Long-term effects of oil dispersants on intertidal benthic invertebrates

L o n g - T e r m Effects of Oil Dispersants on Intertidal Benthic Invertebrates III. Toxicity to Barnacles and Bivalves of Untreated and Dispersant-Treated Fresh and Weathered Condensate FERGUS M. POWER Zoology Department, University of Auckland, Private Bag, Auckland, New Zealand Field studies directed towards evaluating the toxicity of dispersant-treated and untreated fresh and weathered Maui 'D-sand' condensate to intertidal invertebrates were conducted at Anawhata, on the west coast of the North Island of New Zealand. Observations were concentrated on the barnacles Chamaesipho columna and Epopella plicata and the bivalves Xenostrobus pulex and Perna canaliculus. The field studies continued for 83 days after treatment. Two dispersants were tested, Shell SD LTX and BP1100 WD (Synperonic OSD 20). 1. I N T R O D U C T I O N The Maui gas and condensate field lies 30 km west of Cape Egmont, Taranaki, New Zealand, in water depths of around 110 m. This study is one of several (Battershill and Bergquist, 1982; Power, 1983a,b) initiated with the aim of assessing the toxicity of Maui D-sand condensate and of dispersants approved for use in New Zealand to organisms common on the Maui shore. No work of this nature had been previously undertaken in New Zealand. In the event that an accident occurred in the Maui operational area and the biota was exposed to condensate or dispersants, there would be an immediate need for data which could indicate whether the use of dispersants was warranted or advisable. Studies of the toxic effects of dispersants in isolation provide a basis for comparison of the effects of oil in isolation, or of the effects of dispersants on oil-exposed organisms. Of more

direct value in terms of predicting the likely ecological effects of an oil spillage, and assessing the advantages bestowed by chemically dispersing it, is analysis of the effects of oils and dispersanttreated oils on marine organisms in their natural ecosystems. Attempts to achieve this have ranged from scale models of the Baltic littoral system (Ladner and Hagstrom, 1975) and fencing of isolated environments around coral reef outcrops (Kinsey, 1973) to treatment of fixed intertidal quadrats in a manner designed to simulate likely exposure of the organisms during a real spillage and clean-up operation (Crapp, 1971). There is a general lack of information on the comparative toxicity of fresh and weathered oils to field populations of animals and laboratorytested organisms. The present study is an extension of field investigations of toxicity of dispersants to intertidal animals (Power, 1983a, b), and considers the rela-

Fergus Power is presently Senior Biologist with the Taranaki Catchment Commission and Regional Water Board, PO Box 159, Stratford, New Zealand. Oil & Petrochemical Pollution © Graham & Trotman Ltd., 1983 171

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FERGUS M. POWER

tive toxicities of fresh and weathered Maui condensate, and the biological effects of nontreatment or dispersant-treatment of weathered condensate. Marine animals may be more susceptible to oil spills in cooler conditions than they would be in warmer conditions because of direct and indirect effects of low temperatures on the physical behaviour of oil and the sensitivity of the animals. Oilwater solutions probably remain at toxic concentrations for longer periods at lower temperatures because of reduced volatility and biodegradation of oil in seawater. In the present study Maui condensate was sprayed onto the shore before dawn in order to provide cool exposure conditions for oil-treated animals, thus reducing evaporation of volatile aromatics. Following large spills of crude oil as in the Torrey Canyon (Smith, 1968) and the Santa Barbara (Moore and Dwyer, 1974) incidents, the effects of the oil on the marine fauna and flora were not of great magnitude and were of relatively short duration. However, there are records of spills where effects were reported as having been significantly more damaging. Usually, where oil of a high aromatic content has been spilled, the toxic fractions have been transported into intertidal and subtidal substrates (North et al., 1964; Blumer et al., 1970; Rutzler and Sterrer, 1970; Wormald, 1976). The wreck of the Amoco Cadiz, in March 1978, spilled over 100 000 t of Iranian Crude and Light Arabian Crude Oil onto the coast of Brittany. This oil proved very toxic to marine life. The lethal nature of the oil carried by the Amoco Cadiz can be explained by the high content of aromatics (30--50%) and up to 3% benzene. Great mortalities close to the wreck were attributed to these aromatics, while lesser mortalities further away indicated that some evaporation had taken place over the few days before the stranding of this weathered oil (Southward 1978). Thus, apparently disparate effects between oil spillages can be partially explained by considering the chemical nature of the oil, and loss of highly toxic ephemeral components due to weathering. In the present study the toxicity of fresh and weathered Maui condensate can be considered in relation to previously reported research into the behaviour of components of oils with respect to solution, evaporation and interaction with each

OPP VOL. 1 NO. 3, 1983

other. All of these factors can affect the toxicity of an oil, and during the weathering process, each is continually changing. The results of the present study can be directly compared with the results of a previous study aimed at assessing the long-term toxicities of dispersants in isolation (Power, 1983a). 2. MATERIALS AND METHODS A rocky promontory forming the northern boundary of a beach located on the west coast of the North Island of New Zealand at Anawhata (latitude 36° 55' south, longitude 170° 27' east) was chosen as the experimental area. The promontory offered sufficient space for the establishment of several experiments, each of which could be confined in an area of constant aspect. This was necessary to minimise environmental variation. Areas of comparable aspect were selected to keep the natural variation in the growth and mortality of the test animals as constant as possible. Quadrats were positioned so as to avoid runoff of the oils and dispersants from higher to lower areas. The minimum distance between ~iny two quadrats was 2 m. The size of each quadrat was dependent on the species contained and the requirement to isolate a minimum of 100 test animals. All quadrats were less than 50 c m x 50 cm in area. Numbered cards were placed in each quadrat area, then each treatment was assigned using random numbers (Zar, 1974). Permanent site markers, in the form of expocreted, self-drilling bolts, were fixed to allow rapid and positive identification of the position of the numerous quadrats. To reproduce conditions similar to those which would occur in a real spillage, fresh condensate (FC) and weathered condensate (WC) were applied immediately following the recession of an outgoing tide. On the following day, the dispersants were applied during an incoming tide to those quadrats previously treated with WC. The 'weathered' Maui condensate was not weathered in the true sense, but had the more volatile fractions removed by light aeration for 72 h at 20°C. Both the Shell SD LTX and the BP1100 WD were applied according to the manufacturers recommendations. The amount of dispersant applied in the effective dispersal of oil on shores ranges

EFFECTS OF OIL DISPERSANTS. III.

OPP VOL. 1 NO. 3, 1983

from 0.09 1 m 2 to 1.0 1 m 2 but is normally near to 0.4 1 m 2 (Department of Trade and Industry 1975). The rate of application used in this study was 0.6 1 m 2 for both of the dispersants and for the FC and WC. Table I shows the allocation of treatments for each species, and the number of quadrats used per treatment. Table II presents the number of animals of each species which were monitored. Mortality of individual barnacles was recorded with the use of a binocular microscope by laying acetate sheets with randomly placed pinholes over colour transparency photographs of the barnacle quadrat. The bivalves were counted in situ. Because of difficulties previously experienced with migrant P e r n a c a n a l i c u l u s (Power, 1983a), in the present study experimental mussels were marked with an extremely durable rubber-based paint. Mortalities were analysed by performing analyses of variance on a Prime P400 computer using a G L I M statistical package (Netder, 1975). Each data point was weighted according to the

173

original number of animals in each quadrat. In species where differences between the control and the treatments were detected by A N O V A , further analyses were used to establish which treatments differed significantly from each of the others. These selected analyses used 95% simultaneous confidence intervals based on the Maximum Modulus t statistic (0.05 level of significance) (Seber, 1977). Animals were monitored for 83 days after treatment. Treatments involving the application of WC and dispersant will be referred to as WC/Shell SD LTX and WC/ BP100 WD. 3. RESULTS 3.1 BARNACLES

Epopella plicata Figures la to le display control E . p l i c a t a and the response of E . p l i c a t a to WC, FC, WC/Shell SD LTX and WC/BP1100 WD treatments, respectively. Analyses of variance performed for days

TABLE I Allocation of treatments for each species and number of quadrats used per treatment Dispersant treatment Species

Weathered Control

condensate

Fresh condensate

Weathered conden- Weathered condensate sate treated with treated with BPllO0 Shell SD L T X WD

Chamaesipho columna

4

4

4

4

4

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4

4

4

4

4

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4

4

4

4

4

Perna canaliculus

4

4

4

4

4

TABLE II Numbers of animals per species monitored Dispersant treatment Control

Weathered condensate

Fresh condensate

Chamaesipho columna

400

400

400

400

400

Epopella plicata

400 596 275

400 611 309

400 444 315

400 306 278

400 400 403

Species

Xenostrobus pulex Perna canaliculus

Weathered conden- Weathered condensate sate treated with treated with BPllO0 Shell SD L T X WD

FERGUS M. POWER

174

11, 50 and 83 all produced significant results (F0.05 (4, 15) = 3.056, day 11 F = 6.92, day 50 F = 7.11, day 83 F = 9.08). The results of Maximum Modulus t analyses for day 83 (Table III) show a significant difference between control and WCand FC-treated E. plicata populations, both treatments causing elevated mortalities. TABLE III

Epopella pIicata: selected comparisons between treatments on day 83 using 95% simultaneous confidence intervals based on the maximum modulus t statistic (Seber, 1977) Comparison

Result

Control x WC Control x FC Control x WC/Shell SD LTX Control x WC/BP1100 W D WC x FC WC x WC/Shell SD LTX WC x WC/BPl100 WD WC/Shell SD LTX × WC/ BPl100 W D FC x WC/Shell SD LTX FC x WC/BP1100 WD

Significant Significant Not Significant Not significant Not significant Significant Not significant Not significant Not significant Not significant

There was no significant difference between FC and WC treatments. Treatment of WC-exposed E. plicata with Shell SD LTX significantly reduced WC-induced mortality, whereas treatment with BPll00 WD did not. After dispersant treatment, survival of WC-exposed populations was improved to the extent that no significant difference existed between survival of animals in WC/ Shell SD LTX or WC/BPll00 WD populations and that in control populations. Shell SD LTX was superior to BPll00 WD in reducing the toxic effect of WC-treatment of E. plicata. No significant difference was evident between survival in FC-treated populations and WC/dispersanttreated populations. In order of decreasing toxicity the treatments were ranked: WC = FC>WC/BPll00 WD>WC/Shell LTX. Chamaesipho columna Figures 2a to 2e display control C. columna and the response of C. columna to WC, FC, WC/Shell SD LTX and WC/BPll00 WD treatments, respectively. Analyses of variance performed for

OPP VOL. 1 NO. 3, 1983

days 11, 50 and 83 all produced significant results (F0.05 (4, 15) = 3.056, day 11 F = 5.26, day 50 F = 9.14, day 83 F = 3.94). The results of Maximum Modulus t-analyses for day 83 (Table IV) show that WC- and FC-treatment equally elevated mortalities significantly in C. columna. There was a significant improvement in survival of WC-exposed animals after treatment with BPll00 WD, whereas treatment with Shell SD LTX caused only a small but insignificant improvement. There was a significant difference between the effects of the dispersants. Survival in FC- and WC/Shell SD LTX-treated populations was similar.

TABLE I V

Chamaesipho columna: selected comparisons between treatments on day 83 using 95% simultaneous confidence intervals based on the maximum modulus t statistic (Seber, 1977) Comparison

Result

Control x WC Control x FC Control x WC/Shell SD LTX Control x W C / B P l l 0 0 WD WC x FC WC x FC/Shell SD LTX WC x W C / B P l l 0 0 WD WC/Shell SD LTX x WC/ B P l l 0 0 WD FC x WC/Shell SD LTX FC x WC/BP1100 WD

Significant Significant Significant Significant Not significant Not significant Significant Significant Not significant Significant

Survival in WC/BP1100 WD-treated populations was significantly higher than in FC-treated populations. In order of decreasing toxicity the treatments were ranked: WC = FC>WC/Shell SD LTX>WC/BPll00 WD. 3.2 BIVALVES Xenostrobus pulex Figures 3a to 3e display control X. pulex and the response of X. pulex to WC, FC, WC/Shell SD LTX and WC/BP1100 WD treatments respectively. Analyses of variance performed for days 11, 50 and 83 all produced significant results (F0.05 (4, 15) = 3.056, day 11 F = 14.8, day 50 F = 13.98, day 83 F = 14.33). The results of Maxi-

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mum Modulus t analyses for day 83 (Table V) show that while FC caused significantly high mortalities in X. pulex, WC did not cause mortalities any higher than those found in control populations. TABLE V

Xenostrobus pulex: selected comparisons between treatments on day 83 using 95% simultaneous confidence intervals based on the maximum modulus t statistic (Seber, 1977) Comparison

Result

Control x W C Control x FC Control × WC/Shell SD LTX Control x W C / B P l l 0 0 W D W C × FC W C x WC/Shell SD L T X W C × WC/BP1100 W D WC/Shell SD L T X × W C / BPll00 WD FC × WC/Shell SD L T X FC x WC/BP1100 W D

Not significant Significant Significant Significant Significant Significant Significant Significant Not significant Significant

Treatment of WC-exposed animals with dispersant caused greater mortalities than those found in populations which had been treated with WC alone. Shell SD LTX was significantly more toxic to WC-exposed animals than BPll00 WD, though these mortalities exceeded those of FCtreated animals. There was no significant difference between survival of FC-treated and WC/ Shell SD LTX-treated animals. In order of decreasing toxicity, the treatments were ranked: WC/Shell SD LTX>WC/BPll00 W D > F C > W C .

OPP VOL. 1 NO. 3, 1983

treatments. This is a significant factor to be taken into account when options for oil-spill clean-up are being considered. When exposed to WC, Epopella plicata suffered elevated mortalities, test populations dropping to about 60% of their original size by the end of the 83-day recovery period. WC did not prove significantly toxic to Xenostrobus pulex, while FC proved extremely so. When exposed to WC, Chamaesipho columna suffered complete eradication by day 70. This was the only instance of complete destruction in all species and treatments tested. FC-treated Epopella plicata populations dropped to about 67% of their original size by the end of the 83-day recovery period. FC-treated Xenostrobus pulex and Chamaesipho columna suffered very similar mortalities, populations dropping to about 17% and 12%, respectively, of their original sizes by the end of the 83-day recovery period. For each treatment, the rank order of species resistance (in order of decreasing resistance) is set out in Table VI.

TABLE VI Rank order of species resistance to the four treatments (in order of decreasing resistance) Comparison W e a t h e r e d condensate Fresh condensate W e a t h e r e d condensate/ Shell SD L T X W e a t h e r e d condensate/ BPll00 WD

Rank 1 > 2, 4 > 3 1> 2 > 3, 4 1> 2 > 3, 4 1> 2

> 3 > 4

1 = Perna canaliculus; 2 = Epopella plicata; 3 = Chamaesipho columna; 4 = Xenostrobus pulex

Perna canaliculus Figures 4a to 4e display control P. canaliculus and There was no significant difference between the the response of P. canaliculus to WC, FC, WC/ Shell SD LTX and WC/BPll00 WD treatment, effects of WC- or FC-treatment on either Eporespectively. Analyses of variance performed for pella plicata or Chamaesipho columna. FC proved days 11, 50 and 83 all produced non-significant considerably more toxic than WC to Xenostrobus pulex. Treatment of WC-exposed E. plicata with results. Shell SD LTX significantly improved survival to a level equivalent with control populations, while 4. DISCUSSION treatment with BPll00 WD did not significantly P. canaliculus is the major intertidal edible mar- improve survival of WC-exposed animals. WC ine species gathered on the Taranaki coast, and and FC proved very toxic to C. columna, there it was not significantly affected by any of the being no significant difference in their effects.

OPP VOL. 1 NO. 3, 1983

EFFECTS OF OIL DISPERSANTS. III.

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Treatment of WC-exposed C. columna with Shell SD LTX did not significantly alter survivorship of test animals. There was a significant difference between the effects of the two dispersants; treatment with BPll00 WD mitigating the toxic effect of WC-exposure to a greater degree than Shell SD LTX. Treatment of WC-exposed populations with Shell SD LTX produced similar mortalities to those in FC-treated populations. WC did not prove significantly toxic to X. puIex, while FC proved extremely so. Treatment of WC-exposed X. pulex with either Shell SD LTX or BPll00 WD significantly increased mortalities above those expected for WC-treatment alone. There was a significant difference between the effects of the two dispersants, Shell SD LTX causing higher mortalities than BPll00 WD. Treatment of WC-exposed populations with Shell SD LTX increased mortalities to a level similar to that of FC-treated populations. In the present study, both WC and FC caused significant field mortalities of Epopella plicata. Treatment of WC-exposed animals with either Shell SD LTX or BPll00 WD produced mortalities similar to those shown by field populations treated with the dispersants alone in a previous study (Power 1983a). In contrast, all four treatments (WC, FC, WC/Shell SD LTX and WC/BPll00 WD) caused heavy mortalities in field populations of Chamaesipho columna. Treatment of WC-exposed animals with dispersants caused much heavier mortalities than those which had previously been recorded for C. columna treated with the dispersants alone (Power, 1983a). Epopella plicata proved more resistant to all treatments than C. columna. The superior resistance of this higher-shore barnacle has previously been attributed to its greater ability to isolate itself from the environment (Power, 1983a). It is possible that X. pulex proved more susceptible to FC than P. canaliculus because of either an innate susceptibility to hydrocarbons or a more rapid assimilation of the ephemeral, highly toxic lower molecular weight aromatic fractions. However, any additional stress on surviving treated animals may prove sufficient to induce mortality. Synergistic ecological interactions are important when assessing the effects of pollutants. X. pulex, which lives much higher on the shore than P. canaliculus, may have been subject to sufficiently more acute adverse conditions than P. canaliculus to

OPP VOL. 1 NO. 3, 1983

cause a synergistic increase in mortality. The majority of laboratory investigations of the toxicity of oil/dispersant mixtures are of little relevance to the present study. In laboratory experiments, test organisms are invariably exposed to oil and dispersant simultaneously, and under physical conditions favourable to the test organlsms. In the present study, condensate was first applied to the emersed organisms, the tide then returned and rinsed the polluted shore, and subsequently, 24 hours after condensate application, dispersants were used to cleanse the shore. In the present study, while treatment of WCexposed X. pulex with dispersants elevated mortalities, treatment of E. plicata and C. columna with Shell SD LTX and BPll00 WD respectively, significantly mitigated the toxic effect of the WC. The latter results clearly point to the invalidity of extrapolating the results of laboratory toxicity tests with oil/dispersant mixtures to the field. Such studies may be of use for prediction of likely ecological effects of oil/dispersant mixtures on plankton, or on intertidal pool organisms contaminated with freshly-spilt oil and treated immediately with dispersant. The present results show that these laboratory studies are no substitute for field trials in which the likely time sequence of oil and dispersant exposure is observed. Improvement in survival of WC-exposed barnacles by dispersant treatment may have been due to removal by the dispersants of persistent oil components which had absorbed to the exterior of the animals and the surrounding substrate. After initial application, the WC was subjected to a further 24 hour period of evaporation, solution, and physical water turbulence during immersion. This would have further depleted the concentration of lower MW hydrocarbons in the remaining condensate. Consequently, improval of survival by chemical dispersal of these persistent compounds argues that they were toxic to the barnacles. The heavy mortalities experienced by WC-exposed X. pulex treated with dispersants may have been due to a synergistic effect of the dispersants acting on condensatestressed animals. Shell SD LTX and BPll00 WD are not toxic to X. pulex when applied in isolation (Power 1983a). The present study provides evidence both for and against the use of dispersants for cleansing shores polluted with Maui D-sand condensate.

OPP VOL. 1 NO. 3, 1983

EFFECTS OF OIL DISPERSANTS. III.

ACKNOWLEDGEMENTS I wish to thank Professor Pat Bergquist for helpful discussion, D r Brian McArdle for statistical assistance, and the Shell, BP and T o d d (New Zealand) Oil Consortium for financial assistance. This p a p e r is an abbreviated version of research

181

p e r f o r m e d for Shell BP and Todd and presented as an interim report: Long-term effects on intertidal animals of untreated and dispersant-treated fresh and weathered Maui condensate. University of Auckland Maui D e v e l o p m e n t Environmental Study, Phase II. Submitted to Shell BP and Todd Oil Services Limited, New Plymouth, New Zealand, O c t o b e r 1980. R e p o r t No. 80-4.

REFERENCES Battershill, C. B. and Bergquist, P. R. 1982. Responses of an intertidal gastropod to field exposure of an oil and a dispersant. Mar. Pollut. Bull. 13 (5) 159-162. Beynon, L. R. 1974. Ecological aspect of toxicity testing of oils and dispersants. In: L. R. Beynon and E. B. Cowell (Eds), Proc. Workshop, Inst. Petrol. London. Applied Science Publishers, Barking. p. 122. Blumer, M., Souza, G. and Sass, J. 1970. Hydrocarbon pollution of edible shellfish by an oil spill. Mar. Biol. 5, 195-202. Crapp, G. B. 1971. Field experiments with oil and emulsifiers. In: Annual Report 1969. Field Studies Council, Oil Pollution Research Unit, London. Department of Trade and Industry 1975. Newsletter No. 6 (August). Warren Spring Laboratory. Giere, O. 1980. The impact of crude oil and oil dispersants of the marine oligochaete Marionina subterranea. Cahiers Biol. Mar. 21 (t) 51-60. Kinsey, D. W. 1973. Small-scale experiments to determine the effects of crude oil films on gas exchange over the coral back-reef at Heron Island. Environ. Pollut. 4, 167-182. Ladner, L. and Hagstom, A. 1975. Oil spill protection in the Baltic Sea. Jour. Water Pollut. Control Fed. 47 (4) 796-809. Levell, D. 1976. The effect of Kuwait crude oil and the dispersant BPl100X on the lugworm, Arenicola marina L. In: J. M. Baker (Ed.), Marine Ecology and Oil Pollution. Applied Science Publishers, Barking. Nelder, J. A. 1975. GLIM manual (Release 2). General Linear Interactive Modelling Numerical Algorithms Group, Oxford. North, W. J., Neushul, M. and Clendenning, K. A. 1964. Successive biological changes observed in a marine cove exposed to a large spillage of mineral oil. Proc. Symp. Pollution and Marine Microorganisms Prod. Petrol, Monaco, pp. 333-354. Power, F. M. 1983a. Long-term effects of oil dispersants on intertidal benthic invertebrates. I. Survival of barnacles and bivalves. Oil Petrochem. Pollut. 1 (2) 9%108. Power, F. M. 1983b. Long-term effects of oil dispersants on intertidal benthic invertebrates. II. Growth of the barnacle Epopella plicata (Gray) following dispersant application to a shore. Oil Petrochem. Pollut. 1 (2) !09-112. Rutzler, K. and Sterrer, W. 1970. Oil pollution. Damage observed in tropical communities along the Atlantic seaboard of Panama. Bioscience 20 (1) 222-224. Seber, G. A. 1977. In: Linear Regression Analysis. John Wiley and Sons, New York. Smith, J. E. (ed.) 1968. 'Torrey Canyon" Pollution and Marine Life. Cambridge University Press, Cambridge. Southward, A. 1978. Marine life and the Amoco Cadiz. New Scientist, 20 July. Wormald, A. P. 1976. Effects of a spill of marine diesel oil on the meiofauna of a sandy beach at Picnic Bay, Hong Kong. Environ. Pollut. 11 117-130. Zar, J. H. 1974. Biostatistical Analysis. Prentice-Hall International, London, pp. 577-580.