Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
–
Page 16
Figure 11: The Sea Empress incident (UK)
Various studies have been carried out to devise toxicity test methods which expose test
organisms in conditions closer to their real environment. Toxicity tests performed with more
realistic “spike
-
exposure” regimes show that th
e use of dispersants does not cause significant
effects at dispersed oil concentrations of lower than 5-10 ppm with embryos and larvae. A level
of 10-40 ppm-hours (concentration in ppm multiplied by exposure in hours) was found to
produce no significant effects on higher marine life, such as older larvae, fish and shellfish.
However, recent studies (e.g. Discobiol), show that:
lethal concentration on adults and juveniles are much higher than the concentration
observed in real incidents;
sub-lethal effects can be observed after the exposure time (bio-accumulation,
metabolites in leaver, stress indicators…); however most of these observed effects are
reversible in a relatively short delay: after 2 weeks of recovery the observed effects
disappeared or reduced close to the background level.
Figure 12: Assessment of the impact of dispersed oil on fish
–
(Discobiol program)
Provided that dispersants are used to disperse oil in water where there is adequate depth and
water exchange to cause adequate dilution, there is little risk of dispersed oil concentrations
reaching levels for prolonged periods that could cause significant effects to most marine
creatures.
Generally speaking, after incidents where large quantities of oil were dispersed at sea (e.g.
“Sea Empress”), the environmental impact, when observed, has been much lower than
expected, and the overall advantages resulting from the use of dispersants confirmed.
Guidelines for the use of dispersants for combating oil pollution at sea in the Mediterranean region
Part II: Basic information on dispersants and their application
–
Page 17
7.2
Microbial Degradation
Dispersion of oil, either mechanically or chemically, renders oil more available to
microorganisms present in the sea water. The influence of dispersants on microbial
degradation of oil is hence of prime importance.
Microorganisms capable to grow on petroleum hydrocarbons are present in all sea waters, and
the rate of microbial degradation is directly related to the degree of oil dispersion. Paraffinic
and high and medium aromatic fractions of oil are biodegradable, while for polyaromatic
hydrocarbons (4, 5 cycles) and asphaltenes it has not been proven beyond doubt. There is no
evidence of biodegradation of polar fractions, nitrogen-, sulphur- and oxygen-containing
compounds.
Dispersants increase the rate of oil biodegradation through:
increasing surface to volume ratio of oil;
increasing oil bioavailability, (reduce the tendency of oil to form tar balls or mousse;
stabilization of oil droplets in the water column instead of beaching or sedimenting).
However, dispersants may also reduce the rate of biodegradation by adding new bacterial
substrate (the dispersant) that may be more attractive to microorganisms than oil or possibly
increasing dispersed oil concentrations in the water column, which may have temporary toxic
or inhibitory effects on the natural microbial populations.
As for toxicity, most of the knowledge of dispersed oil degradation is limited to results of
laboratory or other small scale studies. Some laboratory studies and all mesocosm studies
have shown an increase in rates of oil biodegradation when dispersants are used. Temporary
inhibition of biodegradation with dispersed oil was also recorded in laboratory tests. However, it
appears to occur at higher dispersed oil concentrations than expected in the field. Data from
pond and mesocosm studies strongly indicate that effective use of dispersants would increase
the biodegradation rate of spilled oil. The question whether dispersants enhance the extent of
biodegradation needs to be further studied, although available information suggests that
refractory compounds would not be degraded despite the addition of dispersants.
7.3
Effects on Seabirds and Marine Mammals
Oil affects seabirds and marine mammals due to:
toxic effects of either direct ingestion of oil from the sea surface or indirect ingestion
through grooming or preening;
effects on the water-repellency of feathers or fur needed for thermal insulation.
Reduction of these effects by use of dispersant has not been studied extensively.
Review of available studies did not indicate differences of the toxicity to seabirds of oil
components in chemically and mechanically dispersed oil. However, there is an obvious need
to reduce surface oiling for bird protection. Exposure to dispersants and dispersed oil seems to
be a greater problem than enhanced toxicity of oil.
It is known that marine mammals are affected by exposure to oil. The effects reported include
the dysfunction of physiological processes such as thermoregulation, balancing and swimming
ability as well as impairment of biochemical processes such as enzyme activity. Other overt
effects such as eye irritation and lesions have also been reported. Exposure of marine