Phosphorus and organic carbon in the sediments of a polluted subtropical estuary, and the influence of coastal reclamation

Marine Pollution Bulletin, Vol. 13, No, 10, pp. 354-359, 1982

0025 3 2 6 X / 8 2 / 1 0 0 3 5 4 - 0 6 $ 0 3 . 0 0 ' 0 © 1982 Pergamon Prc~s I ld.

Printed in Greal Britain

Phosphorus and Organic Carbon in the Sediments of a Polluted Subtropical Estuary, and the Influence of Coastal Redamation G. B. T H O M P S O N and S. K. YEUNG

Fisheries Research Station, Aberdeen, Hong Kong

In July 1978, total phosphorus and organic carbon were determined in the sediments of Tolo Harbour, a sewagepolluted estuary in north-east Hong Kong. Concentrations were correlated with % silt-clay in each of three areas. Phosphorus concentrations were highest in central Tolo Harbour, lower by about 1.5~g at. P g-1 in the outer estuary, Tolo Channel, and lowest in the polluted inner reaches near large coastal reclamations. The latter values, about 3.0 ~g at. P g-t lower than in central Tolo Harbour, might reflect a selective adsorption of phosphate by reclamation sediments. Organic carbon concentrations were high in the inner reaches and decreased towards the outer channel. Correlations between phosphorus and organic carbon were compared with a published correlation for the east coast of England: in Hong Kong, phosphorus concentrations showed a smaller increase as organic carbon increased, and reached only one-third of the English values as organic carbon approached 2.0070. This paper describes the distribution of phosphorus and organic carbon in the sediments of Tolo Harbour, an enclosed and poorly-flushed estuary in north-east Hong Kong. When the survey was made in July 1978, the 15 km long estuary was polluted by untreated sewage from about 90 000 people and their livestock, and reclamation work was under way for the construction of new towns, at Tat Po and Sha Tin, which will eventually house nearly 1 000000 people. Sewage from the new towns is to be given secondary treatment before it is discharged into the inner part of the estuary, and this should prevent an increase in suspended solids and BOD load (Oakley & Cripps, 1972; Preston, 1975). The nutrient load will be greatly increased, however; it may be possible to remove much of the nitrate in the treatment process, but most of the phosphate will be discharged into the sea. In shallow coastal waters, the phosphate cycle is dominated by sediment-water exchanges (Nixon et al., 1980), and these were investigated in Tolo Harbour by Stirling & Wormald (1977). From laboratory experiments, Stirling & Wormald concluded that sediments used in reclamations adsorbed more added phosphate (95°70) than did estuarine sediments (71-88 °7o).Adsorption by sediments should reduce the impact of increased phosphate discharge, especially near large reclamation projects. They also found that, while adsorption and desorption from sediments 354

appeared to act as a buffer system, equilibrium values were in the range 0.1-0.3/ag at. P 1_ 1 in sea-water. These values were lower than the equilibrium values of 0.7-1.5 lag at. P 1_ 1 reported from temperate estuaries by Rochford (1951), P omeroy et al. (1965) and Butler & Tibbitts (1972), and suggest that phosphate-sediment interactions may differ in temperate, subtropical and tropical areas (Johannes & Betzer, 1975). Stirling & Wormald confined their work to laboratory experiments, and did not determine concentrations of phosphorus in sediments. This was done in the present survey which forms part of a broad programme of environmental studies in Tolo Harbour. The results are described here and discussed in relation to Stirling & Wormald's findings, i.e. that coastal reclamation may affect the distribution of phosphorus, and that results from a subtropical area will differ from those obtained in temperate waters.

Materials and M e t h o d s Smith-Mclntyre grab samples were collected at 76 stations (Fig. 1) from 6 to 10 July 1978. The grab was opened from above, and 2 sub-samples were scooped into 200 ml polypropylene containers and frozen at - 2 0 ° C until analysed. One set of sub-samples was used in the work

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Fig. 1 Map of Tolo Harbour and Tolo Channel, Hong Kong, showing the division into areas and sub-areas, and the distribution of phosphorus in surfacesediments.

Volume 13/Number 10/October 1982

described here, and the other set was analysed for trace metal content by Mr W. W. S. Yim of the University of Hong Kong. For determination of total phosphorus, sediment was dried to constant weight at 80°C and ground to pass a 0.5 mm screen. Phosphorus was brought into solution by a nitric-perchloric acid hot digestion and analysed by the method of Strickland & Parsons (1972). Organic carbon and particle size were determined by standard methods described by Buchanan & Kain (1971). Organic carbon was expressed in 'chromic acid oxidation values' and for particle size, rapid partial analysis was used to estimate the combined silt-clay content, by wet sieving on a 62 gm mesh. The precision of the phosphorus determinations was estimated in pairs of replicate samples from 18 randomlyselected stations. The mean difference in each pair of samples was 0.61 ~g at. P g- 1, ot 7.6°7o of the mean concentration. For organic carbon, the same procedure yielded a mean difference or 0.0507o organic carbon between sample pairs, or 3.7 07oof the mean value.

14 (a)

Results The distribution of phosphorus is shown for 75 stations in Fig. 1 (the result for station 37 was lost, as was the silt-clay result for station 16). To analyse this distribution, the study area was divided into three parts. Area 1 comprised 3 subareas, 1A near the Tai P • Reclamation, 1B near the Sha Tin Reclamation, and 1C at Ma Shi Chau dumping ground, which receives marine spoil from the two Reclamation areas. Area 1C was included because its surface sediments consisted of material dredged from Areas 1A and 1B in the preceding months, and similar results were obtained in all three sub-areas. Area 2 contained the central part of Tolo Harbour, which is polluted by sewage and agricultural waste from the existing towns of Sha Tin and Tai P • and the rivers that flow through them. Area 3 contained Tolo Channel, which leads to the open coast and contains no important sources of pollution. For each area, the distribution of total phosphorus was expressed in relation to particle size (as 070 silt-clay), and this yielded three regression lines that were significantly different in an analysis of covariance [

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355

Marine Pollution Bulletin 2.4

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Fig. 3 Relationship of organic carbon and °70 silt-clay in sediments from (a) Tolo Channel, (b) Tolo Harbour and (c) Reclamation areas. Regression lines were calculated by the method of least squares and values are given for correlation coefficients r and probability p. The regression lines are compared at (d), which includes the results of analysis of covariance, n.s. = not significant a t p = 0.05.

(Snedecor & Cochran, 1967; Fig. 2). The highest phosphorus concentrations were found in Tolo Harbour; values in Tolo Channel were lower by about 1.5/~g at. P g- 1, and the lowest values were found in the Reclamation areas, about 3/~g at. P g- 1below Tolo Harbour values. The results for organic carbon were analysed in the same way and yielded significant correlations with %0silt-clay in Tolo Harbour and Tolo Channel, but not in the Reclamation areas (Fig. 3). Values were generally high in the Reclamation areas, and in Tolo Harbour values were about 0.3% higher than in Tolo Channel. Significant correlations were found between total phosphorus and organic carbon in all three areas (Fig. 4). There was no significant difference between the regression lines for Tolo Harbour and Tolo Channel, but the regression line for the Reclamation areas was significantly lower in terms of phosphorus content. Discussion

Adsorption and desorption on sediments appears to be important in buffering the concentration of dissolved phosphate in estuaries (Rochford, 1951; Carritt & Goodgal, 1954; Jitts, 1959; Pomeroy et al., 1965; Butler & Tibbitts, 356

1972). In the inner parts of Tolo Harbour (Areas 1 and 2) Stirling & W ormald (1977) argued that the concentration of phosphate in sea-water had increased as a result of pollution and rarely fell below the equilibrium value of 0.1-0.3/Jg at. P 1_1: phosphorus would therefore accumulate in the sediments. Desorption would occur only in unpolluted waters where phosphate concentrations often fell below the equilibrium value. Thus, it is not surprising that sediment phosphorus concentrations were higher in Tolo Harbour than in Tolo Channel, by about 1.5/~g at. P g-1 (Fig. 2). It is less easy to explain why sediment phosphorus concentrations were lower in the Reclamation areas than in Tolo Harbour, by about 3.0/~g at. P g- 1. On the east coast of England, Aston & Hewitt (1977) found low concentrations of phosphorus and organic carbon in sediments close to a sewer outfaU, and high concentrations at distances of 0.5-1.5 km. This pattern was ascribed to the fact that adsorption is not instantaneous, and maximum deposition of adsorbed pollutants will occur at some distance from the outfall. Such an explanation does not fit the present results, however. Dispersal of effluents is very slow and for Areas 1 and 2, residence periods have been estimated at 28-42 days (Preston, 1975). Also, phosphorus and organic carbon

Volume 13/Number 10/October 1982

14

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should be equally affected; Aston & Hewitt (1977) found a good correlation between phosphorus and organic carbon in all parts of their study area, and we have found a similar correlation in Tolo Harbour and Tolo Channel. For Areas 2 and 3, the regression lines are virtually indistinguishable. In Area 1, however, phosphorus values were significantly lower in relation to organic carbon (Fig. 4). Stirling & Wormald (1977) found that reclamation sediments, similar to those dumped in Areas 1A and 1B, were particularly efficient at absorbing phosphate. Therefore, large-scale reclamation works could have a significant impact on the fate of phosphate discharged into Tolo Harbour. This prediction seems to be confirmed by the present results, in that low phosphorus values were found near reclamations and in the associated dumping ground, but this interpretation raises three problems. First, the distribution of phosphorus may be affected by factors other than reclamation works. For example, desorption of phosphorus from sediments is favoured by increases in temperature, pH and salinity, and by decreases

in turbidity and dissolved oxygen (Rochford, 1951; Carritt & Goodgal, 1954; Stifling & Wormald, 1977; Kelderman, 1980; Krom & Berner, 1980). These factors are probably not important in the present case, however, because conditions in Areas 1A and 1B do not differ markedly from those in the inner part of Area 2 (Trott, 1972; Horikoshi & Thompson, 1980; Thompson & Yeung, 1980; Y. C. Tai, personal communication). Second, the simplest explanation of the low phosphorus values in Area 1 is that phosphate concentrations in water have been reduced by adsorption into reclamation sediments. This explanation is not supported by data on the concentration of phosphate in water samples collected 1 m above the bottom. In 1976-1978, mean values from biweekly sampling were, in/ag at. P 1_1, 0.42 and 0.45 in Areas 1A and 1B, 0.48 in Area 2 and 0.44 in Area 3 (Y. C. Tai, personal communication). Differences between areas were very small, and all values were in excess of the equilibrium value of 0.1-0.3/ag at. P 1_1reported by Stirling & Wormald (1977). 357

Marine Pollution Bulletin

Third, the results are based on a single survey of phosphorus distribution and no observations have been made on the processes that create the distribution pattern. Few in situ studies have been made on phosphate flux between water and sediments and the process is not well understood. Recently, in Narragansett Bay, Nixon et al. (1980) found that in situ phosphate flux varied with temperature and was independent of phosphate concentrations in the overlying water. This was not consistent with sediment buffering of phosphate concentrations in water, and Nixon et al. speculated that in undisturbed sediments, biological activities in the top few millimetres might be more important than physical exchanges. In contrast, the evidence for sediment buffering is based on laboratory experiments with agitated sediments, or on field observations in estuaries where suspended sediments may play an important part (Butler & Tibbitts, 1972). To sum up, the present results show a curious decrease in the total phosphorus content of sediments close to reclamation works, in relation to particle size and organic carbon values. Stirling & Wormald (1977) predicted that these reclamations would adsorb much phosphate, thereby influencing phosphorus distribution, and low sediment phosphorus values were in fact confined to the vicinity of reclamation works (and their associated dumping ground). There was no evidence of low phosphate values in the overlying water, however, and the observed distribution pattern cannot be explained without in situ studies of phosphate flux between water and sediments. Nixon et al. (1980) reported that sediment-to-water phosphate flux increased exponentially with rising temperature in Narragansett Bay, from a negligible value at 3°C to about 10/ag at. P m -2 h -1 at 13°C, and reaching 60/ag at. P m -2 h -~ at 23°C. This increase is relevant to comparisons of phosphorus distributions in temperate and subtropical sediments, but such comparisons are not easy to make because of a lack of comparable data: temperature is not the only variable and sampling strategies differ. It is, however, possible to compare the present results with those of Aston & Hewitt (1977), who made a similar survey in the Walton Backwaters on the east coast of England, where temperatures ranged from 4 to 15°C. In Tolo Harbour and Channel the annual temperature range is from about 15 to 30°C. Aston & Hewitt confined their analyses to the < 180/am sediment fraction, to reduce particle-size effects, and their data should be broadly comparable to the present results, in which the < 62 /am fraction is predominant. Aston & Hewitt reported a significant correlation between total phosphorus and organic carbon and their data and regression line are compared, in Fig. 5, with those from Tolo Harbour and Channel (Areas 2 and 3) combined. The difference between the two sets of data is striking: in Walton Backwaters, concentrations of phosphorus were higher and showed a greater increase with increasing organic carbon. At 2.0°70 organic carbon, total phosphorus concentrations were about three times higher than in Hong Kong. Walton Backwaters and Tolo Harbour and Channel differ in many ways besides temperature, but it is interesting to speculate that the differences shown in Fig. 5 may reflect general differences between temperate and subtropical sediments. Thus, concentrations of 358

32-

• Walton Backwaters

30-

o

Tolo

and

Harbour Channel

2826. 24-

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22-

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~

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phosphorus may be relatively low in subtropical (and tropical) sediments, as a result of the exponential increase in sediment to water phosphate flux with rising temperature, described by Nixon et al. (1980). This exponential increase also accounts for the lower equilibrium values of phosphate in sea-water, found in Hong Kong by Stifling & Wormald (1977), but raises interesting questions about the circulation and fate of phosphorus in subtropical and tropical waters. We thank the coxswain and crew of Marine 62 for their help with sampling, and our colleagues, particularly Y. C. Tai, D. J. H. Phillips and M. L. Chu, for their assistance and advice. Drs R. S. S. Wu, C. K. C. Lee and J. Richards criticized drafts of this paper, the figures were prepared by K. H. Cheung, Y. L. Sin and S. F. Tsang, and Nancy Lui typed the manuscript. We thank the Director of Agriculture and Fisheries, Hong Kong, for permission to publish this paper.

Aston, S. R. & Hewitt, C. N. (1977). Phosphorus and carbon distributions in a polluted coastal environment. Estuar. cstl mar. Sci., 5, 243-254. Buchanan, J. B. & Kain, J. M. (1971). Measurement of the physical and chemical environment. In Methods for the Study o f Marine Benthos (N. A. Holme & A. D. Mclntyre, eds.), pp. 30-58. Blackwell Scientific Publications, Oxford. Butler, E. I. & Tibbitts, S. (1972). Chemical survey of the Tamar Estuary. I. Properties of the waters. J. mar. biol. Ass. U.K., 52, 681-699. Carritt, D. E. & Goodgal, S. (1954). Sorption reactions and some ecological implications. Deep-sea Res., 1,224-243. Horikoshi, M. & Thompson, G. (1980). Distribution of subtidal molluscs collected by trawling in Tolo Harbour and Tolo Channel, Hong Kong, with special reference to habitat segregation in two venerid bivalves. In Proceedings of the First International Workshop on the Malacofauna o f Hong Kong and Southern China, Hong Kong, 1977 (B. Morton, ed.), pp. 149-162. Hong Kong University Press, Hong Kong. Jitts, H. R. (1959). The adsorption of phosphate by estuarine bottom deposits. Aust. J. mar. Freshwat. Res., 10, 7-21. Johannes, R. E. & Betzer, S. B. (1975). Introduction: marine communities respond differently to pollution in the tropics than at higher latitudes. In Tropical Marine Pollution (E. J. F. Wood & R. E. Johannes, eds.), pp. 1-12. Elsevier Scientific, Amsterdam.

Volume 13/Number 10/October 1982 Kelderman, P. (1980). Phosphate budget and sediment-water exchange in Lake Grevelingen (S. W. Netherlands), Neth. J. Sea Res., 14, 229-236. Krom, M. D. & Berner, R. A. (1980). Adsorption of phosphate in anoxic marine sediments. Limnol. Oceanogr., 25, 797-806. Nixon, S. W., Kelly, J. R., Furnas, B. N., Oviatt, C. A. & Hale, S. S. (1980). Phosphorus regeneration and the metabolism of coastal marine bottom communities. In Marine Benthic Dynamics (K. R. Tenore & B. C. Coull, eds.), pp. 219-242. University of South Carolina Press, Columbia, S.C. Oakley, H. R. & Cripps, T. (1972). Marine pollution studies at Hong Kong and Singapore. In Marine Pollution and Sea Life (H. Ruivo, ed.), pp. 83-91. Fishing News (Books), London. Pomeroy, L. R., Smith, E. E. & Grant, C. M. (1965). The exchange of phosphate between estuarine water and sediments. Limnol. Oceanogr., 10, 167-172. Preston, J. R. (1975). Some problems in Hong Kong. In Discharge of

Sewage from Sea Outfalls (A. L. H. Gameson, ed.), pp. 25-30. Pergamon Press, Oxford. Rochford, D. J. (1951). Studies in Australian estuarine hydrology. I. Introductory and comparative features. Aust. J. mar. Freshwat. Res., 2, 1-116. Snedecor, G. W. & Cochran, W. G. (1967). StatisticalMethods, 6th edn. Iowa State University Press, Ames, Iowa. Stifling, H. P. & Wormald, A. P. (1977). Phosphate/sediment interaction in Tolo and Long Harbours, Hong Kong, and its role in estuarine phosphorus availability. Estuar. cstl mar. Sci., 5, 631-642. Strickland, J. D. H. & Parsons, T. R. (1972). A practical handbook of seawater analysis, 2nd. edn. Bull. fish. Res. Bd Can., 167, 1-310. Thompson, G. B. & Yeung, S. K. (1980). Report on low dissolved oxygen values in Tolo Channel in September 1979. Hong Kong Fisheries Occasional Paper, 19, 1-9. Trott, L. B. 0972). Preliminary hydrographic studies of Tolo Harbour, Hong Kong. Chin. Univ. Bull., 1,256-269.

Marine Pollution Bulletin, Vol. 13, No. 10, pp. 359-364, 1982 Printed in Grea! Britain

0025-326X/82/100359-06 $03.00/0 Pergamon Press 1 td.

Sediment Toxicity and the Distribution of Amphipods in Commencement Bay, W l gton, USA R. C. SWARTZ, W. A. DEBEN, K. A. S E R C U a n d J. O. L A M B E R S O N Marine Division, Corvallis E n v i r o n m e n t a l Research Laboratory, US E n v i r o n m e n t a l Protection Agency, Marine Science Center, N e w p o r t , O R 97365, U S A

The toxicity of 175 sediment samples from Commencement Bay, Washington, was measured by the survival of marine infaunal amphipods (Rhepoxynius abronius) during tenday exposure to test sediment. Survival was high in sediment from offshore, deeper parts of the Bay, including two designated dredge material disposal sites. Within each of the major industrialized waterways there was a wide range in amphipod survival. Both acutely toxic and relatively nontoxic samples were collected from various areas within the Hylebos, Blair, Sitcum and City Waterways. Habitat differences, sedimentation rates, proximity to contaminant sources and sinks, and disruption of the seabed by prop scour and dredging could contribute to this variation in toxicity. Community structure data show a correlation between amphipod distribution and sediment toxicity, with lower amphipod density and species richness in the waterways than in the deeper part of the Bay. Phoxocephalid amphipods, a family that includes the bioassay species, were ubiquitous in the deeper Bay, but absent from the waterways. This correlation between laboratory and field results indicates the ecological relevance of the sediment bioassay. T h e E n v i r o n m e n t a l P r o t e c t i o n A g e n c y has recently design a t e d C o m m e n c e m e n t Bay, n e a r T a c o m a , W a s h i n g t o n , U S A (Fig. 1), as o n e o f this n a t i o n ' s w o r s t h a z a r d o u s w a s t e disposal areas. T h e c o n c e n t r a t i o n o f synthetic o r g a n i c c o m p o u n d s in water, s u s p e n d e d m a t e r i a l , s e d i m e n t s a n d a n i m a l tissues is typically h i g h e r in the w a t e r w a y s o f C o m m e n c e m e n t B a y t h a n in o t h e r u r b a n e m b a y m e n t s o f

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Fig. 1 Survival of the infaunal amphipod, Rhepoxynius abronius, in sediment from Commencement Bay. Twenty individuals were seeded in each sample. 359