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Effects of a crude oil and an oil dispersant (Corexit 9527) on populations of the littleneck clam (protothaca staminea)
Marine Environmental Research 6 (1982)291-306
EFFECTS OF A CRUDE OIL AND AN OIL DISPERSANT (COREXIT 9527) ON POPULATIONS OF THE LITTLENECK CLAM (PROTOTHACA STAMINEA)
E. B. HARTWICK,R. S. S. WU* & D. B. PARKER
Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada (Received: 8 December, 1981)
ABSTRACT
Field and laboratory experiments were carried out to investigate the effects of Alberta crude oil and an oil dispersant (Corexit 9527) on the larval settlement, survival, siphon activities and behaviour of the littleneck clam (Protothaca staminea). Corexit 9527 was much more toxic than crude oil, and the highest toxicity was obtained when Corexit 9527 was mixed with crude oil. Siphon activities were impaired and abnormal behaviour was exhibited when adult clams were treated with 100ppm Corexit 9527, lO00ppm crude oil or a combination of both. Larval settlement was not affected when the substratum was treated with lO00ppm crude oil but was retarded when the substratum was treated with a mixture of lOOOppm oil and lOOppm Corexit 9527. Gas chromatograms also showed that the retention time and depth of penetration of hydrocarbons in the substratum was increased when Corexit 9527 was used with crude oil.
INTRODUCTION
Although the major concern with oil pollution is its impact on natural ecosystems, the majority of oil pollution studies have been with single species in the laboratory. However, due to the dissimilarity between laboratory and natural spill conditions, the laboratory results have only limited application value. Studies integrating laboratory and field experiments, which simulate natural spill conditions closely, are therefore desirable (Craddock, 1977). A wide gap also exists in our present knowledge concerning effects of oil pollution and shore cleaning on soft bottom * Present address: Fisheries Research Station, 100A Shek Pai Wan Road, Aberdeen, Hong Kong
291 Marine Environ. Res. 0141-1136/82/0006-0291/$02.75 © Applied Science Publishers Ltd, England, 1982 Printed in Great Britain
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E. B. HARTWICK, R. S. S. WU, D. B. PARKER
infauna (Levell, 1976; Sandborn, 1977), although infauna are usually considered to be more susceptible to oil pollution (Sandborn, 1977). The littleneck clam (Protothaca staminea) has a wide geographical distribution along the Pacific coast, ranging from the Aleutian Islands to southern California (Quayle, 1973). This species supports commercial and recreational fisheries and is abundant in muddy gravel shores and shallow sub-littoral habitats. The recent development of oil industries, associated with a rapid increase in oil transportation by oil tankers, increases the risk of oil pollution along the Pacific coast (Clarke, 1976). This may endanger the clam populations; especially when considering that both adults and larval bivalves are particularly susceptible to hydrocarbons (Renzoni, 1973; Swedmark et al., 1973; Avolizi & Nuwayhid, 1974; Dow, 1975; Spooner, 1977) and gravel-mud beaches are especially vulnerable to oil spills (Grundlach & Haynes, 1978). In the present study, laboratory and field experiments were carried out to study the effects of Alberta crude oil and Corexit 9527 (a dispersant marketed in Canada) on natural littleneck clam populations. Laboratory toxicity tests were carried out to determine the effects of weathered oil and dispersant on the survival and behaviour of the adults. An oil spill was simulated in the field and the mortality of the clams was monitored over a period of 231 days and compared with the laboratory results. Field experiments were also designed to examine the effects ofoil and dispersant on larval settlement.
MATERIALS AND METHODS
Field study sites and the natural population Three intertidal sites (tidal level 0.9 to 1.5 m above chart datum, which is where P. staminea is abundant) at the north end of Lemmen's Inlet, Vancouver Island (49°IYN, 125°51'w) (Fig. 1), were selected for the field experiment and the collection of animals for laboratory experiments. All the sites were protected gravelmud beaches with a gentle slope and uniform substrate. The isolated nature of the sites meant that disturbance from other sources was improbable. Fifteen random samples were taken with a 0.5 m 2 quadrat from each study site to determine the density and structure of the littleneck clam population. The population densities of the clams at Sites A, B and C were found to be 64.8 + 11.0, 77.4 + 7.0 and 60.8 + 6.4 individuals m -2 (~ + SD) respectively. An analysis of variance showed no significant difference (p = 0-05) in the population density at all the sites. The population structures at the three sites were also very similar; typically bimodal with two distinct size classes (umbo-length: 2.7 to 3-4 cm and 3.7 to 4. ! cm). Preparation of test media The oil used in the present study was Alberta crude oil and the dispersant Corexit
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
Scale
293
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Fig. 1.
Map of Lemmen's Inlet and Vancouver Island, showing the locations of the study sites.
9527 (a water-based concentrate). The oil and dispersant, the gas chromatograms of which are shown in Fig. 2, were obtained, in sealed tanks, from Trans-Mountain Oil Company and Imperial Oil Enterprises respectively. Two hundred litres of various nominal concentrations of oil or dispersant or both media were prepared by pipetting an appropriate amount of oil or dispersant or both into sea water (salinity=22%o) in outdoor tanks. This was followed by a 15rain period of thorough mixing (using an electric stirring motor of 3500 rpm and a blade of 8 cm diameter). In preparing the oil-dispersant mixture, Corexit 9527 was added in a 1:10 ratio (dispersant:oil, the recommended application ratio in an oil spill) immediately after the introduction of the oil, followed by a 15 rain mixing period. In order to simulate natural weathering, test media were held in outdoor tanks for five consecutive days, where they were subject to evaporation and sunlight. During this period the test media were stirred for 15 min every day and aliquots for experiments were withdrawn immediately after the stirring. Thus, on day 1, the test
294
E.B. HARTWICK, R. S. S. WU, D. B. PARKER 1
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Gas chromatogramsof(a) Albertacrude oil and (b) Corexit9527usedin the presentstudy.
media were withdrawn after the initial stirring, without weathering. On day 2, the test media were withdrawn after 24 h weathering, and so on. No appreciable change in salinity was found for all the test media at the end of the 5 days weathering period.
Laboratory experiments Clams collected from Lemmen's Inlet were transported to the laboratory in plastic containers covered with kelp to keep the animals damp. They were acclimatised for at least 48 h in outdoor tanks with running sea water prior to experiments (salinity = 22 + 1%0, temperature = 10 + I °C). Two size groups, which were most abundant in the field, were sorted for the experiment: a smaller size group with an umbo-length of 2.7 to 3.4cm, designated as small clams and a larger group with an umbo-length of 3.7 to 4-1 cm, designated as large clams. Twenty individuals of one size group were inserted in their normal upright position in a gravel-mud substratum in a 40 x 35 cm plastic tray and five replicates were prepared for each treatment. After insertion they were allowed to acclimatise in running sea water for another 24 h before treatment. The following nominal concentrations were tested: (1) 100 ppm oil; (2) 1000 ppm oil; (3) 10 ppm Corexit 9527; (4) 100 ppm Corexit 9527; (5) 100 ppm oil plus 10 ppm Corexit 9527; (6) 1000 ppm oil plus 100 ppm Corexit 9527; and (7) sea water control. All laboratory experiments were carried out at a water temperature of 10.5 to 12.5°C. A volume of 3.5 litres of each medium was obtained from the outdoor weathering tank and added to each tray. The test medium was drained from a bottom outlet the next morning and the clams were left exposed for 5 h in order to
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
295
simulate a tidal cycle. This technique of draining also allowed the test medium to percolate through the substratum, simulating the receding of the tide in the intertidal zone after an oil spill.Another 3-5 litres of test medium was obtained from the weathering tank and added to each tray at the end of the 5 h daily exposure. Exposure and treatment were repeated daily for the following 5 days. The percentage of clams showing siphon activities and the mortalities were recorded before and after draining, respectively. Clams with a gaping shell which showed no response to a tactile stimulus on their mantles were considered dead and were discarded. At the end of the 5 day treatment the survivors were kept in clean running water for another 5 days and again the mortality was recorded~
Simulated field oil spill experiments Eighteen 0.5 m 2 quadrats were dug (approximately 16 cm deep) in the intertidal zone at Site A. All the clams in the quadrats were removed. Clams collected from the general areas of the same site were sorted into a large and a small size group, as indicated earlier, and each individual was numbered with a permanent felt-tip pen. Twenty clams of one size group were inserted in their normal upright position in the substratum of each quadrat and were allowed to acclimatise for 24 h. The following media were prepared in outdoor weathering tanks, as described earlier: (1) 1000 ppm oil; (2) 1000 ppm oil plus 100 ppm Corexit 9527; and (3) sea water control. A volume of 7 litres of the appropriate medium was obtained daily from the weathering tank and poured over the quadrats (randomly assigned a test medium) immediately after the tide receded, and was contained for 10min by a 16cm deep, 0.5 m 2 stainless steel quadrat dug into the substratum. Within 10min the media would either percolate into the sediment or were drained off. The above treatment was repeated for the following 5 days. The mortality of the clams was checked on days 5, 10 and 231 after the initial treatment. The results for the field oil spill experiment are recorded as 'mortality' for animals actually found dead, and 'lost' for a category including 'mortality' plus clams disappeared in the next consecutive sampling. Cumulative 'percentage mortality' and 'percentage lost' were then calculated.
Larval settlement experiments Plastic petri dishes (diameter 100 mm, depth 25 ram) were filled with sediments collected approximately 16cm below surface at Site A (to reduce the chance of collecting newly settled larvae). These petri dishes were colour-coded, to differentiate different treatments, and had 5 holes (diameter 6 ram) drilled in the bottom to allow for percolation of test medium. The following test media were prepared in outdoor weathering tanks as described earlier: (1) I 0 ppm oil; (2) 100 ppm oil; (3) 1000ppm oil; (4) 10ppm oil plus 1 ppm Corexit 9527; (5) 100ppm oil plus 10ppm Corexit 9527; (6) 1000 ppm oil plus ! 00 ppm Corexit 9527; and (7) sea water control.
296
E . B . HARTWICK, R. S. S. WU, D. B. PARKER
The sediment filled petri dishes were placed in plastic trays (40 x 35 cm), to which 4 litres of the appropriate freshly prepared medium was added. The test medium was drained from a bottom outlet in the tray after 5 min to let the medium percolate through the sediment; this procedure was repeated five times. Ten replicates of each of the above treatment groups were randomly placed in the intertidal area at both Site A and Site B, and secured with a 117 cm nail through a bottom hole. The petri dishes were exposed for larval settlement from the 19th of June to the 24th of June 1979, when the natural settling of littleneck clam larvae was occurring at Lemmen's Inlet. They were then collected and immediately frozen and transported to the laboratory. The sediment was washed through a 180/~m sieve. Sediment retained was stained with Bengal red and littleneck clam larvae with whole fresh tissue were counted under a dissecting microscope.
Hydrocarbon analysis Preliminary investigations were also carried out to provide some information on the fate of oil and dispersant in the substratum under the field experimental conditions. A volume of 200 litres of the following media were prepared in outdoor weathering tanks: (1) 1000 ppm oil; (2) 1000 ppm oil plus 100 ppm Corexit 9527; and (3) sea water control. A volume of 7 litres of the appropriate medium was administered daily for five consecutive days on 0.5 m: quadrats at Site A, using the same technique as described earlier. Core samples (15 cm long, 2 cm diameter) were taken at 24 h and on the 10th day after the initial treatment and the hydrocarbon profiles of the top 3cm, middle 6cm and bottom 6cm of the core samples were determined. The samples were extracted three times with cyclohexane and made up to a known volume. A volume of extract representing 2 g of dry sediment was injected into the gas chromatograph for hydrocarbon determination. (GC Mode and conditions: Hewlett-Packard 5830A Gas Chromatograph with flame ionisation detector; GC column: 3 ~ dexsi1300 on chromatograph with (AW) 80-100 packing in a stainless steel column; nitrogen flow rate: 30mlmin-1; temperature programmed from 50 ° to 350 ° at 10°C min-l; cyclohexane gave the largest peak at around 1 rain)
RESULTS
Laboratory experiments Cumulative mortalities of the large and small clams generally showed a positive relationship with treatment time and concentration (Table 1). No mortality was observed in the sea water control and upon treatments of 100 ppm oil, 1000 ppm oil and 10 ppm Corexit 9527 (small clams only) throughout the experiment. The LTs0
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values upon the various treatments were interpolated by plotting the probit mortality against log time (Sprague, 1969) and these are also shown in Table 1. The LTso (240 h) was approximately 4.5 days for both size groups in the 1000 ppm oil plus 100 ppm dispersant treatment, and was 5 to 6 days and 2 to 3 days for the small and large groups, respectively, in the 100ppm dispersant treatment. LT~o values were all > l0 in other treatments. In general, the mortality resulting from oil plus dispersant treatment was higher than dispersant or oil treatment alone. Percentages of small and large clams exhibiting siphon activities upon various treatments are shown in Figs. 3 and 4. Percentages of siphon activities were similar in the sea water control for both small and large clams. A Duncan's New Multiple Range test (Li, 1965) showed that the percentage of individuals showing siphon activities was significantly lower (p > 0-05), in both size groups, when treated with 1000 ppm oil plus 100 ppm dispersant, followed by 100 ppm dispersant and then 1000 ppm oil. No significant difference could be found between the siphon activities upon the other treatments and the control, in both size groups. lOO-
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CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
299
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Individuals from the sea water control and 100 ppm oil treatment usually showed a fast siphon retraction resffonse when they were touched. Their shells were also tightly closed during the 5 h exposure period. Individuals from the otlaer treatment groups began to exhibit abnormal behaviour after the second day of treatment. The first observable symptom involved a slow tactile response and a wide gaping of the shell during exposure to air, the next symptom was the tight-closing of their shells, leaving the siphons extended. The tight-closing of the shell eventually led to a complete pinching off of the siphon, followed by death of the animal.
Simulated field oil spill experiments The cumulative percentage mortality and percentage loss of animals in the field experiments are shown in Table 2. Percentage loss generally exhibited a pattern similar to that found for percentage mortality. The cumulative percentage mortality and percentage loss of animals in both size groups (except the percentage mortality of large clams treated with
E. B. HARTWICK, R, S. S. WU, D. B. PARKER
300
TABLE 2 FIELD EXPERIMENTS" CUMULATIVEPERCENTAGE MORTALITYAND PERCENTAGEIA36S(NO. FOUND DEAD PLUS NO. DISAPPEARED)OF Protothaca staminea UPON VARIOUS TREATMENTS
Treatment Control
Size Small
0-5 days ~ mortality
5"0 + 2.8 5-0+2'8 % mortality 0 % loss 0 % mortality 3.3 + 2.3 % loss 5.0 + 2.8 ~ mortality 0 % loss 0 % mortality 28.3+ 5.8" % loss 28.3 + 5.8" % mortality 0 %loss 0 %loss
Large 1000ppm oil
Small Large
1000ppm oil + Small 100ppm Corexit 9527 Large
0-10 days
0-231days
6.8 + 1.8 6.8+i.8 10.0 + 3.9 10.0 + 3.9 8-4 + 3.0 10-0+ 3.0" 0 0 32-5__.3-6" 32.5+ 3-6° 10.5 + 4.0 10.5+4.0
8.3 + 3.6 11.7+4.1 13.3 + 4.4 18-3 + 5-0 15.0 + 4.6* 26.7+ 5.7* 11.7 + 4. I 28-3 + 5-8" 35"0_+6.2" 40.0+ 6.3* 20.0 +__5.2* 31.7+6.0"
° Values significantlydifferent from the respective control at p = 0.05.
1000 ppm oil) was significantly higher than the representative controls. Moreover, the percentage mortality and percentage loss of the small clams in the oil plus dispersant treatment was significantly higher than the oil treatment alone (p > 0.05). Upon treatment of oil or oil plus dispersant, the percentage mortalities of the small clams were significantly higher than those found for the large clams. It is also apparent that, in field experiments, percentage mortalities were always lower than in the equivalent laboratory tests. No abnormal behaviour (e.g. pinched siphon or gaping) was observed in the clams from the control and oil treated quadrats. A few individuals in the oil plus dispersant treated quadrats, however, had pinched siphons and were gaping during low tides. Larval settlement The results of the larval settlement experiments for both Site A and Site B are shown in Table 3. No significant difference (t-test, p = 0.01) could be found in larval settlement between the control and the respective oil-treated dishes at both sites. At site A larval settlement on the dishes treated with 1000 ppm oil plus 100 ppm dispersant was significantly lower compared with that of the control and the other oil plus dispersant treatments. However, an analysis of variance (p =0.01) showed no significant difference between all the treatments at Site B. Hydrocarbon analysis No petroleum products or similar compounds were present in the sediments collected from the study sites and vicinity. Chromatograms of the oil compounds from the core sample taken from the oil treated quadrats 24h after the first
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
301
TABLE 3 THE EFFECTS OF CRUDE OIL AND COlt,EXIT 9527 ON THE LARVAL SETTLEMENT O F Protothaca staminea A T SITE A AND SITE B
Treatment
No. of larvae~dish (Yt + SD) Site A Site B
Control 10 ppm oil 100 ppm oil 1000ppm oil 10ppm oil + 1 ppm dispersant 100ppm oil + 10ppm dispersant 1000ppm oil + 100ppm dispersant
2.9 _+ 1.3 2.5 +_ 2.0 2.5 _+ 1.0 3.0 + 1.3
2.8 _+ 1.5 2.2 _+ 1.0 3-8 _+ 1.5 2.4 + 1.9
3.0 _+ 1.9
4.6 _+ 2.1
3.5 +_2.1
3-2 _+ 2-2
1.8 _+ 1.5°
3.0 _+ 2.5
° Values significantly different from the control at p = 0.01.
treatment are shown in Fig. 5. In comparison with the pure oil sample (Fig. 2), the chromatograms in Fig. 5 show that the light molecular weight fractions were largely absent after a 24 h field exposure. The relative abundance of oil compound also decreased with depth of sediment. No petroleum products were detectable in the sediment profile of the oil treated quadrats after l0 days exposure. Chromatograms of the petroleum compounds from the oil plus dispersant quadrat taken after the first treatment are shown in Fig. 6. Similarly, a decrease in relative abundance of petroleum products with depth is observed. Moreover, the
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302
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Fig. 6. Gas chromatograms of the core sediment samples taken from the oil plus dispersant-treated quadrats, 24h after the first treatment: (a) top 3cm, (b) middle 6cm and (c) bottom 6cm.
relative abundance of petroleum compounds is greater in the bottom portion of the core sample for oil plus dispersant, than in the crude oil treated samples. A measureable amount of hydrocarbons was still identified in the oil plus dispersant quadrats 10 days after the treatment (Fig. 7), while no petroleum hydrocarbon was identified in the oil quadrat. N o hydrocarbon compounds could be identified in any sediment sample 231 days after the initial treatment.
a
b
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Fig. 7. Gas chromatograms of the core sediment samples taken from the oil plus dispersant treated quadrats 10 days after the first treatment: (a) top 3cm, (b) middle 6 c m and (c) bottom 6 c m
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
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DISCUSSION
In the present study, the weathering of the test media and the application procedures in the laboratory and field experiments closely simulated the conditions arising in an actual oil spill, and when Corexit 9527 was used. All of the field experiments were carried out in a typical littleneck clam habitat. The results from the laboratory and field experiments indicate that Alberta crude oil alone, or low concentrations of Corexit 9527 (e.g. < 10 ppm), may not be greatly harmful to Protothaca staminea. In the laboratory experiment no mortality occurred within the first day of any treatment and no mortality was found throughout the experimental period when the clams were treated with 100 ppm oil, 1000ppm oil or 10 ppm Corexit 9527 (small clams only). In the field experiment no significant mortality was found in both size groups when treated with 1000 ppm oil. The present findings therefore contradict those of Avolizi & Nuwayhid (1974), Dow (1975; 1978) and Spooner (1977), in which molluscs were found to be particularly susceptible to hydrocarbons. However, the susceptibility of animals to hydrocarbons may be highly variable within a single taxonomic group (Wu, 1981). It has been well established that dispersants, or oil and dispersant mixtures, may be much more toxic than oil itself (Smith, 1968; Nelson-Smith, 1972). A similar pattern was found in both the laboratory and field experiments in the present study; crude oil had relatively little effect while dispersant or dispersant and oil mixtures had a significant effect on adult P. staminea. Mortality was observed when the clams were treated with 100ppm Corexit 9527 and was highest (in both laboratory and field experiments) when treated with a mixture of 100ppm Corexit 9527 and 1000ppm oil. Such an increase in mortality may be due to an increase in the availability of hydrocarbons when dispersant was used, or an additive toxic effect of the dispersant and the oil. It was also apparent that the percentage mortalities resulting from the field experiments were much lower than those from the equivalent laboratory tests. Such discrepancies demonstrate the difficulty in extrapolating laboratory results to natural spill conditions. In the field experiments percentage loss was calculated in addition to percentage mortality. Loss in a population generally represents emigration and mortality combined. In the study area predators of iittleneck clams included the starfish Pisaster ochraceus, the octopus Octopus dofleini and some shore birds. The difference between the percentage loss and percentage mortality in the field experiments may represent either (1) the emigration of the littleneck clams from the polluted substratum or (2) the loss of clams to predation after the treatment. Unfortunately, these two possibilities cannot be differentiated in the present study. However, in an oil pollution of larger scale (e.g. the whole beach is polluted), emigration of clams may be impossible and correspondingly a higher mortality may be expected. Anderson (1972) found that the filtration rate of the bivalve Mya arenaria
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E . B . HARTWICK, R. S. S. W U , D. B. PARKER
increased in 0.9 ~ Bunker C oil in sea water, but no change of rate was found in 0.9 ~ water soluble fraction of the oil. In this study, the siphon activities of P. staminea were significantly retarded in both size groups when treated with 1000 ppm oil. Siphon activities were further reduced when treated with 100 ppm Corexit 9527, and the lowest siphon activities were found upon treatment with 1000 ppm oil plus 100 ppm Corexit 9527. This indicated that normal respiration and feeding activities of P. staminea may be impaired at these concentrations. One of the most noticeable effects of the oil and dispersant mixtures was the alteration in the behaviour of the clams. The first symptom to appear was slow tactile response, followed by the extension of siphon and gaping of shell during exposure to air. Eventually, siphons were pinched off as shells closed, indicating a loss of coordination of these activities. Such alterations in behaviour, although not directly fatal, may make the animals more vulnerable to natural mortality factors (e.g. predation) in their natural conditions. Swedmark et al. (1973) also found that shell closure of the scallop Pecten opercularis was impaired when treated with 1000ppm crude oil. Ho & Karim (1978) showed that the settlement of barnacles and oysters was severely reduced on oil-coated asbestos. It has been shown that the settlement of larval polychaetes (A renicola marina and Sabellaria spinulosa) were retarded when the substratum was pre-treated with oil and BP 1002 or BP 1100X (Wilson, 1968~ Levell, 1976). Results of the present study indicated that, at both sites, the settlement of P. staminea was not affected on the experimental plots treated with 1000 ppm crude oil. Larval settlement was, however, significantly reduced at Site A when substratum was treated with a mixture of 100ppm crude oil and 100ppm Corexit 9527. However, no significant difference could be found in larval settlement in all the experimental plots at Site B. The inconsistency between the results at the two sites is difficult to explain. It should also be pointed out that larval settlement may not be affected, provided that it occurs at a longer time interval, after a spill since the retention time of petroleum hydrocarbons in the sediment appears to be rather short, as indicated by the gas chromatograms. Scarratt & Zitko (1972) found little decrease in Bunker oil in sediments 26 months after the wreck of the tanker Arrow. Blumer & Sass (1972) also found that oil trapped in sediment degraded slower. In contrast, our results indicated that the volatile fraction of oil was no longer present in the sediment after 24 h exposure in the field conditions and that oil was no longer present in the top of the sediment within 10 days. The petroleum products also showed a decrease in depth. However, a measurable amount of hydrocarbon was still found in the oil plus dispersant plots after 10 days, indicating that the use of Corexit 9527 may increase the retention time of the oil in the substratum. The petroleum products also penetrate deeper into the substratum when Corexit 9527 is applied, and the use of this dispersant in the gravelmud habitat should therefore be avoided. The overall results of our experiments indicate that the impact of oil spill alone on the littleneck clam would be small; a similar conclusion has been reached on marine
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
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invertebrates in the Eleni V oil spill and the Mexican oil spill (Blackman & Law, 1980; Lee et al., 1980). However, the use of Corexit 9527 as a dispersant in an oil spill in such a habitat may augment the harmful effects to P. staminea in several ways: (1) by increasing the mortality of the adult clams; (2) by reducing the recruitment of larvae and therefore a long term effect on the population; and (3) by increasing the retention time and depth of penetration of the hydrocarbons in the substratum.
ACKNOWLEDGEMENTS
We thank Dr C. D. Levings for providing laboratory space and equipment at the Pacific Environment Institute and Dr G. B. Thompson for reading the manuscript. We are also grateful to D. Tupper of the Environmental Quality Laboratories for performing the hydrocarbon analysis and for her helpful comments. This research was financially supported by the Canadian Environmental Protection Service.
REFERENCES ANDERSON, G. E. (1972). The effects of oil on the gill filtration rate of Mya arenaria. Virginia J. Sci., 23, 45-7. AVOLIZ1, R. J. & NuwxYmD, M. (1974). Effects of crude oil and dispersants on bivalves. Mar. Pollut. Bull., 5, 149-53. BLACKMAN,R. A. A. & LAW, R. J. (1980). The Eleni V oil spill: fate and effects of the oil over the first twelve months. Mar. Pollut. Bull., 11, 217-20. BLUma, M. & SASS,J. (1972). Oil pollution: persistence and degradation of spilled fuel oil. Science, 1"/6, ! 120-2. CLARrOB,R. C., JR. (1976). Impact of the transportation of petroleum on the waters of the northeastern Pacific. US Natl. Mar. Fish. Serv. Mar. Fish. Rev., 38, 20-6. CRADDOCK, D. R. (1977). Acute toxic effects of petroleum on arctic and subarctic marine organisms. In: Effects of petroleum on arctic and subarctic marine environments and organisms. Volume 11. Biological effects. (Matins, D. C. (Ed)) Acad. Press, Inc., New York, 1-21. Dow, R. L. (1975). Reduced growth and survival of clams transplanted to an oil spill site. Mar. Pollut. Bull., 6, 124-5. Dow, R. L. (1978). Size selective mortalities of clams in an oil spill site. Mar. Pollut. Bull., 9, 45-8. GRUNDLACH, E. R. & HAYI,~S, M. O. (1978). Vulnerability of coastal environments to oil spill impacts. Mar. Tech. Sac. J., 12, 18-27. Ha, C. L. & KA~JM,H. (1978). Impact of absorbed petroleum hydrocarbons on marine organisms. Mar. Poilut. Bull., 9, 156-62. LEE, W. Y., Moams, A. & BO^TWRIGHT,D. (1980). Mexican oil spill: a toxicity study of oil accomodated in seawater on marine invertebrates. Mar. Pollut. Bull., I1, 231-4. LEVELL, D. (1976). The effect of Kuwait crude oil and the dispersant BP II00X on the lungworm Arenicola marina L. In: Marine ecology and oilpollution. (Baker, J. M. (Ed)) Appl. SCI. Publishers, 131-85. Li, J. C. R. (1965). Statistical inference, Edward Brothers, Inc., Michigan, 658 pp. NELSON-SlmTH, A. (1972). OUpoilution and marine ecology, Elek Science, London, 260pp. QUAYI.~, D. B. (1973). The intertidal bivalves of British Columbia, British Columbia provincial museum handbook No. 17, 104pp. RENZONI, A. (1973). Influence of crude oil, derivatives and dispersants on larvae. Mar. Pollut. Bull., 4, 9-13.
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SANDBORN,H. R. (1977). Effects of petroleum on ecosystems. In: Effects of petroleum on arctic and subarctic marine organisms. Volume H. Biological effects. (Mallin, D. C. (Ed)) Acad. Press, Inc., New York, 337-52. ScAngAT'r, D. J. & ZITKO, V. (1972). Bunker C oil in sediments and benthic animals from shallow depths in Chedabucto Bay, N. S. J. Fish. Res. Bd. Can., 29, 47-1350. S~ITH, J. E. (1968). 'Torrey Canyon' pollution andmarine life, Cambridge Univ. Press, London, 196 pp. SPOONER, M. F. (1977). Oil spill in Hong Kong. Mar. Pollut. Bull., 8, 62-5. SPRAGUE,J. B. (1969). Measurement of pollutant toxicity to fish. I. Bioassay methods for acute toxicity. Wat. Res., 3, 792-821. SWEDMARK, M., GP,ANMO, A. & KOLLBERG, S. (1973). Effects of oil dispersants and oil emulsions on marine animals. War. Res., 7, 1649-72. WILSON, D. P. (1968). Long term effects of low concentrations of an oil spill remover (detergent): study with the larvae of Sabellaria spinulosa. J. Mar. Biol. Ass. UK, 48, 177-82. Wu, R. S. S. (1981). Difference in toxicities of an oil dispersant and a surface active agent to some marine animals, and their implications in the choice of species in toxicity testing. Marine Environ. Res., 5, 157-63.
EFFECTS OF A CRUDE OIL AND AN OIL DISPERSANT (COREXIT 9527) ON POPULATIONS OF THE LITTLENECK CLAM (PROTOTHACA STAMINEA)
E. B. HARTWICK,R. S. S. WU* & D. B. PARKER
Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada (Received: 8 December, 1981)
ABSTRACT
Field and laboratory experiments were carried out to investigate the effects of Alberta crude oil and an oil dispersant (Corexit 9527) on the larval settlement, survival, siphon activities and behaviour of the littleneck clam (Protothaca staminea). Corexit 9527 was much more toxic than crude oil, and the highest toxicity was obtained when Corexit 9527 was mixed with crude oil. Siphon activities were impaired and abnormal behaviour was exhibited when adult clams were treated with 100ppm Corexit 9527, lO00ppm crude oil or a combination of both. Larval settlement was not affected when the substratum was treated with lO00ppm crude oil but was retarded when the substratum was treated with a mixture of lOOOppm oil and lOOppm Corexit 9527. Gas chromatograms also showed that the retention time and depth of penetration of hydrocarbons in the substratum was increased when Corexit 9527 was used with crude oil.
INTRODUCTION
Although the major concern with oil pollution is its impact on natural ecosystems, the majority of oil pollution studies have been with single species in the laboratory. However, due to the dissimilarity between laboratory and natural spill conditions, the laboratory results have only limited application value. Studies integrating laboratory and field experiments, which simulate natural spill conditions closely, are therefore desirable (Craddock, 1977). A wide gap also exists in our present knowledge concerning effects of oil pollution and shore cleaning on soft bottom * Present address: Fisheries Research Station, 100A Shek Pai Wan Road, Aberdeen, Hong Kong
291 Marine Environ. Res. 0141-1136/82/0006-0291/$02.75 © Applied Science Publishers Ltd, England, 1982 Printed in Great Britain
292
E. B. HARTWICK, R. S. S. WU, D. B. PARKER
infauna (Levell, 1976; Sandborn, 1977), although infauna are usually considered to be more susceptible to oil pollution (Sandborn, 1977). The littleneck clam (Protothaca staminea) has a wide geographical distribution along the Pacific coast, ranging from the Aleutian Islands to southern California (Quayle, 1973). This species supports commercial and recreational fisheries and is abundant in muddy gravel shores and shallow sub-littoral habitats. The recent development of oil industries, associated with a rapid increase in oil transportation by oil tankers, increases the risk of oil pollution along the Pacific coast (Clarke, 1976). This may endanger the clam populations; especially when considering that both adults and larval bivalves are particularly susceptible to hydrocarbons (Renzoni, 1973; Swedmark et al., 1973; Avolizi & Nuwayhid, 1974; Dow, 1975; Spooner, 1977) and gravel-mud beaches are especially vulnerable to oil spills (Grundlach & Haynes, 1978). In the present study, laboratory and field experiments were carried out to study the effects of Alberta crude oil and Corexit 9527 (a dispersant marketed in Canada) on natural littleneck clam populations. Laboratory toxicity tests were carried out to determine the effects of weathered oil and dispersant on the survival and behaviour of the adults. An oil spill was simulated in the field and the mortality of the clams was monitored over a period of 231 days and compared with the laboratory results. Field experiments were also designed to examine the effects ofoil and dispersant on larval settlement.
MATERIALS AND METHODS
Field study sites and the natural population Three intertidal sites (tidal level 0.9 to 1.5 m above chart datum, which is where P. staminea is abundant) at the north end of Lemmen's Inlet, Vancouver Island (49°IYN, 125°51'w) (Fig. 1), were selected for the field experiment and the collection of animals for laboratory experiments. All the sites were protected gravelmud beaches with a gentle slope and uniform substrate. The isolated nature of the sites meant that disturbance from other sources was improbable. Fifteen random samples were taken with a 0.5 m 2 quadrat from each study site to determine the density and structure of the littleneck clam population. The population densities of the clams at Sites A, B and C were found to be 64.8 + 11.0, 77.4 + 7.0 and 60.8 + 6.4 individuals m -2 (~ + SD) respectively. An analysis of variance showed no significant difference (p = 0-05) in the population density at all the sites. The population structures at the three sites were also very similar; typically bimodal with two distinct size classes (umbo-length: 2.7 to 3-4 cm and 3.7 to 4. ! cm). Preparation of test media The oil used in the present study was Alberta crude oil and the dispersant Corexit
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
Scale
293
1:40.000
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Fig. 1.
Map of Lemmen's Inlet and Vancouver Island, showing the locations of the study sites.
9527 (a water-based concentrate). The oil and dispersant, the gas chromatograms of which are shown in Fig. 2, were obtained, in sealed tanks, from Trans-Mountain Oil Company and Imperial Oil Enterprises respectively. Two hundred litres of various nominal concentrations of oil or dispersant or both media were prepared by pipetting an appropriate amount of oil or dispersant or both into sea water (salinity=22%o) in outdoor tanks. This was followed by a 15rain period of thorough mixing (using an electric stirring motor of 3500 rpm and a blade of 8 cm diameter). In preparing the oil-dispersant mixture, Corexit 9527 was added in a 1:10 ratio (dispersant:oil, the recommended application ratio in an oil spill) immediately after the introduction of the oil, followed by a 15 rain mixing period. In order to simulate natural weathering, test media were held in outdoor tanks for five consecutive days, where they were subject to evaporation and sunlight. During this period the test media were stirred for 15 min every day and aliquots for experiments were withdrawn immediately after the stirring. Thus, on day 1, the test
294
E.B. HARTWICK, R. S. S. WU, D. B. PARKER 1
;
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~s
~
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Gas chromatogramsof(a) Albertacrude oil and (b) Corexit9527usedin the presentstudy.
media were withdrawn after the initial stirring, without weathering. On day 2, the test media were withdrawn after 24 h weathering, and so on. No appreciable change in salinity was found for all the test media at the end of the 5 days weathering period.
Laboratory experiments Clams collected from Lemmen's Inlet were transported to the laboratory in plastic containers covered with kelp to keep the animals damp. They were acclimatised for at least 48 h in outdoor tanks with running sea water prior to experiments (salinity = 22 + 1%0, temperature = 10 + I °C). Two size groups, which were most abundant in the field, were sorted for the experiment: a smaller size group with an umbo-length of 2.7 to 3.4cm, designated as small clams and a larger group with an umbo-length of 3.7 to 4-1 cm, designated as large clams. Twenty individuals of one size group were inserted in their normal upright position in a gravel-mud substratum in a 40 x 35 cm plastic tray and five replicates were prepared for each treatment. After insertion they were allowed to acclimatise in running sea water for another 24 h before treatment. The following nominal concentrations were tested: (1) 100 ppm oil; (2) 1000 ppm oil; (3) 10 ppm Corexit 9527; (4) 100 ppm Corexit 9527; (5) 100 ppm oil plus 10 ppm Corexit 9527; (6) 1000 ppm oil plus 100 ppm Corexit 9527; and (7) sea water control. All laboratory experiments were carried out at a water temperature of 10.5 to 12.5°C. A volume of 3.5 litres of each medium was obtained from the outdoor weathering tank and added to each tray. The test medium was drained from a bottom outlet the next morning and the clams were left exposed for 5 h in order to
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
295
simulate a tidal cycle. This technique of draining also allowed the test medium to percolate through the substratum, simulating the receding of the tide in the intertidal zone after an oil spill.Another 3-5 litres of test medium was obtained from the weathering tank and added to each tray at the end of the 5 h daily exposure. Exposure and treatment were repeated daily for the following 5 days. The percentage of clams showing siphon activities and the mortalities were recorded before and after draining, respectively. Clams with a gaping shell which showed no response to a tactile stimulus on their mantles were considered dead and were discarded. At the end of the 5 day treatment the survivors were kept in clean running water for another 5 days and again the mortality was recorded~
Simulated field oil spill experiments Eighteen 0.5 m 2 quadrats were dug (approximately 16 cm deep) in the intertidal zone at Site A. All the clams in the quadrats were removed. Clams collected from the general areas of the same site were sorted into a large and a small size group, as indicated earlier, and each individual was numbered with a permanent felt-tip pen. Twenty clams of one size group were inserted in their normal upright position in the substratum of each quadrat and were allowed to acclimatise for 24 h. The following media were prepared in outdoor weathering tanks, as described earlier: (1) 1000 ppm oil; (2) 1000 ppm oil plus 100 ppm Corexit 9527; and (3) sea water control. A volume of 7 litres of the appropriate medium was obtained daily from the weathering tank and poured over the quadrats (randomly assigned a test medium) immediately after the tide receded, and was contained for 10min by a 16cm deep, 0.5 m 2 stainless steel quadrat dug into the substratum. Within 10min the media would either percolate into the sediment or were drained off. The above treatment was repeated for the following 5 days. The mortality of the clams was checked on days 5, 10 and 231 after the initial treatment. The results for the field oil spill experiment are recorded as 'mortality' for animals actually found dead, and 'lost' for a category including 'mortality' plus clams disappeared in the next consecutive sampling. Cumulative 'percentage mortality' and 'percentage lost' were then calculated.
Larval settlement experiments Plastic petri dishes (diameter 100 mm, depth 25 ram) were filled with sediments collected approximately 16cm below surface at Site A (to reduce the chance of collecting newly settled larvae). These petri dishes were colour-coded, to differentiate different treatments, and had 5 holes (diameter 6 ram) drilled in the bottom to allow for percolation of test medium. The following test media were prepared in outdoor weathering tanks as described earlier: (1) I 0 ppm oil; (2) 100 ppm oil; (3) 1000ppm oil; (4) 10ppm oil plus 1 ppm Corexit 9527; (5) 100ppm oil plus 10ppm Corexit 9527; (6) 1000 ppm oil plus ! 00 ppm Corexit 9527; and (7) sea water control.
296
E . B . HARTWICK, R. S. S. WU, D. B. PARKER
The sediment filled petri dishes were placed in plastic trays (40 x 35 cm), to which 4 litres of the appropriate freshly prepared medium was added. The test medium was drained from a bottom outlet in the tray after 5 min to let the medium percolate through the sediment; this procedure was repeated five times. Ten replicates of each of the above treatment groups were randomly placed in the intertidal area at both Site A and Site B, and secured with a 117 cm nail through a bottom hole. The petri dishes were exposed for larval settlement from the 19th of June to the 24th of June 1979, when the natural settling of littleneck clam larvae was occurring at Lemmen's Inlet. They were then collected and immediately frozen and transported to the laboratory. The sediment was washed through a 180/~m sieve. Sediment retained was stained with Bengal red and littleneck clam larvae with whole fresh tissue were counted under a dissecting microscope.
Hydrocarbon analysis Preliminary investigations were also carried out to provide some information on the fate of oil and dispersant in the substratum under the field experimental conditions. A volume of 200 litres of the following media were prepared in outdoor weathering tanks: (1) 1000 ppm oil; (2) 1000 ppm oil plus 100 ppm Corexit 9527; and (3) sea water control. A volume of 7 litres of the appropriate medium was administered daily for five consecutive days on 0.5 m: quadrats at Site A, using the same technique as described earlier. Core samples (15 cm long, 2 cm diameter) were taken at 24 h and on the 10th day after the initial treatment and the hydrocarbon profiles of the top 3cm, middle 6cm and bottom 6cm of the core samples were determined. The samples were extracted three times with cyclohexane and made up to a known volume. A volume of extract representing 2 g of dry sediment was injected into the gas chromatograph for hydrocarbon determination. (GC Mode and conditions: Hewlett-Packard 5830A Gas Chromatograph with flame ionisation detector; GC column: 3 ~ dexsi1300 on chromatograph with (AW) 80-100 packing in a stainless steel column; nitrogen flow rate: 30mlmin-1; temperature programmed from 50 ° to 350 ° at 10°C min-l; cyclohexane gave the largest peak at around 1 rain)
RESULTS
Laboratory experiments Cumulative mortalities of the large and small clams generally showed a positive relationship with treatment time and concentration (Table 1). No mortality was observed in the sea water control and upon treatments of 100 ppm oil, 1000 ppm oil and 10 ppm Corexit 9527 (small clams only) throughout the experiment. The LTs0
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297
298
E.B. HARTWICK, R. S. S. WU, D. B. PARKER
values upon the various treatments were interpolated by plotting the probit mortality against log time (Sprague, 1969) and these are also shown in Table 1. The LTso (240 h) was approximately 4.5 days for both size groups in the 1000 ppm oil plus 100 ppm dispersant treatment, and was 5 to 6 days and 2 to 3 days for the small and large groups, respectively, in the 100ppm dispersant treatment. LT~o values were all > l0 in other treatments. In general, the mortality resulting from oil plus dispersant treatment was higher than dispersant or oil treatment alone. Percentages of small and large clams exhibiting siphon activities upon various treatments are shown in Figs. 3 and 4. Percentages of siphon activities were similar in the sea water control for both small and large clams. A Duncan's New Multiple Range test (Li, 1965) showed that the percentage of individuals showing siphon activities was significantly lower (p > 0-05), in both size groups, when treated with 1000 ppm oil plus 100 ppm dispersant, followed by 100 ppm dispersant and then 1000 ppm oil. No significant difference could be found between the siphon activities upon the other treatments and the control, in both size groups. lOO-
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CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
299
LARGE .
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Individuals from the sea water control and 100 ppm oil treatment usually showed a fast siphon retraction resffonse when they were touched. Their shells were also tightly closed during the 5 h exposure period. Individuals from the otlaer treatment groups began to exhibit abnormal behaviour after the second day of treatment. The first observable symptom involved a slow tactile response and a wide gaping of the shell during exposure to air, the next symptom was the tight-closing of their shells, leaving the siphons extended. The tight-closing of the shell eventually led to a complete pinching off of the siphon, followed by death of the animal.
Simulated field oil spill experiments The cumulative percentage mortality and percentage loss of animals in the field experiments are shown in Table 2. Percentage loss generally exhibited a pattern similar to that found for percentage mortality. The cumulative percentage mortality and percentage loss of animals in both size groups (except the percentage mortality of large clams treated with
E. B. HARTWICK, R, S. S. WU, D. B. PARKER
300
TABLE 2 FIELD EXPERIMENTS" CUMULATIVEPERCENTAGE MORTALITYAND PERCENTAGEIA36S(NO. FOUND DEAD PLUS NO. DISAPPEARED)OF Protothaca staminea UPON VARIOUS TREATMENTS
Treatment Control
Size Small
0-5 days ~ mortality
5"0 + 2.8 5-0+2'8 % mortality 0 % loss 0 % mortality 3.3 + 2.3 % loss 5.0 + 2.8 ~ mortality 0 % loss 0 % mortality 28.3+ 5.8" % loss 28.3 + 5.8" % mortality 0 %loss 0 %loss
Large 1000ppm oil
Small Large
1000ppm oil + Small 100ppm Corexit 9527 Large
0-10 days
0-231days
6.8 + 1.8 6.8+i.8 10.0 + 3.9 10.0 + 3.9 8-4 + 3.0 10-0+ 3.0" 0 0 32-5__.3-6" 32.5+ 3-6° 10.5 + 4.0 10.5+4.0
8.3 + 3.6 11.7+4.1 13.3 + 4.4 18-3 + 5-0 15.0 + 4.6* 26.7+ 5.7* 11.7 + 4. I 28-3 + 5-8" 35"0_+6.2" 40.0+ 6.3* 20.0 +__5.2* 31.7+6.0"
° Values significantlydifferent from the respective control at p = 0.05.
1000 ppm oil) was significantly higher than the representative controls. Moreover, the percentage mortality and percentage loss of the small clams in the oil plus dispersant treatment was significantly higher than the oil treatment alone (p > 0.05). Upon treatment of oil or oil plus dispersant, the percentage mortalities of the small clams were significantly higher than those found for the large clams. It is also apparent that, in field experiments, percentage mortalities were always lower than in the equivalent laboratory tests. No abnormal behaviour (e.g. pinched siphon or gaping) was observed in the clams from the control and oil treated quadrats. A few individuals in the oil plus dispersant treated quadrats, however, had pinched siphons and were gaping during low tides. Larval settlement The results of the larval settlement experiments for both Site A and Site B are shown in Table 3. No significant difference (t-test, p = 0.01) could be found in larval settlement between the control and the respective oil-treated dishes at both sites. At site A larval settlement on the dishes treated with 1000 ppm oil plus 100 ppm dispersant was significantly lower compared with that of the control and the other oil plus dispersant treatments. However, an analysis of variance (p =0.01) showed no significant difference between all the treatments at Site B. Hydrocarbon analysis No petroleum products or similar compounds were present in the sediments collected from the study sites and vicinity. Chromatograms of the oil compounds from the core sample taken from the oil treated quadrats 24h after the first
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
301
TABLE 3 THE EFFECTS OF CRUDE OIL AND COlt,EXIT 9527 ON THE LARVAL SETTLEMENT O F Protothaca staminea A T SITE A AND SITE B
Treatment
No. of larvae~dish (Yt + SD) Site A Site B
Control 10 ppm oil 100 ppm oil 1000ppm oil 10ppm oil + 1 ppm dispersant 100ppm oil + 10ppm dispersant 1000ppm oil + 100ppm dispersant
2.9 _+ 1.3 2.5 +_ 2.0 2.5 _+ 1.0 3.0 + 1.3
2.8 _+ 1.5 2.2 _+ 1.0 3-8 _+ 1.5 2.4 + 1.9
3.0 _+ 1.9
4.6 _+ 2.1
3.5 +_2.1
3-2 _+ 2-2
1.8 _+ 1.5°
3.0 _+ 2.5
° Values significantly different from the control at p = 0.01.
treatment are shown in Fig. 5. In comparison with the pure oil sample (Fig. 2), the chromatograms in Fig. 5 show that the light molecular weight fractions were largely absent after a 24 h field exposure. The relative abundance of oil compound also decreased with depth of sediment. No petroleum products were detectable in the sediment profile of the oil treated quadrats after l0 days exposure. Chromatograms of the petroleum compounds from the oil plus dispersant quadrat taken after the first treatment are shown in Fig. 6. Similarly, a decrease in relative abundance of petroleum products with depth is observed. Moreover, the
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Fig. 5. Gas chromato~'ams of the core sediment samples taken from the oil Lre~ted.quadrats, 24 h after the first treatment: (a) top 3cm, (b) middle 6crn and (c) bottom 6cm.
302
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v~ i'
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relative abundance of petroleum compounds is greater in the bottom portion of the core sample for oil plus dispersant, than in the crude oil treated samples. A measureable amount of hydrocarbons was still identified in the oil plus dispersant quadrats 10 days after the treatment (Fig. 7), while no petroleum hydrocarbon was identified in the oil quadrat. N o hydrocarbon compounds could be identified in any sediment sample 231 days after the initial treatment.
a
b
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i 10
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i 30
Fig. 7. Gas chromatograms of the core sediment samples taken from the oil plus dispersant treated quadrats 10 days after the first treatment: (a) top 3cm, (b) middle 6 c m and (c) bottom 6 c m
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
303
DISCUSSION
In the present study, the weathering of the test media and the application procedures in the laboratory and field experiments closely simulated the conditions arising in an actual oil spill, and when Corexit 9527 was used. All of the field experiments were carried out in a typical littleneck clam habitat. The results from the laboratory and field experiments indicate that Alberta crude oil alone, or low concentrations of Corexit 9527 (e.g. < 10 ppm), may not be greatly harmful to Protothaca staminea. In the laboratory experiment no mortality occurred within the first day of any treatment and no mortality was found throughout the experimental period when the clams were treated with 100 ppm oil, 1000ppm oil or 10 ppm Corexit 9527 (small clams only). In the field experiment no significant mortality was found in both size groups when treated with 1000 ppm oil. The present findings therefore contradict those of Avolizi & Nuwayhid (1974), Dow (1975; 1978) and Spooner (1977), in which molluscs were found to be particularly susceptible to hydrocarbons. However, the susceptibility of animals to hydrocarbons may be highly variable within a single taxonomic group (Wu, 1981). It has been well established that dispersants, or oil and dispersant mixtures, may be much more toxic than oil itself (Smith, 1968; Nelson-Smith, 1972). A similar pattern was found in both the laboratory and field experiments in the present study; crude oil had relatively little effect while dispersant or dispersant and oil mixtures had a significant effect on adult P. staminea. Mortality was observed when the clams were treated with 100ppm Corexit 9527 and was highest (in both laboratory and field experiments) when treated with a mixture of 100ppm Corexit 9527 and 1000ppm oil. Such an increase in mortality may be due to an increase in the availability of hydrocarbons when dispersant was used, or an additive toxic effect of the dispersant and the oil. It was also apparent that the percentage mortalities resulting from the field experiments were much lower than those from the equivalent laboratory tests. Such discrepancies demonstrate the difficulty in extrapolating laboratory results to natural spill conditions. In the field experiments percentage loss was calculated in addition to percentage mortality. Loss in a population generally represents emigration and mortality combined. In the study area predators of iittleneck clams included the starfish Pisaster ochraceus, the octopus Octopus dofleini and some shore birds. The difference between the percentage loss and percentage mortality in the field experiments may represent either (1) the emigration of the littleneck clams from the polluted substratum or (2) the loss of clams to predation after the treatment. Unfortunately, these two possibilities cannot be differentiated in the present study. However, in an oil pollution of larger scale (e.g. the whole beach is polluted), emigration of clams may be impossible and correspondingly a higher mortality may be expected. Anderson (1972) found that the filtration rate of the bivalve Mya arenaria
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E . B . HARTWICK, R. S. S. W U , D. B. PARKER
increased in 0.9 ~ Bunker C oil in sea water, but no change of rate was found in 0.9 ~ water soluble fraction of the oil. In this study, the siphon activities of P. staminea were significantly retarded in both size groups when treated with 1000 ppm oil. Siphon activities were further reduced when treated with 100 ppm Corexit 9527, and the lowest siphon activities were found upon treatment with 1000 ppm oil plus 100 ppm Corexit 9527. This indicated that normal respiration and feeding activities of P. staminea may be impaired at these concentrations. One of the most noticeable effects of the oil and dispersant mixtures was the alteration in the behaviour of the clams. The first symptom to appear was slow tactile response, followed by the extension of siphon and gaping of shell during exposure to air. Eventually, siphons were pinched off as shells closed, indicating a loss of coordination of these activities. Such alterations in behaviour, although not directly fatal, may make the animals more vulnerable to natural mortality factors (e.g. predation) in their natural conditions. Swedmark et al. (1973) also found that shell closure of the scallop Pecten opercularis was impaired when treated with 1000ppm crude oil. Ho & Karim (1978) showed that the settlement of barnacles and oysters was severely reduced on oil-coated asbestos. It has been shown that the settlement of larval polychaetes (A renicola marina and Sabellaria spinulosa) were retarded when the substratum was pre-treated with oil and BP 1002 or BP 1100X (Wilson, 1968~ Levell, 1976). Results of the present study indicated that, at both sites, the settlement of P. staminea was not affected on the experimental plots treated with 1000 ppm crude oil. Larval settlement was, however, significantly reduced at Site A when substratum was treated with a mixture of 100ppm crude oil and 100ppm Corexit 9527. However, no significant difference could be found in larval settlement in all the experimental plots at Site B. The inconsistency between the results at the two sites is difficult to explain. It should also be pointed out that larval settlement may not be affected, provided that it occurs at a longer time interval, after a spill since the retention time of petroleum hydrocarbons in the sediment appears to be rather short, as indicated by the gas chromatograms. Scarratt & Zitko (1972) found little decrease in Bunker oil in sediments 26 months after the wreck of the tanker Arrow. Blumer & Sass (1972) also found that oil trapped in sediment degraded slower. In contrast, our results indicated that the volatile fraction of oil was no longer present in the sediment after 24 h exposure in the field conditions and that oil was no longer present in the top of the sediment within 10 days. The petroleum products also showed a decrease in depth. However, a measurable amount of hydrocarbon was still found in the oil plus dispersant plots after 10 days, indicating that the use of Corexit 9527 may increase the retention time of the oil in the substratum. The petroleum products also penetrate deeper into the substratum when Corexit 9527 is applied, and the use of this dispersant in the gravelmud habitat should therefore be avoided. The overall results of our experiments indicate that the impact of oil spill alone on the littleneck clam would be small; a similar conclusion has been reached on marine
CRUDE OIL AND OIL DISPERSANT EFFECTS ON THE LITTLENECK CLAM
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invertebrates in the Eleni V oil spill and the Mexican oil spill (Blackman & Law, 1980; Lee et al., 1980). However, the use of Corexit 9527 as a dispersant in an oil spill in such a habitat may augment the harmful effects to P. staminea in several ways: (1) by increasing the mortality of the adult clams; (2) by reducing the recruitment of larvae and therefore a long term effect on the population; and (3) by increasing the retention time and depth of penetration of the hydrocarbons in the substratum.
ACKNOWLEDGEMENTS
We thank Dr C. D. Levings for providing laboratory space and equipment at the Pacific Environment Institute and Dr G. B. Thompson for reading the manuscript. We are also grateful to D. Tupper of the Environmental Quality Laboratories for performing the hydrocarbon analysis and for her helpful comments. This research was financially supported by the Canadian Environmental Protection Service.
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