Petroleum introduced to the environment can elicit gross biological damage, physiological (pathological) effects on the biota (both plants and animals) and a brand range of ecological changes. All Marine biota are permeable to hydrocarbons and accumulate them from their environment either directly from the water column or pore water or through their food. Petroleum hydrocarbons can affect and cause changes in many organisms at all levels that is cellular, organismic and ecosystems.
Petroleum exposure, even at very low concentrations, can impart deleterious effect, whether lethal or sub lethal, to an organism population or community. It can also enhance genetic effects and a wide range of deleterious effects on metabolism.
The impact of oil spills to an ecosystem depend on the oil type and amount spilled, the area of spill, biota of the area, and other environmental factors such as sea state, season and weather conditions. Toxic effects on various aquatic lives vary widely as a result of the differences in sensitivity of oil among marine organisms and the complexity of the chemical composition of the oil. Environmental indices for characterization of the impact of an oil spill are however subject to such variables as the test parameters and marine biological factors, that is, contamination history of the organisms.
Apart from the biological damage, other hazards such as fire hazards, possible human health hazards, tourism reduction and economic loss are very significant. Pollution destroy or limit marine life, ruin wildlife habitats, kill birds, limit or destroy beach areas, contaminate water supply, pose environmental problem and create fire hazards.
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Impact of oil on Coastal Activities.
1. Recreational Activities : Contamination of coastal amenity areas is a common feature of many oil spills leading to public disquiet and interference with recreational activities such as bathing, boating, angling and diving. Hotel and restaurant owners, and others who gain their livelihood from the tourist trade can also be affected. Because of their visual impact, persistent oils and their residues cause the most nuisance and concern, with the greatest effects likely to be just before or during the main tourist season. The disturbance to coastal areas and recreational pursuits from a single oil spill is comparatively short-lived and any effect on tourism is largely a question of restoring public confidence once clean up is completed.
2. Industry: Industries that rely on a continuous supply of clean sea water for their normal operations can only be adversely affected by oil spills. Power stations, in particular, are often locates close to the coast in order to have access to very large quantities of water required for cooling purposes. If substantial quantities of oil are drawn through intakes, contamination of the condenser tubes may result, requiring a reduction in output or total shutdown whilst cleaning is carried out. Similarly, the normal operation of desalination plants may be disrupted by oil, causing water supply problems for consumers.
Shipyards with slipways and drylocks for construction and repair work may be affected by oil spills causing damage to unpainted or newly painted surfaces and creating hazardous working conditions. Interference with shipping may result from oil spills and clean up operations, particularly when they take place in harbours and port approaches. The installation of booms or closure of lock gates to contain oil may cause delays. Direct contamination of jetties as well as mooring lines and ships’ hulls is a common occurrence. Other routine harbour activities such as ferry services and lock operations can be disrupted, particularly after a spill of light crude oil, gasoline or other flammable material. Welding and the use of spark generating machinery may have to be suspended as long as a fire hazard persists. In this way, even small spills in a busy port can have considerable repercussions.
Biological effects of oil spills
The effects of oil on marine life can be considered as being caused by either its physical nature ( physical contamination and smothering) or by the chemical components of the oil (toxic effects and accumulation leading to fainting). Marine life may also be affected by clean up operations or indirectly through physical damage to the habitats in which they live.
Population of plants and animals in the sea are subject to considerable natural fluctuations in numbers brought about, for example, by changes in climate and hydrographic conditions and availability of food. Thus, the species composition and age structure of the various populations within a particular marine habitat are far from constant but instead are in a state of dynamic balance. In view of this, it is usually extremely difficult to assess the effects of an oil spill and to distinguish changes caused by the oil from those due to natural variability.
The different life stages of a species may show widely different tolerance and reactions to oil pollution. Usually the eggs, larva and juvenile stages will be more susceptible than the adults. However, many marine species produce very large number of eggs and larva stages to overcome natural losses. Thai will normally result in less than 1 in 100,000 eggs or larvae surviving to maturity but the excess production provides a reservoir to compensate for any extreme losses due to adverse local conditions. These facts make it unlikely that any localised losses of eggs or larvae caused by an oil spill will have a discernible effect on the size or health of future adult populations.
The ability of animal and plant populations to recover from an oil spill and the time taken for normal balance in the habitat to be reestablished depends upon the severity and the duration of the disturbance and the recovery potential of the individual species. Abundant organisms with highly mobile young stages produced regularly in large numbers may repopulate an area rapidly when pre-spill conditions are restored, whereas populations of long-lived slowly maturing specise with low reproductive rates may take years to recover their numbers and age structure. In general, the rate of recovery in triopical regions is much faster than in cold environments.
Whilst it may be possible to restore the physical characteristics of an oiled habitat to near its pre-spill condition, the extent to which its biological recovery can be enhanced is severely limited. Although the cleaning of mangroves and salt marshes, and replanting with seedlings may be feasible in some situations, care needs to be exercised to ensure that the area is not physically damaged since this may be more destructive in the longer term than the loss of the vegetation. The replacement of animals is virtually impossible and although some species can be bred and released or be moved from undamaged areas, (for example certain birds, mammal, reptiles and fish) It is highly improbable that such programmers will accelerate the natural recovery of a complex marine habitat.
Pathological / Ecological Impact.
Oil interferes with structures or function of various organs of plants and animals directly killing the organisms coming in contact with the oil. For example, hatching of fish eggs, germination of seeds, and growth and reproduction in some grasses can be completely inhibited by coating with crude oil. Oil also creates an environment which is unfavorable for life. For example, oil on the water surface forms a layer which prevents oxygen from dissolving in water and also prevents the exchange of gases across the air water surface. Thus marine aerobic organisms either die to shortage of air thereby suffocating or migrate to some other areas. Secondly, oil prevents the transmission of light thus stopping photosynthesis, a vital process for cellular synthesis in green pants, which are at the beginning of the food chain. Thirdly, oil contains many oxidizable components which utilize dissolved oxygen of water and therefore reduce its level.
Toxicity and smothering effects of hydrocarbons caused mortality in all major spills. Mortality was greatest where spills were confined to inshore areas with abundant biota. The effects were generally localised depending on the quantity of oil and the area of spill. Different petroleum products have different effects. Toxicity of refined distillate, which is high in aromatic content is greatest. The effect of different environments may vary considerably due to synergistic interaction between the oil and other environmental stresses.
Impact of Oil on Specific Marine Habitats.
The following summarises the impact that oil spills can have on selected marine habitats. Within each habitat, a wide range of environmental conditions prevail and often there is no clear division between one habitat and another.
1.Open Waters and Seabed: Plankton is a term applied to floating plants and animals carried passively by water currents in the upper layers of the sea. They form the base of the marine food web that includes the eggs and young stages of fish, shellfish, and many bottom living animals. Their sensitivity to oil pollution has been demonstrated experimentally. In the open sea, the rapid dilution of naturally dispersed oil and its soluble components as well as the high natural mortality and patchy, irregular distribution of plankton, make significant effect unlikely.
Large swimming animals such as squid, turtles, whales and dolphins are highly mobile and are rarely affected in offshore waters even in major oil spills. In coastal areas, some Marine mammals such as seals, reptiles and turtles may be particularly vulnerable to adverse effects from oil contamination because of their need to leave the water to breed. Adult fish living in near shore waters and juveniles in shallow water nursery grounds may also be at risk from exposure to dispersed or dissolved oil.
Plants and animals living on the sea bed (benthos) also form an important part of the food web and in nearshore waters many of the animals (shellfish) and some seaweeds, such as kelp, are exploited commercially. The risk of surface slicks affecting the sea bed in offshore waters is minimal, but in shallow waters, oil droplets may reach the bottom, particularly during periods of rough weather. Fresh crude oils and light refined products with a high proportion of toxic components can cause local damage to sea-grass beds, and to various animals such as clams, sea urchins and worms. The incorporation of oil into sediments can lead to residence times of several years in localised areas, with the possibility of sublethal effects and tainting of commercial species. Weathered oil may accumulate sediment particulates and sink, especially after temporary stranding, possibly causing damage to benthic species.
2. Shorelines: Shorelines, more than any other part of the marine environment, are exposed to the effects of floating oil. The impact may be particularly great where large areas of rock, sand, and mud are uncovered at low tide. Whilst intertidal animals and plants are able to withstand short term exposure to adverse conditions, they may be killed by toxic oil components or physically smothered by viscous and weathered oils and emulsions. Animals may also become narcotised by the oil such that they become detached from rock surfaces or emerge from burrows. They are susceptible to predators or to being washed into an area where they cannot survive. Recolonisation of a shoreline by the dominant plant and animal species can be rapid on rocks, the initial stage is usually the settlement of seaweeds followed by the slower return of grazing animals. However, the complete reestablishment of a normal balance may in extreme situations take many years.
3. Wetlands: Salt marshes occurring in sheltered waters in temperate and cold regions are characterised by dense low vegetation on the mud flats drained by a network of channels. The organic input from the marsh provides the basic source of food for a rich and diverse fauna of worms, snails, clams and crabs which in turn are eaten by birds congregating in large numbers at low tide, especially at certain times of the year.
Marsh vegetation shows greater sensitivity to fresh light crude or light refined products than to weathered oil which cause relatively little damage. Oiling of the lower portion of plants and their root systems can be lethal whereas even a severe coating on leaves may be of little consequence, especially of it is outside the growing season. More widespread damage can be expected from repeated contamination or if oil penetrates into sediments where it may persist for several years. Similarly, of oil reaches the inner portion of marshes during a period of extreme high tides, the residence time may be prolonged, affecting the plants as well as birds that feed and roost there.
In tropical regions, mangrove forests are widely distributed and replace salt marshes on sheltered coasts and in estuaries. Mangrove trees have complex breathing roots above the surface of the originally rich and oxygen depleted muds in which they live. The network of roots and trapped sediments create productive habitats for fish, shrimps, crabs, oysters, snails, mussels and other animals living directly or indirectly on the nutrients for fallen mangrove leaves. Mangrove forests also provide food and shelter for the young stages of commercially important fish and prawns. Fishing in the creeks and drainage channels and collection of shellfish from among the prop roots sustain village communications often living at subsistence levels. The wood is also used for burning, building and tanning.
Oil may block openings of the air breathing roots of mangrove trees or interfere with the salt balance causing leaves to drop and the trees to die. The root system can be damaged by fresh oil entering nearby burrows and the effect may persist for sometime inhibiting recolonisation by mangrove seedlings. The long term effects on the associated animals are likely to be less severe.
4. Mangroves: The mangrove system represents a unique problem regarding oiling and oil impact, exceeding that of the salt marsh of the temperate zone. Mangroves are essential to the tropical marine environment representing a major component in the productivity of tropical coastal systems. The mangrove systems in terms of the marine environment, provide two essential functions. They act to protect coastal systems against storm and current erosion through trapping and stabilizing sediments and debris. They also provide food and shelter for a large number of invertebrate and vertebrate species through a complex detrital food web.
Important species depend on mangroves for part of their life cycles. These include spiny lobster, snapper, drum, sea trout, crabs, shrimp, mullet and menhaden. In many tropical regions, the mangrove represents an important source of wood and other products and are a significant feature and staple of local culture and commerce.
The mangrove ecosystem has two unique features which are the aerial root system and the permeability of the mangroves are highly adapted to anaerobic soils and muds, emerging above the surface as aerial prop roots (red mangroves) or pneumatophone (black mangroves). The surfaces of these structures are marked by numerous small pores termed lenticels, through which oxygen passes into the air passages within the root system. The aerial roots are highly susceptible to oiling, when impacted with clogging of the lenticels and inner passages, eventually choking off the respiratory system. The permeability of the mangroves to oiling is occasioned by the low wave energy, large numbers of small channels and fine sediments. Mangrove seedlings show considerable sensitivity to oil. Oil propagules show reduced rooting rates in comparison to non-oiled controls. Young trees generally appear to be more sensitive than the mature trees.
A common feature of oiling of a mangrove forest is leaf loss and complete defoliation in severely impacted areas as a result of either root or leaf exposure to the oil. The leaves are particularly sensitive to direct oiling, possibly because the trichomes on the lower surface of the leaves are damaged, resulting in disruption of the plants ability to regulate water loss through its stomates.
5. Corals: Reef building corals are found off most tropical coastlines and Islands, in shallow warm waters of suitable salinity and clarity. The living coral grows on the calcified remains of dead coral colonies which form overhangs, crevices and other irregularities inhabited by a rich variety of fish and other animals. To the land side of the reef crest and reef flat, a lagoon is often found which is a low energy environment usually with a sand bottom and seagrass beds, protected by the outer reef. If the living coral is destroyed, the reef itself may be subject to wave erosion.
Coral reefs are generally submerged and it is only if they are briefly exposed to air at low tide that they are vulnerable to physical coating by floating oil. Because of the turbulence and wave motion characteristic of the reefs, the corals may be exposed to naturally dispersed oil droplets. The effects of oil on corals and their associated fauna are largely determined by the proportion of toxic components, the duration of oil exposure as well as the degree of other stresses. Observations of oiled corals suggest that several sub lethal effects may occur such as interference with reproductive processes, abnormal behaviour and reduced or suspended growth. Most of the effects are temporary but similar effects on the associated reef fauna can have greater repercussions since narcotised animals may be swept away from protection of the reef by waves and currents.
6. Birds: Birds which congregate in large numbers on the sea or shoreline to breed, feed or moult are particularly vulnerable to oil pollution. They include Oaks, penguins, and ducks, but other more solitary species such as pelicans, cormorants and gannets can also be affected. Although oil ingested by birds during preening may be lethal, the most common cause of death is from drowning, starvation, and loss of body heat following damage to the plumage by oil. Feathers which are down matted with oil loose their waterproofing and insulating properties.
There are many examples of oil spills which have caused large bird mortalities at sea resulting in some concern for survival of populations. Many seabirds have long lives, delayed maturity and low rates of reproduction and these factors taken together have been thought to hinder rapid recovery. However, recent research indicates that not all mature birds breed at any given time and that these individuals form a reservoir for replacement of killed birds. Oil spill moralities may not therefore have a detectable impact in the case of isolated colonies with limited potential for recolonisation from elsewhere and those exposed to other stresses, for example, through being at the limit of their geographical range.
Impact of oil on Fisheries and Mariculture.
An oil spill can directly dangerous the boats and gear used for catching or cultivating marine species. Floating equipment and fixed traps extending above the sea surface are more likely to become contaminated by floating oil whereas submerged nets, pots, lines and bottom trawl are usually well protected, provided they are not lifted through an oily sea surface. However, they may sometimes be affected by sunken oil.
Reduced catches of fish, shell fish and other Marine organism are occasionally reported after an oil spill. Most often this is due to a reduction in fishing effort although on rare occasions moralities can be caused by physical contamination or close contact with poor water exchange. It is sometimes suggested that fish and shellfish stocks will be depleted for a number of years after a spill as a result of damage that eggs and larvae. However, experience from major oil spills has shown that the possibility of such long term effects is remote because the normal over production of eggs provides a reservoir to compensate for any localised losses. Cultivated stocks, such as caged fish, are more at risk from an oil spill. Cultured seaweed is particularly vulnerable in tidal areas where it may become contaminated with oil at low tide. Seaweeds such as kelp, suspended in deep water from floating structures are better protected.
It is comparatively easy to determine oil spill mortalities in a cultivated stock of known post spill production with yields and market values in previous years or in adjacent unaffected areas. The situation in the case of naturally occurring species is frequently far more difficult since accurate stock assessment is impossible and any dead individuals are likely to be consumed by scavengers. Catch statistics are rarely detailed to enable any decline due to an oil spill to be isolated from other changes brought about by other factors such as variable fishing effort and natural fluctuation in the size of the stock.
An oil spill can cause loss of market confidence since the public may be unwilling to purchase marine products from the region irrespective of whether the sea food is actually tainted. Because of the serious economic consequences arising from a loss of sales, considerable care is frequently exercised to prevent contaminated fish and shellfish from reaching the market. Ideally, this should involve organised tasting by qualified personnel at the time of the spill. To eliminate the effect of subjective assessment, the testers must correctly establish the absence and presence of taint in clean and deliberately oiled controls, respectively. The control samples should be introduced amongst the test samples in a random manner so as to give the taster no clues. A number of replicate tests should be made by each taster to ensure that the results are statistically significant.
Bans on the fishing and harvesting of marine products may be imposed following a spill, both to maintain market confidence and contamination. The area covered by a ban should be related directly to the location and extent of oil slicks and the justification for it’s maintenance reviewed frequently in the light of the position of floating oil and the results of taste tests. Such ban should not be kept in force beyond the relatively short time normally necessary while oil is present. Rearing of cultured species often follows a strict time table and delays designed to protect the culture from oil can prove counter productive if growth is impaired and the subsequent harvest is poor as a result.
Long term effect of petroleum spills.
Concerns regarding the long term environmental effects of petroleum spills informed detailed assessment by a group of experts and this effort was supported by the National Oceanic and Atmospheric Administration and the National Science foundation. The issues addressed by this group concerned the evaluation of the significance of subtle and long term effects of offshore oil and gas development. Long term is operational considered to include time periods greater than two years. There are fundamental problems in identifying the nature and extent of environmental effects and in determining causality. Uncovering subtle effects in coastal ocean requires long term observation and difficult and imaginative experimentation to overcome obstacles provided by natural variability, statistical limits of detection, the effects of other human activities, recognition of recovery, unknown relationships within ecosystems and their role in supporting human resources.
After a spill, the relatively undegraded petroleum hydrocarbons are gradually released from anaerobic sediments or under ice in cold environments resulting in long term effects on benthic and demersal species.
Further more, aromatic hydrocarbons are often incompletely degraded, producing among other compounds, oxygenated aromatics which are highly toxic and persistent.
Sublethal effects, which have subtle consequences to populations of exposed species result from sediment contamination by persistent aromatic hydrocarbons, heterocyclics and degradation products.
Nine categories of potential long term environmental effects of offshore oil and gas development activities were identified as
1. Chronic biological effects resulting from the persistence of medium and high molecular weight aromatic hydrocarbons and heterocyclics and their degradation products in sediments and cold environment: Long term effects result from acute damage due to an oil spill on biogenically structured habitats such as coral reefs, mangrove swamps, salt marshes, oyster reefs, seagrass beds and kelp forests.
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 which provide physical structure of the habitat will be many years. In some cases where the structure forming species actually stabilizes the habitat, it is conceivable that permanent modification of that habitat could result from as acute accident.
2. The residual damage from oil spills to biogenically structured communities, such as coastal wetlands, reefs and vegetation beds.
3. Effects of channelization for pipeline routing and navigation in coastal wetlands.
4. Effects of physical fouling by oil of aggregations of birds, mammals, and turtles.
5. Effects of benthos of drilling discharges accumulated through field development rather than from exploratory drillings.
6. Effects of produced water discharges into nearshore rather than open shelf environments.
7. Effects of noise and other physical disturbances on populations of birds, mammals and turtles.
8. The reduction of fishery stocks due to mortality of eggs and larvae as a result of oil spill.
9. Effects of artificial Islands and causeways in the Arctic on benthos and anadromous fish species.