4_SD
Range
41.9+20.4 b 7.6-72.7 45.14_12.4 ab 23.4-82.2 62.5___48.9a 11.4-240.9 48.9+ 16.2ab 18.2-81.9 18.29
95.5__ 29.5b 107.44-40.3 b 70.9+ 19.1c 160.0+ 51.7a 17.81
38.2-151.5 61.7-221.8 34.0-106.8 61.5-262.6
87.1_ 16.5a 48.8+16.1 e 73.3_+12.6b 12.55
64.7_8.7 c 84.1+18.4 b 102.9__+53.7 a 6.79
50.3-80.6 63.6-120.1 49.8-223.1
65.7-110.5 33.2-83.2 49.6-96.3
*The mean and standard deviation values of each transect followed by different superscripts within each column indicated that there were significant differences between sampling times at a probability level of 0.05 according to ANOVA. Two extremely high concentrations of total Pb in both transects and one extreme value of total Mn in transect A were deleted from the calculation.
256
Volume 31/Numbers 4--12/April-December 1995 Transect
A
Transect Total
B
Cu 50
30
25 40
20
J
30 =L v
v
~D
.w
-~
•
io
20
0
10 5
W 0
1
I
I
I
I
20
40
60
80
i00 Total
160
140
140
100
I
I
I
i
I
14
21
28
35
42
.-, 120
v
~
// . ,,
ao
I
7
Zn
160
120 L
0
100 80
_.% _
o\
z
19
20
\-\
4o 20
O'
0
J
L
,
,
20
40
60
80
Distance
)
100
(rn)
0
1
I
I
7
14
21
I
28
Distance
I
I
35
42
(m)
Fig. 1 Changes in total Cu and Zn concentrations in sediments collected from Transects A and B of Sai Keng Mangrove: December (O), March (V), June (V), September (D) sampling times. TABLE 4 Ammonium acetate extractable Zn and Mn concentrations (lag g - t dry wt) of sediments in Sai Keng mangrove stand.* Extractable Zn (lag g-1) Sampling month
Mean ± SD
Range
Extractable fraction of Zn (%)
Extractable Mn (lag g - t ) Mean ± SD
Range
Extractable fraction of Mn (%)
Transect A
December March June September LSD
1.47±2.11 b 2.51 ± 1.26a 1.71 ±0.81 b 1.38 ± 0.80 b 0.33
0.01-9.70 0.14--4.92 0.56--3.62 0.27-3.46
3.99 + 4.25 a 3.22± 1.65b 2.29± 1.25~ 3.23± 1.79b 0.52
9.69 ± 4.87 b 8.11±6.46 ¢ 4.27±3.61 d 12.09± 13.01a 1.16
3.59+2.31 ~b 0.97±0.85 b 5.95 ± 4.68" 2.95
1.13-8.49 0.22-3.02 1.04--16.9
7.23 ± 3.34a 2.74+ 1.52b 8.10+2.86 a 2.73
3.22-1-1.83 c 6.74+4.43 a 4.54+2.39 b 1.17
0.35-24.39 1.54-27.53 0.71-16.59 2.64--62.71
10.03 -I-6.39 a 6.79±3.83 b 6.59±5.60 b 6.28±3.27 b 1.15
Transect B
December March June LSD (p = 0.05)
1.05-7.99 2.18-16.46 1.68-8.57
4.91 + 2.46 b 7.30-1-3.16 a 5.07_+2.81 b 1.43
*The mean and standard deviation values of each transect followed by different superscripts within each column indicated that there were significant differences between sampling times at a probability level of 0.05 according to ANOVA test. 257
Marine Pollution Bulletin
oo[
Transect B
Transect A Total Pb 650
1
475 45O
~-. 240
625 f
60O 140
.
I
:¢
200
'~
160
.o0
120
::h ,~ la,
"~
120 I00
8o
0
~
s0
v,
~ •
\
60 40 20
0
0 0
20
40
60
80
100
0
I
I
i
I
I
I
7
14
21
28
35
42
Total M n
85°I
240 []
640
630
200
620 240
[]I
t
I
/
160 v
2OO 120
"~ leo 0
0
120
F
80 40 0 0
20
80
I
I
I
I
40
60
80
100
40
I
I
I
1
I
I
7
14
21
28
35
42
Distance (m) Distance (m) Fig. 2 Changes in total Pb and Mn concentrations in sediments collected from Transects A and B of Sai Keng Mangrove: December(Q), March(V), June (V), September(I-1)sampling times. Sai Keng mangrove sediments are not significantly polluted by heavy metals, and the concentrations recorded can be considered as 'natural background' levels, Comparison of data obtained in the present study with metal levels in other coastal sediments might be difficult, however, as heavy metal concentrations, even background levels, vary from area to area, depending on sediment characeristics (Pardue et aL, 1992). Origin and composition of the sediment, particle size distribution and feasible post-depositional reactions all play an important role in determining metal concentrations, and they preclude any simple comparisons of metal levels in sediments with different characteristics (Barreiro et al., 1994). The hydrodynamic and chemical complexity of each particular estuary, together with the singular 258
behaviour of each metal, make it difficult to compare the results from one estuary to another. To facilitate comparisons between ecosystems, heavy metal concentrations should be normalized or correlated to a reference element such as A1 and Fe (Pardue et al., 1992). In order to evaluate pollution, 'hot spots' of contamination within a site could be identified by comparing the concentration at a specific site with the average concentration at the adjacent location; a site with a concentration of at least twice the average would be considered as a 'hot spot' (Everaarts & Fischer, 1992). Spatial variations of heavy metal concentrations were significant in this study, with lower metal concentrations towards the seaward regions. This seaward decrease in
Volume 31/Numbers 4-12/April-December 1995
Transect A
Transect B Extractable
10
Zn 18
1
15
6 I~t) 4
•
=L "-" 9
\,
v
N
I
~
0
0
Extractable
[]
60
,
,
7
14
0
20 40 60 80 I00
65 I
6
,"IV v,X7 X3-V x7 21
28
35
42
Mn
18
F
15
55
50
5
@..@
r~v
0
0 0
20
40
60
fl0
100
0
DisLanee (m)
7
~ 14
, 21
I 28
I 35
, 42
Distance (m)
Fig. 3 Changes in NH4OAc (pH4) extractableZn and Mn concentrations in sediments collected from Transects A and B of Sai Keng Mangrove: December (0), March (V), June (V), September ([-1) sampling times. metal concentrations of estuarine sediments may be related to the mobilization of metals from sediments, as well as the mixing o f fluvial and marine particulate material (Barreiro et al., 1994). Salinity also influences levels o f heavy metals accumulated in surface sediments. Real et aL (1993) reported that in sediments located seaward, increased salinity impedes the oxidation process and some heavy metals such as Mn are able to cross the oxic layer and return to the water column, leading to lower concentrations at sites nearer to the sea. Sediment analysis offers certain advantages over water analysis for the control and detection of metal pollution in estuaries, as sediments act as integrators and amplifiers o f the concentrations o f these elements
(Real et al., 1993; Barreiro et al., 1994). On the other hand, surface sediment often exchanges with suspended material, thereby affecting the release of metals to the overlying water. Therefore, the top few centimetres of the sediments reflect the continuously changing presentday degree of contamination, instead of recording the history of contamination. This helps to explain why spatial and temporal variations are significantly high in this small mangrove ecosystem in H o n g Kong.
Conclusion The present study indicates that heavy metal contamination has not been a serious problem in the Sai Keng mangrove ecosystem. Total Cu, Zn, Pb and 259
Marine Pollution Bulletin TABLE 5 Correlation coefficients between heavy metals concentrations and other related parameters in mangrove sediments along two transects. Organic matter content
Total Cu
Total Zn
Total Mn
Total Pb
Extractable Zn
-0.12 0.25 0.16 0.13 0.25
-0.15 0.14 0.31' -0.31"
-0.12 -0.07 0.49**
-0.05 -0.12
0.19
0.03 0.68** 0.43*
0.28 -0.21
-0.01
Transect A (76 samples)
Total Cu Total Zn Total Mn Total Pb Extractable Zn Extractable Mn
0.49** 0.13 -0.07 0.14 0.41'* 0.08
Transect B (32 samples)
Total Cu Total Zn Total Mn Total Pb Extractable Zn Extractable Mn
0.45** 0.65** 0.51'* 0.28 0.59** 0.34
0.67** 0.49* 0.32 0.63** 0.14
0.78** 0.37 0.95** 0.13
* and ** indicate that the correlations were significant at probability levels of 0.01 and 0.001 (one-tailed), respectively. TABLE 6 Concentrations of heavy metals in sediments reported by previous studies (mean values with the range given in parentheses). Metal concentrations (lag g-l) Location and types of sediments
Cu
Zn
Sai Keng Mangrove, Hong Kong
12.4 (1-31)
53.3 (17-147)
Brisbane Mangrove, Australia
22.4 (3-30)
97.9 (41-144)
66.8 (20-82)
7.3 (2-17)
Saudi Mangrove, Arabian Gulf
1.8 (0.1--4)
Pb
Source of reference
Comments
Present study
Relatively clean
ND*
MacKey et al. ( 1 9 9 2 )
Industrial and urban discharge
11.8 (6-19)
28.7 (2-69)
Sadiq & Zaidi (1994)
Nil
58.2 (8-241)
Mn 97.9 (34-223)
Sepetiba Bay Mangrove
12.4
311
17.8
ND
Lacerda et al. (1993)
With Avicennia covered
Sepetiba Bay Mangrove, SE Brazil
23.9
823
39.4
ND
Lacerda et al. ( 1 9 9 3 )
Mud-fiat area
Mangrove, S. Australia
(30-80)
(142-190)
(85-112)
ND
Harbinson (1986)
Nil
18.4 (1-89)
69.3 (9-321)
57.3 (7--467)
ND
Maehiwa (1992)
Harbour sediments affected by human activities
Coastal of Dares Salaam, Tanzania Ria de Arousa, NW Spain
(25--365)
(123-281)
ND
Estuarine, Java Sea
(6-54)
(33-122)
(5-46)
ND
Everaarts (1989)
North Sea
(25-240)
(400-4000)
(75-630)
ND
Everaarts & Fischer (1992) Contaminated due to anthropogenie inputs
Saguenay Fjord, Canada
24.4 (21-26)
114.5 (88-133)
ND
ND
Gagnon et al. (1993)
Nil
River Tees, UK
120 (10-1100)
473 (40-2900)
249 (40-990)
397 (160-1800)
Davies et al. (1991)
Highly polluted
2500
700
ND
Japenga et al. (1990)
Signal value
ND
Katz & Kaplan (1981)
Pollution free 'baseline' concentration
Standard Dutch sediments Coastal area, Southern California
400 9.0
44.4
10.5
(195-3590) Real et al. (1993)
Effluent discharges Nil
*ND, not determined. M n c o n c e n t r a t i o n s were g e n e r a l l y c o m p a r a b l e to a n d / o r b e l o w the values r e p o r t e d b y p r e v i o u s w o r k e r s on m a n g r o v e sediments. H e a v y m e t a l s such as Cr, C d a n d N i were n o t d e t e c t e d in this ecosystem. O n the o t h e r h a n d , ' h o t s p o t s ' o f c o n t a m i n a t i o n were identified w h o s e d i s t r i b u t i o n s were p a t c h y a n d v a r i e d w i t h different m e t a l s , b e t w e e n transects, as well as o v e r time. T h e significant c o r r e l a t i o n coefficients f o u n d b e t w e e n h e a v y m e t a l c o n c e n t r a t i o n s in the sediments 260
o f T r a n s e c t B indicate t h a t the h e a v y m e t a l s m i g h t have c o m e f r o m the s a m e source o f p o l l u t i o n , p r o b a b l y h u m a n input. A m m o n i u m acetate d i d n o t e x t r a c t m o s t o f the a c c u m u l a t e d metals. Z n a n d M n c o n c e n t r a t i o n s , which were extracted, a c c o u n t for less t h a n 10% o f the t o t a l m e t a l c o n c e n t r a t i o n s , i n d i c a t i n g t h a t the h e a v y m e t a l s a c c u m u l a t e d are s t r o n g l y b o u n d w i t h o r g a n i c m a t t e r . Positive c o r r e l a t i o n s were f o u n d b e t w e e n m e t a l c o n c e n t r a t i o n s a n d o r g a n i c m a t t e r c o n t e n t s in the
Volume 31/Numbers 4--12/April-December 1995 sediments. Spatial a n d t e m p o r a l v a r i a t i o n s o f total a n d extractable h e a v y metals were significant in Sai K e n g m a n g r o v e sediments, a l t h o u g h this forest was relatively small, i m p l y i n g t h a t m o r e samples are required for a m e a n i n g f u l record o f the actual v a r i a t i o n o f m e t a l contamination. Research grants awarded to the authors by the Croucher Foundation and the Hong Kong University Grant Council are acknowledged. Barreiro, R., Real, C. & Carballeira, A. (1994). Heavy-metal horizontal distribution in surface sediments from a small estuary (Pontedeume, Spain). Sci. Total Environ. 154, 87-100. Davies, C. A., Tomlinson, K. & Stephenson, T. (1991). Heavy metals in River Tees estuary sediments. Environ. TechnoL 12, 961-972. Everaarts, J. M. (1989). Heavy metals (Cu, Zn, Cd, Pb) in sediment of the Java sea, estuarine and coastal areas of east Java and some deep-sea areas. Neth. J. Sea Res. 23(4), 403--413. Everaarts, J. M. & Fischer, C. V. (1992). The distribution of heavy metals (Cu~Zn, Cd, Pb) in the fine fractions of surface sediments of the North Sea. Neth. J. Sea Res. 29(4), 323--331. Gagnon, C., Pelletier, E. & Maheu, S. (1993). Distribution of trace metals and some major constituents in sediments of the Saguenay Fjord, Canada. Mar. Pollut. Bull. 26(2), 107-110. Harbinson, P. (1986). Mangrove muds--a sink and a source for trace metals. Mar. Pollut. Bull. 17, 246-250. Japenga, J., Zschuppe, K. H., DeGroot, A. J. & Salomons, W. (1990). Heavy metals and organic micropollutants in flood plain of the
River Waal, a distributary of the River Rhine, 1958-1981. Neth ,i.. Agric. Sci. 38, 381-397. Katz, A. & Kaplan, I. R. (1981). Heavy metals behaviour in coastal sediments of southern California: a critical review and synthesis. Mar. Chem. 10, 261-299. Lacerda, L. D., Martinelli, L. A., Rezende, C. A., Mozetto, A. A., Ovalle, A. R. C., Victoria, R. I., Silva, C. A. R. & Nogueira, F. B. (1988). The fate of heavy metals in suspended matter in a mangrove creek during a tidal cycle. Sci. Total Environ. 75, 249-259. Lacerda, L. D., Carvalho, C. E. V., Tanizaki, K, F., Ovalle, A. R. C. & Rezende, C. E. (1993). The biogeochemistry and trace metals distribution of mangrove rhizospbers. Biotropica 25(3), 252-257. MacKey, A. P., Hodgkinson, M. & Nardella, R. (1992). Nutrient levels and heavy metals in mangrove sediments from the Brisbane River, Australia. Mar. Pollut. Bull. 7,4(8), 418-420. Machiwa, J. F. (1992). Heavy metal content in coastal sediments off Dar Es Salaam, Tanzania. Environ. lnterp. 18, 409-415. Page, A. L., Miller, R. H. & Keeney, D. R. (1982). Methods of Soil Analysis, Part 2 Chemical and Microbiological Properties. American Society of Agronomy, WI, USA. Pardue, J. H., DeLaune, R. D. & Patrick Jr, W. H. (1992). Metal to aluminum correlation in Louisiana coastal wetlands: identification of elevated metal concentrations. J. Environ. Qual. 21, 539-545. Real, C., Barreiro, R. & Carballeira, A. (1993). Heavy metal mixing behaviour in estuarine sediments in the Ria de Arousa (NW Spain). Difference between metals. Sci. Total Environ. 128, 51--67. Sadiq, M. & Zaidi, T. H. (1994). Sediment composition and metal concentrations in mangrove leaves from the Saudi coast of the Arabian Gulf. Sci. Total Environ. 155, 1-8. Silva, C. A. R., Lacerda, L. D. & Rezende, C. E. (1990). Heavy metal reservoirs in a red mangrove forest. Biotropica 22, 339-345.
261
0025-326X(95)00141-7
Marine Pollution Bulletin, Vol. 31, Nos 4-12, pp. 254-261, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0025-326X/95 $9.50 + 0.00
Spatial and Temporal Variations of Heavy Metal Contamination in Sediments of a Mangrove Swamp in Hong Kong N. F. Y. TAM* and Y. S. WONGI"
*Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong "~Research Centre, The Hong Kong University of Science and Technology, Hong Kong
Spatial and temporal variations of heavy metal contamination in sediments of a small mangrove stand in Hong Kong were examined by laying two transects perpendicular across the shore. Surface sediment samples were taken along the two transects running landward to seaward at intervals of 5 or I0 m during December 1989, and March, July and September 1990. Total concentrations of Cu, Zn, Mn and Pb did not show any specific trend along each transect, although the maximum concentration of heavy metals tended to occur at the landward edge. There was a high level of variability among locations within each transect; for instance, the Cu concentrations fluctuated from 1 to 42 Ixg g-1. Certain sites contained exceptionally high levels of total metals. Total concentrations of Cu, Zn, Mn and Pb as high as 42, 150, 640 and 650 pg g - l , respectively, were recorded, implying contaminated sediment. A comparison of the two transects indicated that the sediments of Transect B seemed to contain higher total Zn but lower Cu and Mn concentrations than those of Transect A. Most of the heavy metals accumulated in the sediments were not extractable with ammonium acetate and no Cu or Pb was detected in these extracts. The concentrations of extractable Zn and Mn were low, less than 10% of the total metal concentration in the sediment, and appeared to decrease from the landward to seaward samples. For both total and extractable metals, there were significant seasonal fluctuations for both transects, but no specific trends could be identified. These spatial and temporal variations suggest that the scale and representativeness of sampling require careful planning, and a single sample might not give a satisfactory evaluation of the levels of heavy metal contamination in mangrove ecosystems.
The continual development of agricultural, industrial and urban activities has given rise to a number of environmental problems. Heavy metal contamination in aquatic environments is of critical concern, due to the toxicity of metals and their accumulation in aquatic habitats. Heavy metals, in contrast to most pollutants, 254
are not biodegradable, and they undergo a global ecological cycle in which natural waters are the main pathways. Salt marshes, particularly those located near or along estuaries such as mangrove forests, are often polluted by river-borne and marine-derived particles and pollutants. As in other coastal marshes, mangrove sediments are reduced and act either as sources or sinks of heavy metals (Harbinson, 1986; Silva et al., 1990). Knowledge of the levels of such contaminants in mangrove sediments is important for understanding the degree of heavy metal pollution in aquatic systems, throwing light on the degree and sources of metal pollution, the transport, fate and bioavailability of the metals concerned (Japenga et al., 1990). However, information on heavy metal concentrations in mangrove sediments, especially in sub-tropical regions, is scarce (Harbinson, 1986; Lacerda et al., 1988). No 'natural' or 'background' data on heavy metal levels are available, which makes it difficult to determine whether contamination has occurred. It is essential to assess contaminant levels, identify 'hot spots' and attempt to predict the behaviour of heavy metals in mangrove sediments. The mangrove ecosystem, being intertidal, has a very varied sediment composition. As the chemical behaviour and quantities of trace metals are highly dependent on the physico-chemical conditions of sediments and pore-water, sediment heavy metal concentrations fluctuate from place to place, even within an apparently homogenous habitat (Harbinson, 1986; Lacerda et al., 1988). Different plant cover also results in different sediment conditions, patterns of water movement and heavy metal concentrations. Significant spatial and temporal variability is often a major problem in providing an adequate description of heavy metal contamination in this type of ecosystem. Different degrees of human activity and anthropogenic input further aggravates the problems of spatial heterogeneity. Therefore, a single or even a few samples might give an unsatisfactory evaluation of contaminant levels in a mangrove ecosystem, even though the study site may be relatively small. The present study aims to 1.
Volume 31/Numbers 4-12/April-December 1995 investigate the heavy metal concentrations in sediments of a small sub-tropical mangrove in Hong Kong, 2. evaluate the spatial and temporal variation, and 3. identify any 'hot spots' of heavy metal contamination.
Materials and Methods Study site The study was carried out in a mangrove ecosystem located at Sai Keng, along the north-east facing coast of Three Fathoms Cove (22°25'N, 114°16'E), on the eastern side of Hong Kong. The study site was 0.6 km wide, covering an area of 0.03 km 2. The mangrove plants were located in an enclosed bay, inundated by the incoming tide twice a day, with the largest tidal difference during spring at 2.8 m. The effects of wave action were relatively small. The substrata of the mangrove floor varied from hard to soft, comprising large pebbles and clay particles. The dominant plant species found were Kandelia candel (L.) Druce, Aegiceras corniculatum (L.) Blanco and Avicennia marina (Forsk.) Vierh. The plants were dwarf, immature or marginal, with a height seldom greater than 2 m. Sampling and analyses Two transects, labelled as A and B, were laid perpendicular across the shore, stretching over the entire zone occupied by mangrove plants. Transect A was located in the northern part of the mangrove ecosystem and crossed a relatively wide band of mangrove vegetation (100 m), while Transect B was in the south-eastern region with a narrow coverage of mangrove vegetation (40 m) which was more disturbed by human activities. Transect B was selected specifically to explore the relationship between human activity and metal contamination as the transect was close to a residential area and a small recreational centre. Along each transect, duplicate samples (approximately 1 kg) of surface sediment (2-10 cm deep) were collected every 5 or 10 m intervals, from landward to seaward. Samples were collected on four occasions for Transect A: December 1989, and March, July and September 1990. For Transect B, the September sampling was omitted due to the presence of wasps, which made it dangerous to collect samples. Sediment samples were air-dried, sieved using a 2-mm mesh size and analysed for general chemical properties according to Page et al. (1982). For measurement of the total concentrations of heavy metals, samples (0.5 g air-dried weight) were digested using 10 ml concentrated nitric acid and 1 ml sulphuric acid at 160°C. The filtered digests were then analysed for Cu, Zn, Mn, Pb, Cd, Cr and Ni by flame atomic absorption spectrophotometry (AA-680, Schimadzu Corporation). The detection limits of this AAS for Cu, Zn, Mn, Pb, Cd, Cr and Ni were 0.15, 0.04, 0.10, 0.50, 0.05, 0.20 and 0.30 ~tg g - l dry wt, respectively. The less strongly bound heavy metal concentration was determined by extracting the samples with 1M ammonium acetate (pH 4) at a 1:5 sediment:extractingsolution ratio (w/v) followed by AAS measurements. The mean, standard deviation and range of the heavy metal concentrations were calculated. All data were
expressed in terms of 105°C oven-dried weight. The spatial and temporal variations in sediment heavy metal content of each transect were determined by two-way analysis of variance (ANOVA). Any difference between the two transects was assessed using a one-way ANOVA. Simple correlation coefficients between heavy metals and organic matter were also calculated. All statistical analyses were carried out using the statistical package SPSS on an IBM-compatible PC.
Results Total heavy metal concentrations The general chemical properties of sediments collected from the two transects are different (Table 1). Transect A, with more plant coverage, higher production and decomposition of litter, had higher levels of organic matter, conductivity and nutrients but a lower pH than Transect B. Among the heavy metals determined, the concentrations of Cr, Ni and Cd in sediments were below the detection limits of AAS. In general, Transect A had higher total Zn and Mn but lower Cu concentrations than Transect B (Table 2). There was no significant difference between the total Pb concentrations in samples from the two transects. The significant spatial variations of the total Cu, Zn, Mn and Pb concentrations along and between the two transects were found to be significantly different (Figs 1 and 2). Although no specific trend was identified, the heavy metal concentrations were higher in the landward sites, decreasing gradually seaward especially in Transect B. Heavy metal concentrations recorded in this study were generally below levels found in polluted estuaries (Everaarts & Fischer, 1992; MacKey et aL, 1992; Machiwa, 1992; Real et al., 1993). In both transects, however, there were sites with unusual and exceptionally high concentrations of total Cu, Zn, Pb and Mn, with values at 40, 150, 640 and 650 ~tg g-~, respectively (Figs 1 and 2). These particular sites could be considered as local contaminated 'hot spots'. The distribution of these 'hot spots' was patchy and their specific locations varied from transect to transect, sample to sample, and from one metal to another. Nevertheless, it seems that most of the contaminated sites were found closer to the landward region, implying that the contamination might be due to anthropogenic input. The Sai Keng mangrove ecosystem is located in an enclosed bay with no industry, mining or dredging TABLE 1 Chemicalproperties of mangrovesedimentsalong TransectsA and B (mean+ 1 SD). Properties
Transect A
TransectB
pH Conductivity(mmhoscm- ~) Organicmater (%) Total K (%) Extractable K (lagg- J) Total KjeldahlN (%) NI-I~-N (I.tgg-l) NO~--N (lagg- ~) Total P (%) Olsen P (lagg-l)
5.86+-0.44 90.6+-37.5 9.43+__3.48 2.44-t- 1.76 147.3_ 184.5 0.182+-0.048 31.7+-18.2 2.73+-1.47 0.039-1-0.019 26.2_+14.6
6.51 4-0.65 69.9___9.89 5.46+ 1.02 0.57_+0.28 299.9__.102.4 0.116_+0.021 9.27:1:1.95 4.64+-4.99 0.021+-0.005 19.9+-5.68 255
Marine Pollution Bulletin TABLE 2 Results of ANOVA tests showing spatial and temporal differences in sediment heavy metal concentrations of Sai Keng mangrove stand.*
Mean of Mean of Transect A Transect B (lag g-~) (lag g-~)
Heavy metals Total Cu
9.27
16.44
Total Zn
57.65
47.84
Total Mn
107.92
83.92
Total Pb
49.93
69.21
Extractable Zn
1.77
3.51
Extractable Mn
8.49
4.84
F value between Transects A and B
F value of Transect A Between time
52.5 (0.000) 7.6 (0.006) 9.3
26.1 (0.000) 78.4 (0.000) 80.3
2.7 (0.103) 26.4 (0.000) 9.6 (0.002)
3.3 (0.024) 31.2 (0.000) 82.1 (0.000)
(0.003)
Between sites
(0.000)
F value of Transect B
Interaction
Between time
Between sites
Interaction
2.7 (0.001) 2.4 (0.004) 10,1
3.4 (0.000) 1.2 (0.179) 5.7
2.2 (0.133) 79.4 (0.000) 94.4
2.6 (0.021) 28.3 (0.000) 40.8
1.4 (0.178) 6,2 (0.000) 21.4
4.7 (0.000) 10.1 (0.000) 44.3 (0.000)
5.0 (0.000) 5.5 (0.000) 17.6 (0.000)
26.6 (0.000) 53.6 (0.000) 4.4 (0.020)
3.7 (0.002) 16.9 (0.000) 1.6 (0.158)
0.6 (0.864) 3.6 (0.001) 1.1 (0.359)
(0.000)
(0.000)
(0.000)
(0.000)
(0.000)
*The bracketed values show the significance of the calculated F values from the ANOVA tests.
activities in the vicinity. The input might come from the construction industry, waste dumped at the back of the shore and/or from holiday/weekend visitors who might dump domestic and metal-containing waste. This would explain why the 'hot spots' appeared sporadically. Total Pb, Zn and Mn concentrations varied significantly with sampling time, but no specific seasonal trends could be identified, except that sediments of Transect A collected in June 1990 seemed to have lower concentration of total Cu and Mn (Fig. 3, Table 3).
Mn than Transect B (Table 2). Exceptionally high concentrations of extractable Zn and Mn were observed at certain sites of Transect A (Fig. 3). Significant correlations were found between total and extractable Zn and Mn concentrations in both transects (Table 5). Positive correlations were observed between concentrations of Cu, Zn and Mn and organic matter taken from the sediments of Transect B (Table 5).
Extractable Zn and M n content
In this study, the degree of heavy metal contamination in mangrove sediments was assessed by analysing the total and extractable metal concentrations in the bulk surface sediments, as recommended by previous researchers (Everaarts & Fischer, 1992). Except for a handful of 'hot spots', the concentrations of heavy metals in mangrove sediments were low, and most were not extractable and therefore assumed not bioavailable. Although there are very few data on sediment heavy metal concentrations in mangrove ecosystems available, the mean and maximum heavy metal concentrations recorded in this study are generally comparable to or lower than the reported values for surface sediments of estuarine habitats (Table 6). These findings suggest that
The NH4OAc-extractable Pb and Cu concentrations were below detection limits (<0.15 and <0.05 lxg g-~ dry wt, respectively). The mean values of extractable Zn and Mn were 2.51 and 6.95 ~tg g - l , respectively, accounting for less than 10% of the total metal concentrations (Table 4). Spatial and temporal fluctuations in the concentrations of extractable Zn and Mn were similar to those of the total metal concentrations (Fig. 3). In Transect B, the extractable Zn and Mn concentrations appeared to be higher at the landward end, decreasing towards the sea; any such trend was less obvious in Transect A. Transect A had lower extractable Zn but higher concentrations of extractable
Discussion
TABLE 3
Spatial and temporal variations of concentrations of total heavy metals (lag g-1 dry wt) in Sai Keng mangrove sediments.* Total Cu
Total Zn
Total Pb
Sampling month
Mean 4- SD
Range
Mean __.SD
Range
Transect A December March June September LSD
12.4 4- 5.6a 8.9 4- 2.2b 7.4+2.6 c 8.5_+3.0 ~ 1.33
5.6-28.6 4.6-12.7 1.1-11.3 2.8-13.7
30,1_9.9 d 70,74_14.l b 81.6+21.3 a 46.9_6.6 c 8.29
16.6-53.6 34.9-94.2 60.9-147.4 33.2-57.2
Transect B December March June LSD (p = 0.05)
16.4_ 5.7ab 12.1 -t-7.2b 20.8 ___8.7a 7.49
10.9-27.2 4.7-30.8 11.6-41.2
47.5+ 10.8c 31.1+13.1 b 64.9_34.5 a 6.53
33.5-72.9 21.3~8.7 30.3-143.8
Mean _+SD
Total Mn Range
Mean 4_SD
Range
41.9+20.4 b 7.6-72.7 45.14_12.4 ab 23.4-82.2 62.5___48.9a 11.4-240.9 48.9+ 16.2ab 18.2-81.9 18.29
95.5__ 29.5b 107.44-40.3 b 70.9+ 19.1c 160.0+ 51.7a 17.81
38.2-151.5 61.7-221.8 34.0-106.8 61.5-262.6
87.1_ 16.5a 48.8+16.1 e 73.3_+12.6b 12.55
64.7_8.7 c 84.1+18.4 b 102.9__+53.7 a 6.79
50.3-80.6 63.6-120.1 49.8-223.1
65.7-110.5 33.2-83.2 49.6-96.3
*The mean and standard deviation values of each transect followed by different superscripts within each column indicated that there were significant differences between sampling times at a probability level of 0.05 according to ANOVA. Two extremely high concentrations of total Pb in both transects and one extreme value of total Mn in transect A were deleted from the calculation.
256
Volume 31/Numbers 4--12/April-December 1995 Transect
A
Transect Total
B
Cu 50
30
25 40
20
J
30 =L v
v
~D
.w
-~
•
io
20
0
10 5
W 0
1
I
I
I
I
20
40
60
80
i00 Total
160
140
140
100
I
I
I
i
I
14
21
28
35
42
.-, 120
v
~
// . ,,
ao
I
7
Zn
160
120 L
0
100 80
_.% _
o\
z
19
20
\-\
4o 20
O'
0
J
L
,
,
20
40
60
80
Distance
)
100
(rn)
0
1
I
I
7
14
21
I
28
Distance
I
I
35
42
(m)
Fig. 1 Changes in total Cu and Zn concentrations in sediments collected from Transects A and B of Sai Keng Mangrove: December (O), March (V), June (V), September (D) sampling times. TABLE 4 Ammonium acetate extractable Zn and Mn concentrations (lag g - t dry wt) of sediments in Sai Keng mangrove stand.* Extractable Zn (lag g-1) Sampling month
Mean ± SD
Range
Extractable fraction of Zn (%)
Extractable Mn (lag g - t ) Mean ± SD
Range
Extractable fraction of Mn (%)
Transect A
December March June September LSD
1.47±2.11 b 2.51 ± 1.26a 1.71 ±0.81 b 1.38 ± 0.80 b 0.33
0.01-9.70 0.14--4.92 0.56--3.62 0.27-3.46
3.99 + 4.25 a 3.22± 1.65b 2.29± 1.25~ 3.23± 1.79b 0.52
9.69 ± 4.87 b 8.11±6.46 ¢ 4.27±3.61 d 12.09± 13.01a 1.16
3.59+2.31 ~b 0.97±0.85 b 5.95 ± 4.68" 2.95
1.13-8.49 0.22-3.02 1.04--16.9
7.23 ± 3.34a 2.74+ 1.52b 8.10+2.86 a 2.73
3.22-1-1.83 c 6.74+4.43 a 4.54+2.39 b 1.17
0.35-24.39 1.54-27.53 0.71-16.59 2.64--62.71
10.03 -I-6.39 a 6.79±3.83 b 6.59±5.60 b 6.28±3.27 b 1.15
Transect B
December March June LSD (p = 0.05)
1.05-7.99 2.18-16.46 1.68-8.57
4.91 + 2.46 b 7.30-1-3.16 a 5.07_+2.81 b 1.43
*The mean and standard deviation values of each transect followed by different superscripts within each column indicated that there were significant differences between sampling times at a probability level of 0.05 according to ANOVA test. 257
Marine Pollution Bulletin
oo[
Transect B
Transect A Total Pb 650
1
475 45O
~-. 240
625 f
60O 140
.
I
:¢
200
'~
160
.o0
120
::h ,~ la,
"~
120 I00
8o
0
~
s0
v,
~ •
\
60 40 20
0
0 0
20
40
60
80
100
0
I
I
i
I
I
I
7
14
21
28
35
42
Total M n
85°I
240 []
640
630
200
620 240
[]I
t
I
/
160 v
2OO 120
"~ leo 0
0
120
F
80 40 0 0
20
80
I
I
I
I
40
60
80
100
40
I
I
I
1
I
I
7
14
21
28
35
42
Distance (m) Distance (m) Fig. 2 Changes in total Pb and Mn concentrations in sediments collected from Transects A and B of Sai Keng Mangrove: December(Q), March(V), June (V), September(I-1)sampling times. Sai Keng mangrove sediments are not significantly polluted by heavy metals, and the concentrations recorded can be considered as 'natural background' levels, Comparison of data obtained in the present study with metal levels in other coastal sediments might be difficult, however, as heavy metal concentrations, even background levels, vary from area to area, depending on sediment characeristics (Pardue et aL, 1992). Origin and composition of the sediment, particle size distribution and feasible post-depositional reactions all play an important role in determining metal concentrations, and they preclude any simple comparisons of metal levels in sediments with different characteristics (Barreiro et al., 1994). The hydrodynamic and chemical complexity of each particular estuary, together with the singular 258
behaviour of each metal, make it difficult to compare the results from one estuary to another. To facilitate comparisons between ecosystems, heavy metal concentrations should be normalized or correlated to a reference element such as A1 and Fe (Pardue et al., 1992). In order to evaluate pollution, 'hot spots' of contamination within a site could be identified by comparing the concentration at a specific site with the average concentration at the adjacent location; a site with a concentration of at least twice the average would be considered as a 'hot spot' (Everaarts & Fischer, 1992). Spatial variations of heavy metal concentrations were significant in this study, with lower metal concentrations towards the seaward regions. This seaward decrease in
Volume 31/Numbers 4-12/April-December 1995
Transect A
Transect B Extractable
10
Zn 18
1
15
6 I~t) 4
•
=L "-" 9
\,
v
N
I
~
0
0
Extractable
[]
60
,
,
7
14
0
20 40 60 80 I00
65 I
6
,"IV v,X7 X3-V x7 21
28
35
42
Mn
18
F
15
55
50
5
@..@
r~v
0
0 0
20
40
60
fl0
100
0
DisLanee (m)
7
~ 14
, 21
I 28
I 35
, 42
Distance (m)
Fig. 3 Changes in NH4OAc (pH4) extractableZn and Mn concentrations in sediments collected from Transects A and B of Sai Keng Mangrove: December (0), March (V), June (V), September ([-1) sampling times. metal concentrations of estuarine sediments may be related to the mobilization of metals from sediments, as well as the mixing o f fluvial and marine particulate material (Barreiro et al., 1994). Salinity also influences levels o f heavy metals accumulated in surface sediments. Real et aL (1993) reported that in sediments located seaward, increased salinity impedes the oxidation process and some heavy metals such as Mn are able to cross the oxic layer and return to the water column, leading to lower concentrations at sites nearer to the sea. Sediment analysis offers certain advantages over water analysis for the control and detection of metal pollution in estuaries, as sediments act as integrators and amplifiers o f the concentrations o f these elements
(Real et al., 1993; Barreiro et al., 1994). On the other hand, surface sediment often exchanges with suspended material, thereby affecting the release of metals to the overlying water. Therefore, the top few centimetres of the sediments reflect the continuously changing presentday degree of contamination, instead of recording the history of contamination. This helps to explain why spatial and temporal variations are significantly high in this small mangrove ecosystem in H o n g Kong.
Conclusion The present study indicates that heavy metal contamination has not been a serious problem in the Sai Keng mangrove ecosystem. Total Cu, Zn, Pb and 259
Marine Pollution Bulletin TABLE 5 Correlation coefficients between heavy metals concentrations and other related parameters in mangrove sediments along two transects. Organic matter content
Total Cu
Total Zn
Total Mn
Total Pb
Extractable Zn
-0.12 0.25 0.16 0.13 0.25
-0.15 0.14 0.31' -0.31"
-0.12 -0.07 0.49**
-0.05 -0.12
0.19
0.03 0.68** 0.43*
0.28 -0.21
-0.01
Transect A (76 samples)
Total Cu Total Zn Total Mn Total Pb Extractable Zn Extractable Mn
0.49** 0.13 -0.07 0.14 0.41'* 0.08
Transect B (32 samples)
Total Cu Total Zn Total Mn Total Pb Extractable Zn Extractable Mn
0.45** 0.65** 0.51'* 0.28 0.59** 0.34
0.67** 0.49* 0.32 0.63** 0.14
0.78** 0.37 0.95** 0.13
* and ** indicate that the correlations were significant at probability levels of 0.01 and 0.001 (one-tailed), respectively. TABLE 6 Concentrations of heavy metals in sediments reported by previous studies (mean values with the range given in parentheses). Metal concentrations (lag g-l) Location and types of sediments
Cu
Zn
Sai Keng Mangrove, Hong Kong
12.4 (1-31)
53.3 (17-147)
Brisbane Mangrove, Australia
22.4 (3-30)
97.9 (41-144)
66.8 (20-82)
7.3 (2-17)
Saudi Mangrove, Arabian Gulf
1.8 (0.1--4)
Pb
Source of reference
Comments
Present study
Relatively clean
ND*
MacKey et al. ( 1 9 9 2 )
Industrial and urban discharge
11.8 (6-19)
28.7 (2-69)
Sadiq & Zaidi (1994)
Nil
58.2 (8-241)
Mn 97.9 (34-223)
Sepetiba Bay Mangrove
12.4
311
17.8
ND
Lacerda et al. (1993)
With Avicennia covered
Sepetiba Bay Mangrove, SE Brazil
23.9
823
39.4
ND
Lacerda et al. ( 1 9 9 3 )
Mud-fiat area
Mangrove, S. Australia
(30-80)
(142-190)
(85-112)
ND
Harbinson (1986)
Nil
18.4 (1-89)
69.3 (9-321)
57.3 (7--467)
ND
Maehiwa (1992)
Harbour sediments affected by human activities
Coastal of Dares Salaam, Tanzania Ria de Arousa, NW Spain
(25--365)
(123-281)
ND
Estuarine, Java Sea
(6-54)
(33-122)
(5-46)
ND
Everaarts (1989)
North Sea
(25-240)
(400-4000)
(75-630)
ND
Everaarts & Fischer (1992) Contaminated due to anthropogenie inputs
Saguenay Fjord, Canada
24.4 (21-26)
114.5 (88-133)
ND
ND
Gagnon et al. (1993)
Nil
River Tees, UK
120 (10-1100)
473 (40-2900)
249 (40-990)
397 (160-1800)
Davies et al. (1991)
Highly polluted
2500
700
ND
Japenga et al. (1990)
Signal value
ND
Katz & Kaplan (1981)
Pollution free 'baseline' concentration
Standard Dutch sediments Coastal area, Southern California
400 9.0
44.4
10.5
(195-3590) Real et al. (1993)
Effluent discharges Nil
*ND, not determined. M n c o n c e n t r a t i o n s were g e n e r a l l y c o m p a r a b l e to a n d / o r b e l o w the values r e p o r t e d b y p r e v i o u s w o r k e r s on m a n g r o v e sediments. H e a v y m e t a l s such as Cr, C d a n d N i were n o t d e t e c t e d in this ecosystem. O n the o t h e r h a n d , ' h o t s p o t s ' o f c o n t a m i n a t i o n were identified w h o s e d i s t r i b u t i o n s were p a t c h y a n d v a r i e d w i t h different m e t a l s , b e t w e e n transects, as well as o v e r time. T h e significant c o r r e l a t i o n coefficients f o u n d b e t w e e n h e a v y m e t a l c o n c e n t r a t i o n s in the sediments 260
o f T r a n s e c t B indicate t h a t the h e a v y m e t a l s m i g h t have c o m e f r o m the s a m e source o f p o l l u t i o n , p r o b a b l y h u m a n input. A m m o n i u m acetate d i d n o t e x t r a c t m o s t o f the a c c u m u l a t e d metals. Z n a n d M n c o n c e n t r a t i o n s , which were extracted, a c c o u n t for less t h a n 10% o f the t o t a l m e t a l c o n c e n t r a t i o n s , i n d i c a t i n g t h a t the h e a v y m e t a l s a c c u m u l a t e d are s t r o n g l y b o u n d w i t h o r g a n i c m a t t e r . Positive c o r r e l a t i o n s were f o u n d b e t w e e n m e t a l c o n c e n t r a t i o n s a n d o r g a n i c m a t t e r c o n t e n t s in the
Volume 31/Numbers 4--12/April-December 1995 sediments. Spatial a n d t e m p o r a l v a r i a t i o n s o f total a n d extractable h e a v y metals were significant in Sai K e n g m a n g r o v e sediments, a l t h o u g h this forest was relatively small, i m p l y i n g t h a t m o r e samples are required for a m e a n i n g f u l record o f the actual v a r i a t i o n o f m e t a l contamination. Research grants awarded to the authors by the Croucher Foundation and the Hong Kong University Grant Council are acknowledged. Barreiro, R., Real, C. & Carballeira, A. (1994). Heavy-metal horizontal distribution in surface sediments from a small estuary (Pontedeume, Spain). Sci. Total Environ. 154, 87-100. Davies, C. A., Tomlinson, K. & Stephenson, T. (1991). Heavy metals in River Tees estuary sediments. Environ. TechnoL 12, 961-972. Everaarts, J. M. (1989). Heavy metals (Cu, Zn, Cd, Pb) in sediment of the Java sea, estuarine and coastal areas of east Java and some deep-sea areas. Neth. J. Sea Res. 23(4), 403--413. Everaarts, J. M. & Fischer, C. V. (1992). The distribution of heavy metals (Cu~Zn, Cd, Pb) in the fine fractions of surface sediments of the North Sea. Neth. J. Sea Res. 29(4), 323--331. Gagnon, C., Pelletier, E. & Maheu, S. (1993). Distribution of trace metals and some major constituents in sediments of the Saguenay Fjord, Canada. Mar. Pollut. Bull. 26(2), 107-110. Harbinson, P. (1986). Mangrove muds--a sink and a source for trace metals. Mar. Pollut. Bull. 17, 246-250. Japenga, J., Zschuppe, K. H., DeGroot, A. J. & Salomons, W. (1990). Heavy metals and organic micropollutants in flood plain of the
River Waal, a distributary of the River Rhine, 1958-1981. Neth ,i.. Agric. Sci. 38, 381-397. Katz, A. & Kaplan, I. R. (1981). Heavy metals behaviour in coastal sediments of southern California: a critical review and synthesis. Mar. Chem. 10, 261-299. Lacerda, L. D., Martinelli, L. A., Rezende, C. A., Mozetto, A. A., Ovalle, A. R. C., Victoria, R. I., Silva, C. A. R. & Nogueira, F. B. (1988). The fate of heavy metals in suspended matter in a mangrove creek during a tidal cycle. Sci. Total Environ. 75, 249-259. Lacerda, L. D., Carvalho, C. E. V., Tanizaki, K, F., Ovalle, A. R. C. & Rezende, C. E. (1993). The biogeochemistry and trace metals distribution of mangrove rhizospbers. Biotropica 25(3), 252-257. MacKey, A. P., Hodgkinson, M. & Nardella, R. (1992). Nutrient levels and heavy metals in mangrove sediments from the Brisbane River, Australia. Mar. Pollut. Bull. 7,4(8), 418-420. Machiwa, J. F. (1992). Heavy metal content in coastal sediments off Dar Es Salaam, Tanzania. Environ. lnterp. 18, 409-415. Page, A. L., Miller, R. H. & Keeney, D. R. (1982). Methods of Soil Analysis, Part 2 Chemical and Microbiological Properties. American Society of Agronomy, WI, USA. Pardue, J. H., DeLaune, R. D. & Patrick Jr, W. H. (1992). Metal to aluminum correlation in Louisiana coastal wetlands: identification of elevated metal concentrations. J. Environ. Qual. 21, 539-545. Real, C., Barreiro, R. & Carballeira, A. (1993). Heavy metal mixing behaviour in estuarine sediments in the Ria de Arousa (NW Spain). Difference between metals. Sci. Total Environ. 128, 51--67. Sadiq, M. & Zaidi, T. H. (1994). Sediment composition and metal concentrations in mangrove leaves from the Saudi coast of the Arabian Gulf. Sci. Total Environ. 155, 1-8. Silva, C. A. R., Lacerda, L. D. & Rezende, C. E. (1990). Heavy metal reservoirs in a red mangrove forest. Biotropica 22, 339-345.
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