The fate of oil spill in the marine environment is primarily controlled by the energy of the environment as defined by exposure to waves and currents and the intertidal substrate receiving the residue of the spilled oil. The case histories of some major spills, their fate and effects are discussed here. There are more details in the reports of spills that occurred in international waters, especially with regard to determined environmental damage, because these are reported in the open literature. The paucity of reports in the open literature of reported cases of spills in Nigerian waters accounts for the limitation of the discussion here.
1. The Argo Merchant Spill: The Argo Merchant spill occurred in winter, on the northwest coast of North America with Bunker C fuel oil. The Argo Merchant ran aground on the Nantucket Shoals, off Massachusetts on December 15, 1976 and broke up in open waters, with prevailing offshore winds for most of the spilled period. About 29,000 tons of No. 6 fuel oil was spilled. Immediately after the spill, burning of the oil was tried without success. No dispersants were used. We shall now discuss the fate of the spilled and its impact on biota.
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Oil fate: The resulting slick soon broke up into a mixture of thick pancakes surrounded by sheen. The pancakes varied in thickness, averaging, about 1 cm with smooth surface. Dispersion of oil as droplets into the water column was studied on three cruises. Droplets of pure oil were found in five out of forty-two samples. Some samples taken 6 m above the bottom contained oil-coated sediment particles. Winds during the spill period were offshore from Massachusetts and as a result, no oil from Argo Merchant ever reached the shoreline and no coastal impact was incurred.
Hydrocarbon contamination of the bottoms sediments seems to have been restricted to an area immediately around the wreck and apparently was shortlived. Sediment analyses 1 and 2 months after the incident showed oiling in the form of small tar particles or droplets, in a 10-15 km area around the site.
Impact on Biota.
Impact on Birds: Bird observation showed that for a 9-day period following the wreck, about 1,120 birds from different species passed through the area, mostly (92%) gulls. Oiled birds were seen near the wreck as well as in the vicinity of the slicks and shore on Cape Cod and Nantucket.
Impact on Plankton.
Copepods at most stations within 10 km of the wreck were found to have oil either on their mandibles or in their intestines. There was no clear correlation, however, between occurrence of oil in the copepods and the presence of detectable oil concentrations in the water column. On the other hand, a high correlations was found between the oiling of copepods and the occurrence of dead fish eggs. Both cod and Pollock eggs were found contaminated with oil adhering to their surfaces. Twenty per cent of fish eggs and embryos of the collected cod eggs sampled were either moribund or dead.
Sand launce larvae were low in number under the slick, but their abundance was not correlated with other biological effects of the oil.
Impact on Benthos:
Oil was found in an interstitial harpacticoid copepod and in a polychaete species and on the appendages of a burrowing amphipod taken from samples near the wreck. A few other organisms were contaminated with tar, but there was no evidence to suggest that it came from the Argo Merchant. It was concluded that generally there was no indication of significant impact of the Argo Merchant on the benthos.
2. The Exxon Valdez Spill.
The Exxon Valdex rammed into a reef in remote Alaska on March 24, 1989. Eleven million gallons of oil was spilled through the torn bottom of the tanker Exxon Valdez. A powerful spring storm blew in and spread the stain beyond any semblance of man’s control, polluting a vast coastline of the Alaskan Bay.
Although the clean up exercise was the most energetic and single minded any private industry could activate, yet it also showed the limits of what even unlimited amounts of money could accomplish as 2.6 million gallons of petroleum was recovered and a substantial fraction washed ashore. Chemical sprays were used to disperse the oil.
Oil Fate: The slick from the 11 million gallon of petroleum soon broke up into floating ponds, the ponds reduced to puddles, the puddles in turn eroded into sticky, amorphous globs of tar or whipped by waves into frothy emulsion called mousse.
A few island in the direct path of the slick were thoroughly soaked. Oil tended to collect in scattered segment of a few miles or less, in cores and bays within the formidably three-dimensional coast, where steep wooded slopes run down to narrow beaches of gravel and cobblestone. Unfortunately, though, that is where animals congregate.
Application of chemical dispersants together with the natural action of the waves, pulverize the slick into droplets of about 10 microns in diameter which then disperse and dissipate into the water column to be metabolized by bacteria.
Impact on Biota.
Impact on Birds: More than a thousand miles of Alaska’s coastline was contaminated by oil. Bird observation showed that about 200 birds died everyday. A rough count of about 33,000 birds were estimated to have died and some 140 protected eagles were found to have been poisoned and nearly a thousand others were suffocated and died.
Vast stretches of wild land was defaced but the food chain survived. Others are a “keystone” species in Prince William Sound, they eat sea urchins, which in turn feed on the kelp beds that are the foundation of the coastal ecosystem. About 980 otters were found to have died.
Impact on Plankton:
Plankton were beginning their annual bloom, setting mouths watering all the way up the food chain that early spring when Exxon Valdez let 11 million gallons of oil loose onto the water. The hatchling were preserved as the oil slick was advancing into the Sawmill Bay. And whether or not the hatchery fish will be preserved will not be immediately clear until the succeeding years when fishermen will be waiting with open nets for the brood to return to their spawning ground. The plankton bloom that year, the oil spill notwithstanding, was actually one of the best on record. As for the year’s catch of adult salmon, with the season over, the take in the sound stood at 25 million fishes, somewhat higher than the five year average. In the hierarchy of environmental disasters, the impact on the salmon would seem to rank fairly low.
3. The Amoco Cadiz.
The Amoco Cadiz occurred in March 16, 1978, due to steering failure, 13km north of Ile d’Ouessant, in the English channel west of the Brittany Coast. The tanker grounded at high tide on rocks north of porstall, within sight of the shore. As the tide ebbed, the ship broke into two, releasing 223,000 tons of crude oil into the shallow winter beach slope. The oil was driven ashore by a strong northwest wind.
Oil fate : The oil released from the Amoco Cadiz was converted to reddish-brown water-in-oil emulsion (mousse) by tide and wave-induced mixing with water entering the ruptured tanks. Adjacent to the wreck, mousse contained 40-60% water, whereas mousse collected from the beach contained up to 75% water. Concentrations of oil in the water column varied rather widely. Evaporation of the more volatile components is thought to have carried form 20-40% of the spilled oil from the sea surface into the atmosphere. Due to the high wave energy and the vertical mixing of the water column typifying the English Channel, there was greater incorporation of the oil into the water column leading to unexpectedly high mortality of subtidal organisms.
A considerable portion of the oil that did come ashore and was not removed manually was eventually buried in the sediments or entrapped in the low energy salt marches and estuaries.
Impact on Biota:
Impact on Birds.
Birds were migrating through the area at the time of the spill and over 4,500 oiled birds were recovered although 3,200 died on the beaches. Of the 33 species found dead, most were alcids and cormorants.
Impact on plankton: Decreases in biomass of phytoplakton were observed for several weeks in the immediate vicinity of the wreck and in the highly contaminated tidal waters, the Aber Benoit and the Aber Wrach. In contrast, at a further distance from the wreck, the phytoplankton production was elevated, perhaps stimulated as a result of either low levels of petroleum hydrocarbons into the water column or as a result of nutrient release from dead oiled organisms.
Zooplankton experienced high mortalities. The Aber Benoit river contained large amounts of zooplankton debris, coinciding with high oil levels and the surviving copepod Temora Longicornis showed depressed levels of digestive enzymes.
Impact on Fish: There was some mortality of fish, generally within 10 km of the wreck and consisting mainly of rockfish, gobies and one gadis species. Moralities among commercially important species were insignificant. Growth of flatfish, plaice and sole in the oiled river was found to be gradually reduced the level to that of the year of the spill. The effect was greatest among the younger sole and in the adult plaice.
Impact on Benthos: Oil reached the bottom over a large area, including both the bottom sediments of the tidal rivers and a large portion of the western English Channel. In one area of the English Channel, ampeliscid amphipods, comprising about 40% of the bottom biomass in fine sand sediments were totally eliminated. Inshore, there was massive mortality of some species such as heart urchins, razor clams and the amphipod Bathyporeia. Other species, such as the clam Telling tenin and the polychaete owenia survived. However, T. Fabula gradually disappeared in the 2 years after the wreck.
In the tidal rivers, there was a strong correlation between the benthic populations and sediment-hydrocarbon concentrations. There was a reduction in total number of animals, but with no evidence of a change in population structure. At higher concentrations of oil, the appearance of certain opportunistic polychaete species such as Cirratulids and Spionids was observed. At the highest levels of pollution, only the Cirratulids and Capitellids were present.
Impact on Intertidal Communities.
Sandy beaches retained oil as buried layers for several years after the spill, for the oil came ashore during the transition in beach profile from the erosional slope to the beach depositional period. Beaches were littered with dead animals immediately after the spill, including both intertidal and subtidal species.
Exposed intertidal mudflats has almost their entire fauna killed by oil from the water column. In most sheltered areas, the lugworm Arenicola was commonly found alive after the spill.
Marshes and other low energy environments were severely affected where oil came ashore, with complete killing of higher plants and animals. There was no recovery in two years at most of the heavily oiled sites.
4. The Oxtoc I: The Oxtoc I blowout on June 3, 1979 released an estimated half a million tons of oil into an open ocean/continental shelf environment for over a period of nine months.
Oil Fate: The oil emerged in a plume, together with natural gas from the top of the blowout preventer some 38 m beneath the surface and 14 m above the sea floor, in the bay of Campeche in Mexican waters. The turbulent flow mixed the oil and water to form an emulsion of about 70% water suspended in oil, the familiar “Chocolate Mousse.” At the surface, a gas fire burned continually, consuming some of the lighter molecular weight compounds. Evaporation further reduced many of the low ends.
The combination of water circulation and wind resulted in a general movement of large patches of oil slick and mousse of several tens of meters across, towards the west-northwest along the coast of Mexico towards the coast of Texas.
The well was finally capped on March 23, 1980. Less than 10% of the oil was recovered and large amounts of dispersants were used on the slicks approaching within 30 km of the Mexican coastline.
An intensive sampling and analysis effort near the well site in September, 1979, showed that relatively high concentrations of gas, volatile liquid hydrocarbons, and high molecular weight compounds were transported subsurface away from the well site for distances of up to 20-30 km. The oil slick on the surface changed appearance with increasing distance from the well. Concentration of oil slicks by langmuir ciculation cells and evaporative weathering resulted in the formation of windows of mousse several meters in diameter. Patches of mousse up to 20-20 m in length and width and up to a meter thick were observed outside the main slick. The mechanism of formation for these large patches is not known, but microbial and photochemical processes may be involved in addition to surface currents and evaporation.
Impact on Biota.
Impact on Microbed: The major processes acting in the Ixtoc oil, removing it from the environment, appeared to be physical-chemical weathering. Little microbial degradation of the oil was noted and this was in agreement with chemical analyses.
Impact on Fish and Shrimp: Faint petroleum odours were noticed in some shrimp samples, suggesting contamination.
Impact on Benthos: A total of 72 crab samples were taken in 1980 from the South Texas shelf area for benthic ecological studies. Although there were major decreases in number of tax and number of individual at the twelve stations sampled, no quantitative cause and effect relationship with the Ixtoc oil spill could be established due to lack of sufficient background data on life histories and normal cycles of abundance for benthic in fauna of the area.
5. The Arrow Spill: The Arrow spill occurred on February 4, 1970 in Chebabucto Bay, Nova Scotia, when the tanker ran aground on Cerberus Rock on her way into the off-loading facilities in the strait of Canso. She was carrying 15,000 tons of bunker C fuel oil of which about two-thirds were released into the water of the bay.
At the time of the accident, the prevailing winds caused high sea state conditions within the bay. As a result, oil driven by wind and wave action coated over 300 km of the bay into the Atlantic. Eventually, oil from the Arrow was traced as far as south as Halifax, N.S. and Bermuda.
Oil Fate: Oiling along the Southern and Western shores of the bay resulted in a mixture of oil with sand, gravel and rocks to yield a resistant pavement of tar along much of the coastline. Chemical analysis of the subsurface sediments indicated high concentrations of Arrow oil persisting below the surface within the beaches, representing a potential long term source re-entry of spill oil.
Impact on Biota.
Studies on benthic organisms were carried out in the rocky and sedimentary intertidal area. The rock weed Fucus Vesiculosus was reduced in vertical distribution for about five years. Fucus Spiralis, which is confined to the region up to the high tide line was killed off completely. In sheltered areas, the marsh grass Spartina Alterniflora population declined steadily after the spill, with few surviving plants remaining one year later, however, it recovered in two years. Rocky shore animals including barnacles and periwinkles did not change in abundance or in distribution except where their habitat had been altered by changes in rockweed, demonstrating the significance of community associations. Larvae of the common Balanus Balanoides apparently settled and grew normally even during 1970 the spill year.
6. Baki Creek Oil Spill.
The Baki Spill occurred on the 27th February 1988, along Baki Creed at SPDC (Shell Petroleum Development Company) Nun River Flow Station in Oporoma, Rivers State, Nigeria. Baki is a very long water channel of about two kilometers situated along Silver Creek in Rivers State. The water is stagnant at all times except during the flood season. When the spillage occurred, the petroleum spreads from the source of about two kilometres long to the silver Creek and flow suddenly to various communities in less than seven days. The consequences was the high degree of petroleum pollution and the danger to the affected communities and other aquatic lives.
The effects of oil spills the fish catch rate of Choba in the brackish-freshwater transitional zone of the new Calaba River in Nigeria at early post spilled period and one year later from GENECO barge spillage of 1985. Fish catch rate was drastically less at early post spill period than one year later creating economic imbalance of the affected littoral communities.