Comparative Toxicity of Crude oil, Dispersant and Crude Oil-Plus-Dispersant to Tilapia guineensis
The acute toxicity of Nigeria crude oil (Bonny Light), reference compound (Sodium Dodecyl Sulphate (SDS)), dispersant (Nalco-D4106) and dispersant-plus-crude oil to Tilapia guineensis were studied. Tests were conducted over a 96 h period after acclimatization of individual in the laboratory. There were initial ranges finding test to determine the concentrations of the toxicants to be administered on the test organisms in the definitive tests. The tests were semi-static bioassays in which the exposure media were replaced every 24 h, at which the T. guineensis were also examined for mortality. Tilapia guineensis were exposed to he following concentrations of crude oil (0, 40, 80, 160, 240 and 320 mg L-1), SDS (0, 5, 25, 50, 75 and 100 mg L-1), dispersant (0, 1500, 2000, 4000, 7500 and 10000 mg L-1) and dispersant-plus-crude oil (0, 540, 1080, 2160, 3240 and 4320 mg L-1). The 96 h LC50 of Bonny Light crude oil, SDS, dispersant and dispersant-plus-crude oil were 125.89, 25.12, 3162.28 and 1995.26 mg L-1, respectively. Sodium dodecyl sulphate was the most toxic of the toxicants and the dispersant (Nalco-D4106) reduced the toxicity of Bonny Light crude oil by 16 folds.
Oil spillage as a result of petroleum industry activities and pipe-line vandalization
by saboteurs is a frequent occurrence in Nigeria. There have been over three
thousand eight hundred and fifty four (3,854) reported cases of oil spillage
in Nigeria from 1986 to 2000 (Adeyemi, 2004). These
incidences of oil spillage have had profound negative effects on the aquatic
flora and fauna of the oil-producing areas. Some aquatic species have become
endangered while others have gone into extinction. Apart from this, the socio-economic
lives of the indigenes of the oil-producing areas have been adversely affected,
which have resulted in militancy in Nigeria in the last decade.
One of the methods used in oil spill clean-up is the application of dispersants.
Dispersants are group of chemicals designed to be sprayed onto oil slicks to
accelerate the process of natural dispersion. Chemical dispersants are widely
used on waters in Nigeria during routine cleaning of oil spillage to break-up
the oil into emulsion, which are rapidly diluted by water bodies thereby preventing
recombination of the oil to form a slick (Odiete, 1999).
Spraying dispersants may be the only means of removing oil from the sea surface
particularly when mechanical recovery is not possible.
The use of dispersants to combat oil spills is a controversial decision because
it introduces a new pollutant into the aquatic environment and does not remove
the oil but only disperses it (Lin and Mendelssohn, 1998).
Other opinions are in favour of its use because they remove the oil from the
surface and facilitate degradation and dilution thus, preventing the oil from
coming a shore (Riepsaite and Stankevicius, 2005).
Dispersant toxicity to aquatic organisms has been well documented since they
were first developed as a spill response technology (Hall
et al., 1989). Dispersants are intended to enhance oil mobilization
into water column, thus, potentially resulting in increased biological stresses
in affected areas because of elevated oil concentrations (Fuller
et al., 2004). Supporting laboratory studies have shown that surfactants
can increase crude oil concentrations in the water column, thereby increasing
the observed toxicity compared to no-surfactant controls (Kanga
et al., 1997). However, a lot of studies have also shown that application
of dispersant reduces the toxicity of crude oil (Singer
et al., 1991; Fuller et al., 1999,
2004; Page et al., 2000, 2002;
Harris et al., 2002; Mueller
et al., 2003).
The explicit consideration of toxicological effects resulting from dispersant
use is not a straightforward task. In fact, it can be quite confusing and confounding
to decision-making process. The bottom line of toxicity to an ecosystem or a
specific living resource is a function of at least five components: the dispersant,
the oil being dispersed, the nature of the exposure, the organism in question
and life stage of the organism in question. The combination of these factors,
as well as others that may be relevant in specific situations, will determine
the ultimate impact on resources (Singer et al.,
The objectives of this study was to investigate the toxicity of Bonny Light
crude oil, dispersant (Nalco-D4106) and a mixture of Bonny Light crude oil-plus-dispersant
(Nalco-D4106) on Tilapia sdguineensis by the determination of median
lethal concentration (96 h LC50), median lethal time (LT50),
toxicity factor of dispersant and the synergistic or joint action factor of
oil-plus-dispersant among other variables.
MATERIALS AND METHODS
Experimental Set-up and Fish Management
Acute toxicity tests were conducted to compare the toxicities of a crude
oil (Bonny Light), an oil dispersant (Nalco-D4106), a reference compound (Sodium
dodecyl sulphate) and dispersant-plus-oil mixture on Tilapia guineensis.
The bioassay was carried out in the Department of Wildlife and Fisheries Management,
University of Ibadan, Nigeria in 2007. The method of bioassay employed was the
one outlined by APHA (1998).
Acclimatization of Test Organisms
The fish (Tilapia guineensis) were collected from Lagos Lagoon, Lagos,
Nigeria in thick transparent polyethylene bags with artificial oxygen to sustain
the fish during transportation to the laboratory. The fish were held in glass
tanks (60x30x45 cm3) for 14 days prior to the start of the experiment.
Each acclimatization tank had the habitat water from where the fish were collected.
The habitat water in the tanks was replaced every 2 days. During acclimatization,
the temperature was maintained at 29+2°C while aeration was continued throughout
the period with aquarium pumps. The photo-period was 12 h light and 12 h darkness.
The fish were fed with commercial pelleted fish feed (40% protein) at 3% body
weight ad libitum (Odiete, 1999).
Preparation of Test Medium and Application of Test Chemicals
A preliminary (range-finding) test as described by Solbe
(1995) and Rahman et al. (2002) was conducted
to determine the main experimental concentrations for the crude oil (Bonny Light),
dispersant (Nalco-D4106), reference compound (Sodium dodecyl sulphate) and dispersant-plus-crude
oil. The main experimental concentrations for the toxicants (crude oil, dispersant,
reference compound and dispersant-plus-crude oil) above were determined based
on 0-100% mortality of Tilapia guineensis in 24 h.
The second stage of the experiment gives details of the main experimental
concentrations for the toxicants as described by Solbe (1995)
and Daka and Ekweozor (2004).
Crude Oil (Bonny Light) against T. guineensis
The followings were set up: 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8 mL crude
oil (Specific gravity: 0.7942 g mL-1) in 2 L of Lagos Lagoon water
corresponding to 0.0, 40.0, 80.0, 160.0, 240.0 and 320.0 mg crude oil, respectively
per liter of water.
Dispersant (Nalco-D4106) against T. guineensis
The following were set up: 0.0, 3.0, 4.0, 8.0, 15.0 and 20.0 mL of dispersant
in 2 L of Lagos Lagoon water corresponding to 0, 1500, 2000, 4000, 7500 and
10,000 mg of dispersant, respectively per liter of water.
Sodium Dodecyl Sulphate (SDS) against T. guineensis
The followings were set up: 0.0, 10.0, 50.0, 100.0, 150.0 and 200.0
mg SDS in 2 L of Lagos Lagoon water corresponding to 0.0, 5.0, 25.0, 50.0, 75.0
and 100.0 mg SDS, respectively per liter of water.
Crude Oil (Bonny Light)-Plus-Dispersant (Nalco-D4106) against T. guineensis
The followings were set up: 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8 mL Bonny
Light crude oil and 0.0, 1.0, 2.0, 4.0, 6.0 and 8.0 mL dispersant in 2 L of
Lagos Lagoon water corresponding to a combined concentration of 0.0, 540.0,
1080.0, 2160.0, 3240.0 and 4320.0 mg crude oil-plus- dispersant, respectively
per liter of water.
Selection of Organisms (T. guineensis) for the Bioassay
Crude Oil (Bonny Light)
One hundred and twenty (120) fingerlings of T. guineensis of mean
length 7.50±1.30 cm and mean weight 12.17±1.55 g were randomly
assigned in equal number (20) into six test tanks (60x30x45 cm3)
separately, containing the following concentration of the crude oil 0.0 Mg L
-1 (control), 40.0, 80.0, 160.0, 240.0 and 320.0 mg L-1.
Each of these experimental units were replicated thrice to give a total of 18
experimental units (test tanks) containing 360 fingerlings of T. guineensis.
The control (0.0 mg L-1) contained only 20 fingerlings of T. guineensis from Lagos Lagoon without the test toxicant (Bonny Light crude oil). During the bioassay, the test solution in each tank was renewed every 24 h.
Dispersant (Nalco-D4106), Sodium Dodecyl Sulphate and Crude Oil-Plus-Dispersant
A similar experiment as the one described above was set up for dispersant,
sodium dodecyl sulphate and crude oil-plus-dispersant using the concentrations
determined after the preliminary tests (Experiment 1). Six experimental units
were each set up for the toxicants (dispersant, sodium dodecyl sulphate and
crude oil-plus-dispersant). These were the concentration of the toxicants: 0
mg L-1 (Control) and 1500, 2000, 4000, 7500 and 10000 mg L-1
for dispersant; 0.0 mg L-1 (control), 5, 25, 50, 75 and 100 mg L-1
for sodium dodecyl sulphate and 0 mg L-1 (control), 540, 1080, 2160,
3240 and 4320 mg L-1 for the crude oil-plus-dispersant. Each of these
experimental units was replicated thrice to give a total of 54 experimental
units containing 1,080 fingerlings of T. guineensis. The controls (0
mg L-1) contained 20 fingerlings of T. guineensis without
the toxicant (dispersant, sodium dodecyl sulphate or crude oil-plus-dispersant).
In all, there was 72 experimental units containing 1440 fingerlings of T.
Monofilament nettings were used to cover the tanks to prevent the specimen
from jumping out of test solutions. The behavior of specimens was observed and
death was recorded for the 24, 48, 72 and 96 h test periods. Death was defined
as complete immobility with no flexion of the abdomen upon forced extensions
Each test concentration was converted into a logarithm and the corresponding
percentage (%) mortality was transformed into probit (Sprague,
1969). The median lethal toxicity (LC50), median lethal time
(LT50), minimum lethal concentration and minimum lethal time were
determined according to the method described by Finney (1971).
Analysis of Variance (ANOAV) was used to test for significant differences in
the number of survivors in the concentrations of the test toxicants (crude oil,
dispersant, sodium dodecyl sulphate and dispersant-plus-crude oil).
The toxicity factor for dispersant and synergistic or joint action factor was
determined using the formula described by Odiete (1999).
The results of the 96 h median lethal concentrations (96 h LC50) obtained for the toxicants (Dispersant (Nalco-D4106), Reference compound (sodium dodecyl sulphate), crude oil (Bonny Light) and a mixture of Dispersant (Nalco-D4106)-plus-crude oil (Bonny Light) is shown in Fig. 1-4 while, the LC50 values for the toxicants is shown in Table 1. There were very strong and positive correlations between log concentration of crude oil and probit mortality as shown by the values of regression analysis in Fig. 1 (r2 = 0.80, N = 5, α = 0.05), Fig. 2 (r2 = 0.86, N = 5, α = 0.05), Fig. 3 (r2 = 0.98, N = 5, α = 0.05) and Fig. 4 (r2 = 0.88, N = 5, α = 0.05). Coefficient of determination (r2) varied from the value (r2 = 0.80, N = 5, α = 0.05) to (r2 = 0.98 N = 5 α = 0.05) which shows that 80-98% of the association is dependent on the variable (Log concentration and probit mortality) for Tilapia guineensis.
Table 2 shows the median lethal time (LT50) of
the toxicants to T. guineensis within a 96 h period. The values were
obtained by plotting probit mortality against log time for each of the concentration
(Odiete, 1999). The highest (61 h) LT50 for
the dispersant (Nalco-D4106) was obtained at a concentration of 2000 mg L-1
while the lowest (24 h) occurred at 10000 mg L-1.
96 h LC50 of dispersant (Nalco-d4106) to Tilapia guineensis
96 h LC50 of reference compound (sodium dodecyl sulphate) to
96 h LC50 of crude oil (Bonny Light) to Tilapia guineensis
96 h LC50 of dispersant (Nalco-d4106)-plus-crude oil (Bonny
Light) to Tilapia guineensis
||96 h LC50 Toxicants to Tilipia guineensis
||Median Lethal Time (LT50) of toxicants to Tilapia
concentration and minimum time of toxicants to Tilapia guineensis
of fish (T. guineensis) exposed to different concentration (mg
L-1) of toxicants
Ninety six hours was the highest LT50 value obtained for the reference
compound (sodium dodecy sulphate) at a concentration of 50 mg L-1
while, 30 h was the lowest and it occurred at a concentration of 100 mg L-1.
The highest LT50 values for Bonny Light crude oil (90 h) and a mixture
of dispersant (Nalco-D4106) and Bonny Light crude oil (90 h) occurred at concentrations
of 160 and 3240 mg L-1, respectively, while, the lowest values were
49 h and 78 h and they occurred at 320 and 4320 mg L-1, respectively.
Table 3 shows the minimum concentration of each toxicant
that can cause death of T. guineensis and the minimum time that these
concentrations can cause death. These values were obtained by plotting median
lethal time (LT50) values of each toxicant against the corresponding
log of concentration (Table 2) (Odiete,
1999). The values obtained showed that the toxicant that can cause death
of T. guineensis within the shortest time among the toxicants investigated
is the dispersant (Nalco-D4106) and this occurred in 24 h while Bonny Light
crude oil had the highest minimal time (90 h) to cause death of T. guineensis.
The reference compound (sodium dodecyl sulphate) had the least minimal concentration
(50.12 mg L-1) that could cause the death of T. guineensis
while the mixture of dispersant (Nalco-D4106) and Bonny Light crude oil had
the highest minimal concentration (3235.93 mg L-1) that can cause
death of T. guineensis.
The results of the survivors of T. guineensis exposed to different concentrations of the toxicants are shown in Table 4. The number of survivors in each concentration of the dispersant (Nalco-D4106) is significantly (p<0.05) different from the others. A similar result was obtained for the other toxicants; sodium dodecyl sulphate, Bonny Light crude oil and dispersant (Nalco-D4106) (Table 4).
The toxicity factor of the dispersant (Nalco-D4106) to T. guineensis was 125.89 while the synergistic or joint action factor of the mixture of the dispersant (Nalco-D4106) and Bonny Light crude oil to T. guineensis was 15.85.
The 96 h median lethal concentration (LC50) of the investigated
toxicants to T. guineensis revealed that the most toxic of the toxicants
was the reference compound (sodium dodecyl sulphate (SDS)} with a 96 h LC50
value of 25.12 mg L-1. This value agrees with the 96 h LC50
value (22.5 mg L-1) obtained in young Pimephales promelas
by Newsome (1982). However, the above value (that is,
96 h LC50 of SDS on T. guineensis) is lower than the 96 h
LC50 of Bonny Light crude oil (125. 89 mg L-1) on T.
guineensis recorded in this study (Table 1).
The results of the 96 h LC50 of Bonny Light crude oil (125.89 mg
L-1) recorded in this study is similar to the values obtained in
previous study by Daka and Ekweozor (2004). The study
was on the effect of size on the acute toxicity of Nigerian crude oil (Egbogoro
Liner II) to the mangrove oyster (Crassostrea gasar) using semi-static
renewal bioassay (Reish and Oshida, 1987). The LC50
value obtained was 135 mg L-1 for small size (10-30 mm) oyster.
However, Akbari et al. (2004) use a static condition
to test the acute toxicity of Malaysian crude oil on seabass fry under tropical
conditions. The results showed that the 96 h LC50 of Water-Soluble
Fractions (WSF) of crude oil to seabass was 23.1 mg L-1. The difference
in median lethal concentrations obtained in these studies might be due to the
method of bioassay employed and the type of crude oil administered (Vanderhorst
et al., 1976; Fuller et al., 2004).
The results of median lethal concentration (96 h LC50) of the Bonny
Light crude oil (125.89 mg L-1) and the dispersant (Nalco-D4106)
(3162.28 mg L-1) show that the Bonny Light crude oil is more toxic
to T. guineensis than the dispersant (Nalco-D4106). This is not in agreement
with the study of Otitoloju and Popoola (2009) which
reported that Biosolve (a dispersant) was about 27, 284 times more toxic than
crude oil while another dispersant (OSD 9460) was just about 4 times more toxic
than crude oil when acting alone against Clarias gariepinus. The reason
for the variation in the toxicity of the dispersant might be due to their chemical
The mixture of the dispersant (Nalco-D4106) and crude oil (Bonny Light) was
not as toxic as the reference compound (SDS) and the crude oil (Bonny Light)
only but was more toxic than the dispersant only. The median lethal concentrations
of the dispersant-plus-crude oil and crude oil only were 1995.26 and 3162.28
mg L-1, respectively. This result is in agreement with the study
carried out by Fuller et al. (2004). In this
study, the toxicity of Arabian medium crude oil, dispersant and oil-plus- dispersant
to four organisms was carried out. The organisms were two fish species (Cyprinodon
variegatus and Menidia beryllina), one shrimp species (Americamysis
bahia) and a luminescent bacteria (Vibrio fisheri). The results indicated
that the 96 h LC50 of the dispersant only was higher than the mixture
of dispersant-plus-oil for the four organisms indicating that the mixture of
Arabian medium crude oil-plus-dispersant was more toxic than the dispersant
only. However, the study carried out by Kanga et al.
(1997) showed that dispersants increases the toxicity of crude oil to aquatic
The number of organisms that survived in each concentration of the dispersant (Nalco-D4106) differ significantly (p<0.05). A similar result was also obtained for the reference compound (SDS), crude oil (Bonny Light) and the mixture of dispersant-plus-crude oil.
The toxicity factor of the dispersant which is a measure of how much more or less toxic a dispersant is to the test organism than the reference compound was 125.89. It Indicates that the reference compound (SDS) is about 126 times were toxic than the dispersant (Nalco-D4106). The synergistic or joint action factor of the mixture of dispersant-plus-crude oil which indicates how much more (potentiation) or less (reduction) toxic the dispersant-plus- crude oil is to the test organism than the crude oil was 15.85. This shows that crude oil only is about 16 times more toxic than the dispersant-plus-crude oil mixture. The implication is that the dispersant (Nalco-D4106) was able to reduce the toxicity of crude oil to T. guineensis by 16 folds.
Although, the use of dispersants in oil spill clean up have been discouraged because of their adverse effects on aquatic organisms. However, the dispersant (Nalco-D4106) investigated in this study appears to reduce the toxicity of crude oil to T. guineensis. While scientists are seeking biological remedies to oil spillage, relatively low toxic dispersants like Nalco-D4106 might still be used if research shows that they do not have serious and profound effects on that aquatic ecosystem. Ultimately, the use of dispersants will be discontinued when biological remedial technique like bioremediation and phytoremediation are fully adaptive in real case scenario.
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