Burger found a statistically significant but. FIGURE Summary of the effects on mammals of exposure to an inhalation of various petroleum vapors, principally those of gasoline from Geraci and St. Aubin, , Academic Press. In another analysis of 98 spills, there was no correlation Dagmar Etkin, Environmental Research Consulting, personal communication. Large spills that occur over the deep ocean in open water that has little bird life will have a lesser effect on seabirds than a small spill in a critical habitat where high numbers of birds are aggregated on the water. The season in which a spill occurs is also critical Hunt, If the spill occurs when birds are aggregated during breeding or migration, the impact will be much greater than if they are widely dispersed at sea.
Recent evidence, however, suggests that the incidence of seabird mortality from small spills may be declining in the North Sea region Camphuysen, This shift may reflect the reduction of small discharges of oil noted earlier in Chapter 3. Burger , however, found that there was no justification for this assumption; the mean estimate for 21 spills for which the number of birds found dead and that the overall mortality were determined to be between four and five times the number of birds actually counted.
The relationship among the number of carcasses recovered and the total mortality will vary between species, being dependent among other factors on body size, average distance to shore carcass buoyancy, and the prevailing winds during and after the spill. Few studies of oil spills have provided the information necessary to assess delayed or long-term effects on community-level processes in the affected communities or ecosystems.
Where keystone predators are removed, competitive dominants at the next lower trophic level can increase and change the structure of the community. For example, in areas of Prince William Sound where sea otters were removed by oiling, their preferred prey, sea urchins, have increased in some locations Peterson, Elsewhere in Alaska where sea urchin populations have been able to increase in the absence of sea otters, severe damage to kelp beds have resulted Estes, ; Estes and Duggins, In addition to the strong evidence for the impact of massive contamination associated with an oil spill, there is increasing evidence that chronic, low-level exposures to hydrocarbons in the sea can have a significant effect on the survival and reproductive performance of seabirds and some marine mammals.
Sublethal effects of oil on seabirds include reduced reproductive success, and physiological impairment, including increased vulnerability to stress reviewed in Fry and Addiego, ,; Hunt, ; Briggs et al. In contrast, in marine mammals, sublethal exposure to petroleum hydrocarbons has been shown to cause minimal damage to pinnipeds and cetaceans e. Aubin, b , although sea otters appear to be more sensitive Geraci and Williams, ; Monson et al. Because both marine birds and marine mammals have the enzymes necessary for the detoxification and elimination of petroleum hydrocarbons, parent compounds of petroleum hydrocarbons are not accumulated and sequestered in tissues as chlorinated hydrocarbons.
Toxic metabolites produced by metabolism of polycyclic aromatic hydrocarbons, however, may accumulate and induce toxic effects Brunstrom et al. Chronic pathologies would not be expected once oil ceased to be ingested. There is mixed evidence that oil pollution can have demonstrable effects on the population trajectories of marine birds and mammals.
Spilled oil has had and still poses a potentially devastating effect on African Penguins Spheniscus demersus in southern Africa Westphal and Rowan, ; Vermeer and Vermeer, ; Clark, ; Dagmar Etkin, Environmental Research Consulting, personal communication. In northern Europe between and , percent of beached alcids were oiled Camphuysen, , and Hudson found that oiling accounted for between 18 and 28 percent of mortality of banded alcids.
In addition, there is conflicting sentiment that populations of sea ducks in the Baltic declined as a result of oil pollution Lemmetyinen, ; Vermeer and Vermeer, ; Clark, ; vs. Joensen and Hansen, In the cases of two of the largest recent spills, the Exxon Valdez spill in Prince William Sound, Alaska and the Persian Gulf War Box release of oil in the northern Arabian Sea, the population-level impacts on seabirds are not clear. There has been considerable variability in the estimates of the number of seabirds killed in the Exxon Valdez oil spill, which has led to much contention e.
Piatt et al. Even more difficult has been the determination of population-level impacts. Irons obtained evidence of lower rates of production of young in the surface-foraging black-legged kittiwake Rissa tridactyla , but this did not translate into a decrease in the size of colonies in Prince William Sound, or even in the oiled portion of the Sound.
The Common Murre Urea aalge was the species that sustained the highest mortality Piatt and Anderson, ; Piatt and Ford, , and it might have been expected that a population-level effect of this mortality. Over a period of about four months from January-March , crude oil was released into the Arabian Gulf from five tankers, a major tank field, and several offshore terminals, refineries, and battle-damaged tankers as part of the Iraq-Kuwait conflict.
Though the actual volume of release will never be known, the best estimate is about 1,, tonnes ,, gallons Tawfiq and Olsen, , making it the largest oil spill in history and three times as large as the next largest spill the Ixtoc well blowout in the Gulf of Mexico. Although the massive slicks were initially predicted to spread throughout the Arabian Gulf and out through the Gulf of Hormuz, a seasonal shift in wind patterns held the bulk of the oil along the shoreline between the Kuwait border and Abu Ali Island near Al Jubail, a distance of about km.
There was concern that a significant portion of the unaccounted for oil sank; however, Michel et al. None of the researchers studying the Arabian Gulf after the spill reported large-scale oil contamination of bottom sediments Price and Robinson, The spill significantly affected shoreline habitats, with km of shoreline oiled in Saudi Arabia alone, including km of marshes Gundlach et al.
Very little shoreline cleanup was attempted. An estimated percent of the intertidal biota were killed Jones et al. Followup shoreline surveys in and showed that the stranded oil had penetrated up to 40 cm into the sediments, with liquid oil filling burrows in muddy sediments Hayes et al. The heavy surface oiling formed persistent pavements along the upper intertidal zone and on the tops of mid-tide bars that showed little evidence of erosion six years after the spill.
The surface pavements slowed the rate of subsurface oil weathering and physical removal, effectively sealing the subsurface oil in place. Intertidal species diversity in the lower intertidal zone on sandy and muddy substrates was percent of controls by , whereas in the upper intertidal zone, species density and density was percent of unpolluted sites Jones et al. As of , there was little evidence of recovery of heavily oiled marshes. Much of the heavily oiled shoreline occurred along sheltered bays with little exposure to waves and currents.
Thus, natural removal of the stranded oil will be very slow, and full recovery of intertidal communities will likely require decades. Amazingly, no significant long-term impacts to subtidal habitats and communities were observed, including seagrass beds, coral patch and fringing reefs, unvegetated sandy and silty substrates, and rocky outcrops Kenworthy et al.
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Kuwait crude forms a very stable emulsion that resulted in thick surface slicks that stranded onshore rather than mixed into the water column. Impacts to shrimp stocks, however, were severe; in spawning biomass dropped to 1 percent and total biomass dropped to 27 percent of pre-war levels Matthews et al. Causes of this collapse were attributed to a combination of mass mortality of eggs, larvae, and postlarvae resulting from oil exposure during the entire spawning season, emigration of adults out of the oiled areas, mortality of adults, heavy fishing of adults and juveniles thus further reducing the spawning biomass, and decrease in water temperatures and light intensity because of oil fires smoke and haze.
At least 30, seabirds are estimated to have died as a result of the spill. Although the oil spill killed an estimated 25 percent of the Saudi Arabian breeding population of the endemic Socotra cormorant, these colonies tripled in population by Symens and Werner, Internationally important breeding tern populations in Saudi Arabia and Kuwait escaped direct oiling impacts in 70, pairs breed on offshore islands in summer , but severe declines in breeding success in and resulted from an acute shortage of food that was attributed to the oil impacts on fish recruitment Symens and Alsuhaibany, In , breeding success was high.
During the spill, shorebird populations were reduced by up to 97 percent; however, it is not known whether the birds avoided the noxious oil or were driven away by a lack of food and found good feeding areas elsewhere, became oiled and died, or died from starvation Evans et al. The greatest shorebird impacts, however, were likely the indirect effects of long-term degradation of intertidal habitats and the loss of their food supply.
Natural variability and the precision of population estimates, however, complicated the determination of impact to Common Murres, and it remained impossible to assign, with certainty, the population-level effects of the spill in this species Boersma et al. Erikson also reported no evidence of depressed numbers of murres attending colonies in , as compared to historic data.
A lack of up-to-date monitoring in the murre colonies prior to the spill exacerbated the difficulties attendant on determining the effects of the spill. In other species, there was little evidence of significant population-level damage from the spill Kuletz, ; Oakley and Kuletz, ; Sharp et al. Controversy as to the magnitude and duration of the effects of the spill is ongoing e. In addition, some studies have argued that other sources of PAH in both the east and west Prince William Sound, including vessel traffic and PAH from coal and possibly from oil seeps further south in the Gulf of Alaska, may play a role Page et al.
There has also been consid. Some studies found that, within two years of the spill 23 of 42 species showed no evidence of negative impacts from the spill e. Some of these differences reflect methodologies used, whereas others appear to be matters of interpretation Day et al. Effects differed between avian species that were apparently chronically exposed to oil residues through their epibenthic prey.
Trust et al. A demographic model suggested that the differences in over-winter survival between oiled and unoiled areas was sufficient to account for continued declines in the populations of Harlequin Ducks in the oiled areas. These effects reflect loss of individuals from habitually used wintering or foraging sites. Wiens et al. However, all species recovered at the same rate, so community composition was affected over time, though the consequences of these effects are unknown. It is also less than clear that the immense discharges of petroleum into the marine environment during the Persian Gulf War in had a lasting effect on the populations of seabirds breeding in the northern Arabian Gulf Case History For example, during the war, an estimated 8, to 10, Socotra Cormorants Phalacrocorax nigrogularis were killed, approximately 50 percent of the Saudi Arabian population Symens and Werner, As of , the population had rebounded to 30, pairs, suggesting that the losses to oil during the war had little population-level effect, except possibly in slowing the rate of post-war increase.
In contrast, four species of terns nesting on the offshore islands of the northern Gulf of Arabia showed little evidence of oiling during Although about 1 percent of the total adult tern population was moderately to heavily. Cormorants, black-legged kittiwakes, murres, pigeon guillemot, mergansers, and loon. Oiling apparently occurred when terns encountered tar balls while plunge diving in pursuit of small fish. Small spots of oil transferred from adults to eggs caused no decline in hatching success Symens and Alsuhaibany, There was evidence that the oil spilled during the Persian Gulf War had an indirect effect on tern reproductive success.
The clutch sizes of the White-cheeked Tern Sterna repressa were reduced in and , and the breeding success chicks per pair of Lesser Crested Terns Sterna bengalensis , White-cheeked Terns and Bridled Terns Sterna anaethetus were less in and than those in either or This decline in and was apparently caused by a lack of small fish on which to forage. Exposure to the massive spills during the Persian Gulf War significantly reduced the abundance of fish eggs and larvae McCain and Hassan, ; Symens and Alsuhaibany suggest that this mortality of forage fishes resulted in a scarcity of fish prey for the terns in and In those two years, the diets of the terns shifted, and one of the larger species, the Swift Tern Sterna bergii , resorted to eating the chicks of the smaller White-cheeked Terns, and stealing food from Lesser Crested Terns returning to their colonies.
Although this example shows effects of an oil spill on the reproductive ecology of marine birds up to two years after the spill, Symens and Alsuhaibany suggested that this two-year interruption would have a negligible effect on the population biology of these long-lived seabirds. Among marine mammals, river otters Lutra lutra in the British Isles and Alaska, and sea otters Enhydra lutris and harbor seals Phoca vitulina in Prince William Sound, Alaska, all showed short-term population declines after oiling of their inshore marine habitats Baker et al.
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For some species, these effects may have persisted over ten years e. However, in the case of the Exxon Valdez oil spill in Prince William Sound, Alaska, considerable controversy remains concerning the magnitude of the initial losses and the duration of population-level effects e. These uncertainties stem from the lack of sufficient pre-spill data to characterize the population status of these species and difficulties in obtaining adequate post-spill data to distinguish between local movements of animals and area-wide population effects. Chronic or delayed responses of marine bird and mammal populations to petroleum hydrocarbons in the sea can occur because of continued ingestion of oil via contaminated prey, or because of failure of prey populations to recover subsequent to injury.
In the 10 years since the Exxon Valdez oil spill, several species of birds and marine mammals have demonstrated indirect or delayed responses to the spill. These responses were found in sea ducks and shorebirds, species. PHOTO 23 Spills from coastal facilities such as marine terminals and tank farms make up nearly one quarter of the spills by volume associated with the transportation of petroleum. Photo courtesy of Environmental Research Consulting. Seabird responses were of three types: reduced use of oiled habitats as compared to use of unoiled habitats for up to nine years post-spill, reduced numbers post-spill as compared to pre-spill, and lower growth and delayed fledging in a species that fed contaminated mussels to its young.
Marine mammal populations that may have exhibited prolonged effects subsequent to an oil spill include sea otter and harbor seal populations in Prince William Sound Garshelis and Johnson, ; Hoover-Miller et al. In some regions of the Prince William Sound, sea otter abundance had not recovered as of Dean et al.
Other results indicating damage to otters that persisted for more than several years include the finding that the over-wintering mortality of juveniles was higher at oiled as compared to unoiled sites in the winters of and Monson et al. The presumed cause of the failure of the sea otter population to recover is continued contamination via their prey. Some of the measured increases or decreases in sea otter populations may have resulted from local movements of otters or other behavioral or demographic phenomena, and assessment of long-term population effects of the oil spill to sea otters remains difficult and controversial.
In the case of harbor seals, there is some controversy as to whether they have declined in Prince William Sound Peterson, , or whether the apparent declines are the result of movement of seals that were avoiding or moving away from oil contaminated haulout Hoover-Miller et al. Harbor seals were declining before the spill, and if there has been a continued decline, it may be a continuation of the past decline, or it may be the result of a decrease in the abundance of near-shore fish prey, but the available evidence is inconclusive Peterson What is important here is that sub-lethal effects can be identified in marine birds and mammals for several years after the acute effects of a spill have passed.
In summary, it has proven difficult, except in a few notable exceptions, to demonstrate population-level effects of oil spills for either marine birds or marine mammals based on censuses. Although many individuals may be killed, it is frequently difficult to demonstrate commensurate declines in local or regional populations, or to show significant demographic effects, because the power of present techniques to detect change is weak.
Without more complete knowledge of the structure of populations of marine birds and mammals and their demography, it may remain beyond our reach to assign damage or recovery except in cases where ongoing monitoring provides an adequate basis for comparative studies. The temporal and spatial variability found in ecosystems makes even the most sophisticated statistical approaches open to individual interpretation and controversy Wiens and Parker, ; Day et al.
As both Nisbet and Piatt and Anderson point out, even though we often cannot demonstrate statistically that oil pollution has caused population-level effects in marine birds, given what we know of their life history patterns, including long life spans, low adult mortality, and low rates of reproduction, it is risky to assume that increased rates of mortality are without population-level effect.
Total population size, including breeders and non-breeders, has not been determined for any seabird species, and thus it is impossible to determine directly whether pollution is affecting global populations Nisbet, Only if the effects of oil pollution are compensatory and not additive to other, natural, causes of mortality can we hope that large removals of individuals are without population-level consequences.
The same arguments should hold true for marine mammals. Modeling has been used in many ways to assess the impacts of oil spills on living resources and habitats:. To evaluate the impacts of an oil spill using a model, the fate of the oil must first be quantified. Surface trajectory models are used to calculate the intersection of the trajectory path with maps of resources of concern including biota and habitats e.
This approach is appropriate for quantification of impacts to birds, mammals, and shoreline habitats. Bird and mammal impacts have also been modeled by backtracking from locations where oiled animals have stranded on beaches, accounting for losses at sea Seip et al. To evaluate the effects of subsurface oil, subsurface oil must be explicitly tracked French et al.
A prime example is the North Cape oil spill of January that occurred during a severe winter storm French, a,b,c Case History, To evaluate impacts on aquatic biota, oil entrainment and dissolution into the water must be simulated. The relevant concentrations are of those components that might have an impact on aquatic organisms and habitats. The concentrations of main concern are the soluble and volatile lower- and intermediate-molecular-weight compounds that are acutely toxic to biota, primarily monoaromatic hydrocarbons MAHs and polycyclic aromatic hydrocarbons PAH French, ; French McCay, Other compounds in oil may also contribute to toxicity.
Submerged oil and oil smothering on shorelines are also important exposure pathways. Thus, the model must consider the entire fate of the oil and all its components over time, both on and in the water, and in sediments. Oil trajectory models have been used to determine where oil will intersect the shoreline and impacts are presumed if oil reaches a location.
The problem with this approach is that impacts are related to the amount and weathering state of the oil. Thus, this simply identifies areas that might be exposed to some amount of oil, but does not quantify an impact. Computer models of the physical fates, biological effects, and economic damages resulting from releases of oil and. The NRDA models simulate spreading and shoreline stranding of oil. The amount of oil remaining on the shoreline is a function of oil viscosity and shoreline type. Stranded oil is assumed to be removed by waves and other physical processes at a constant rate.
The holding capacities and removal rates are based on data collected after spills. Impacts on intertidal habitats, such as salt marshes, mangroves, macroalgal beds and coral reefs, are assumed to be a percent loss if a threshold thickness dose is exceeded for any interval of time. The threshold is based on observational data for salt marsh impacts French et al. Wildlife birds, mammals, and reptiles are primarily impacted by direct exposure to floating oil, ingestion of contaminated prey or depletion of food resources.
Models used to assess impacts of oil on wildlife populations are summarized in Table In evaluating the wildlife impacts of the Exxon Valdez , Ford et al. Oiled and dead birds are scavenged and may sink at sea. The percent stranded is related to the trajectory of the carcasses. Ford et al.
Wildlife are assumed to move randomly within the habitats they normally use for foraging. The dose is estimated from the oil thickness, path length through the oil, and the width of a swimming bird. A portion of wildlife in the area swept by the slick is assumed to die based on the probability of encounter with the slick, dosage, and mortality once oiled. Estimates for these probabilities are derived from information on behavior and field observations of mortality after oil spills.
French and Rines performed hind-casts on 27 oil spills to validate the wildlife impact model. The results showed that the model is capable of hind-casting the oil trajectory and shoreline oiling, given 1 accurate observed wind data following the spill, and 2 a reasonable depiction of surface currents. Since winds and currents are the primary forcing variables on oil fate, obtaining accurate data on these is very important to the accuracy of any simulation. The accuracy of the impact model is primarily dependent on the accuracy of the wildlife abundance data for the time and location of the event.
In the validation study, regional mean abundances from literature sources were assumed. In nearly all cases, impact information for a spill consists primarily of counts of rescued or dead wildlife. Model validation is necessary to illustrate where the model predicts reasonable estimates of impacts on wildlife. Modeling results show that the wildlife impact algorithm in the model is valid when input data on abundance are accurate French and Rines, In a few cases, the model estimated more birds killed than were observed. These cases were for species impacts not normally assessed or reported.
Even in cases where large efforts were made to recover oiled wildlife, such as following the Exxon Valdez , it is well recognized that many oiled animals are lost at sea or scavenged and not counted directly as oiled. Small and less visible species and. Sea birds and marine mammals—Oil slick encounter and subsequent mortality. Fur seal model—Simulated population processes and mortality due to oiling. Sea birds—Estimate numbers oiled from strandings of oiled animals on beaches. Exxon Valdez —Experimental bird drift and loss rates to estimate the percent of oiled animals that would reach a beach and be stranded.
Thus, it is not possible to verify some of the model estimates of impacts. The model results point to where additional observations are needed to evaluate impacts to these less obvious species French and Rines, Oil toxicity models have been developed to estimate water column toxicity after an oil spill French, ; French McCay, As discussed above oil toxicity may be attributed to many different compounds.
Exposure concentrations of each compound in the mixture, as well as their toxicities, must be estimated to quantify the toxicity of oil to water column organisms French et al. Typically, for surface releases of fuel and crude oils, only the PAH are dissolved in sufficient quantity and remain in the water long enough for their toxic effects to be significant.
The more turbulent the release i. For a subsurface release deep in the water column or for a gasoline or other product spill where the MAHs and lower molecular weight aliphatics are significant fractions of the oil, all of these compounds may cause significant acute toxic effects French, ; French McCay, Other, less familiar models may address the challenges of modeling oil spill fates and effects at least as well.
Fish and their eggs and larvae are affected by dissolved contaminant concentration in the water or sediment. Mortality is calculated using LC 50 , corrected for temperature and duration of exposure, and assuming a log-normal relationship between percent mortality and dissolved concentration. Movements of biota, either active or by current transport, are accounted for in determining concentration and duration of exposure. Organisms killed are integrated over space and time by habitat type to calculate a total kill.
Lost production of plants and animals at the base of the food chain is also computed. Lost production of fish, shellfish, birds, and mammals due to reduction or contamination of food supply is estimated using a simple food web model French et al. In addition to the direct kill and food-web losses of eggs and larvae, young-of-the-year fish may be lost via habitat disruption.
This is included in the model for wetland and other nursery habitats destroyed by lethal concentrations or oiling. Losses are related to the habitat loss. Thus, recovery of spawning and nursery habitat in wetlands follows recovery of plant biomass and production French et al.
Success of a model simulation is dependent on both the algorithms and the accuracy of the input data. Results of the validation exercises have shown the algorithms provide reasonably accurate results. The most important input data in determining accuracy of results are winds, currents, and biological abundance of the affected species. These data inputs need to be site- and event-specific for an accurate model estimate of impacts of a spill. Thus, the limitations of modeling are largely driven by the availability of these input data French and Rines, ; French, a,b,c.
While oiled wildlife and shoreline habitats may be observed and quantified in the field after a spill, it is difficult and often infeasible to measure directly impacts to aquatic biota in the water column. To characterize fully the impact by field sampling, water and sediment samples would be needed at frequent time intervals over the first few weeks after the release and especially in the first hours , and with enough spatial coverage to characterize the extent of contamination. In addition, comprehensive sampling of each of the species affected is needed in the exposed and unaffected areas.
Because marine organisms are patchy in their distribution, large numbers of stations and samples within stations are needed to map abundance accurately. Such extensive sampling of all or even selected species affected is often not feasible, given the rapidity at which the evidence disappears by scavenging of killed organisms and by migration of animals into the impacted area. Modeling may be used in combination with field sampling to quantify oil fate and impacts French, a,b,c.
The weaknesses of modeling are related to our incomplete scientific knowledge of the impacts of oil spills. Because oil spills are infrequent and unplanned events, which have most of their effects on organisms over a very short time, it is very difficult to obtain quantitative information with which to develop and verify models.
The implementation of NRDA regulations under OPA has facilitated the gathering of quantitative data on spills, and provided opportunities for improving and verifying models. Effects on communities will be discussed from the standpoint of habitat types in which they occur. Two broad habitat categories are considered: 1 biogenically-structured habitats, and 2 inorganic substrates, such as intertidal rock, sand, and subtidal muds.
Long-term and chronic effects are likely to be expressed as residual damage from oil spills to biogenically-structured communities, such as coastal wetlands, reefs, and vegetation beds. Effects of oiling on biogenically-structured habitats may result from acute damage on habitats such as coral. Here the concern is that even though oil may not persist following an oil spill, the time required for recovery of damaged populations of organisms that provide the physical structure of the habitat may be many years.
In some biogenic habitats, such as mangroves and mussel beds, oil can sometimes penetrate into the lower-energy sediments associated with these habitats and have potentially long-lasting effects. Biological communities that are integrally dependent on physical structures, which are themselves formed by living organisms, may be inherently slow to recover from severe impacts. In some cases where the structure-forming species actually stabilize the habitat, it is conceivable that permanent modification of that habitat could result from an acute incident that kills the key structuring species.
Recovery from the effects of an oil spill in a community in which organisms provide the physical structure of the habitat depends on structural damage incurred during cleanup operations, the persistence of contamination, and the inherent ability of the community to recover. The Oil in the Sea report focused extensively on the effects of oil spills on tropical habitats including coral reef ecosystems and mangroves. At the time, there were multiple field studies documenting effects on corals including decreased feeding response, coral colonization and premature expulsion of coral planula.
One lament of the Oil in the Sea report was the lack of information on concentrations and composition of oil in the water that prevented comparison of spill effects between coral sites. Since , a wealth of field and laboratory studies have increased our knowledge of the effects of oil on coral reefs. The Galeta spill into Bahia las Minas, Panama is arguably the most studied oil spill in the tropics. Large amounts of medium weight crude oil see Box spilled into mangroves, seagrass beds, and coral reefs on the Caribbean coast of Panama Burns and Knap, ; Jackson et al.
Another notable tropical oil spill was the consequence of the Persian Gulf War in where 1,, tonnes of oil were spilled into the marine environment Price and Robinson, Despite a fold difference in total volume of oil spilled, the long-term effects greater than five years of oil in Panama were more pronounced and detrimental due likely to repeat inoculation of oil from the surrounding mangroves into the coral ecosystem.
In contrast, no long-lasting effects to the coral reef ecosystem were reported from the Persian Gulf War spills Price and Robinson, Corals located in intertidal reef flats are exposed to oil slicks and are more susceptible to damage and death than corals in subtidal reefs.
Coral located subtidally or in areas with high wave action are not directly exposed to the marine surface layer where oil slicks can coat them. Instead, only the water-soluble fraction of oil generally affects submerged coral. The water-soluble fraction is primarily composed of benzene, toluene, ethylbenzene, and xylene, which can rapidly evaporate to the atmosphere.
One laboratory study found that 15 percent of the benzene and toluene and 80 percent of the xylene were lost after 24 hours of exposure to the atmosphere Michel and Fitt, Acute and chronic exposures of oil on coral have been studied in the laboratory and field reviewed by Peters et al, The symbiotic algae associated with coral are affected after 24 hours of exposure to the water-soluble fraction of oil benzene, toluene, ethylbenzene, and xylene; see Box Photosynthetic capacity can recover fully if there is only short-term exposure to oil less than 72 hours , and no adverse affects were measured for exposure of less than one hour Michel and Fitt, Mixtures of dispersants and oil are more toxic to coral than just the oil Peters et al, Branching coral e.
Montastre , Bak, Mussels often occur in dense intertidal aggregations and their interlocking byssal threads provide a low-energy habitat with protection from the rigors of breaking waves above the bed. The interstices of mussel beds are micro-habitats rich in intertidal life Ricketts and Calvin, As with other bivalves, mussels effectively accumulate high concentrations of a variety of contaminants including petroleum hydrocarbons from the water and their food.
Mussels can be affected by the accumulation of petroleum compounds. Low concentrations of petroleum hydrocarbons can interfere with cellular and physiological processes like cellular immunity McCormick-Ray, ; Dyrynda et al. Thus, there is a basis for expecting population impact under some conditions.