Oil pollution of marine habitats
How oil enters the sea
The public generally takes notice of the problem of marine oil pollution when an oil tanker breaks up in heavy seas or a disaster occurs at an oil platform, one example being the Deepwater Horizon incident in the Gulf of Mexico in spring 2010. In such cases, oil slicks often drift towards the coasts and kill seabirds and marine mammals such as seals. Yet in reality, spectacular oil tanker disasters account for only around 10 per cent of global marine oil pollution.
Most of the oil enters the seas along less obvious pathways, making it correspondingly difficult to precisely estimate global oil inputs into the marine environment. Around 5 per cent comes from natural sources, and approximately 35 per cent comes from tanker traffic and other shipping operations, including illegal discharges and tank cleaning. Oil inputs also include volatile oil constituents which are emitted into the atmosphere during various types of burning processes and then enter the water. This atmospheric share, together with inputs from municipal and industrial effluents and from oil rigs, accounts for 45 per cent. A further 5 per cent comes from undefined sources.
Although vegetable oils such as palm oil are now being produced in increasing quantities and are therefore also entering the atmosphere, oil pollution still mainly consists of various types of oil from fossil sources, created over millions of years from deposits of microscopically small marine organisms, mainly diatoms (Chapter 7).
4.13 > Oil enters the sea along various pathways. Around one third comes from regular accident-free shipping operations.
- This crude oil consists of around 10,000 individual substances, with hydrocarbons being the main component (more than 95 per cent). However, the precise composition can vary considerably according to the place of origin. Crude oil also contains heavy metals and nitrogen compounds.
The extent to which mineral oils and their components adversely affect the various marine habitats and their flora and fauna varies considerably from case to case. Major oil spills have the greatest and most disruptive impact, although their effects are in most cases regionally limited. Since the Torrey Canyon tanker disaster in 1967, when around 115,000 tonnes of crude oil were spilled on a reef off the southern English coast, resulting in the largest oil pollution incident ever recorded up to that time, numerous field studies have been carried out which now provide a very clear overview of the impacts of various types of oil on organisms and habitats. However, one oil disaster is quite never the same as another, and the precise effects of an accidental oil spill depend on a variety of conditions.
A crucial factor, for example, is how quickly the oil breaks down or sinks from the surface of the sea to the lower depths, where the damage it causes is likely to be relatively limited. This breakdown is influenced by various physical, chemical and biological processes. Depending on a variety of different environmental conditions such as temperature, nutrient content in the water, wave action etc., the breakdown of the petroleum hydrocarbons may take shorter or longer periods of time. During the first few hours or even during the first few weeks, the oil is modified by the following chemical and physical processes:
- evaporation of volatile constituents;
- spreading of the spilled oil in large oil slicks drifting on the surface waters;
- formation of dispersions (small oil droplets in the water column) and emulsions (larger droplets of oil-in-water or water-in-oil);
- photooxidation (molecular changes to the oil constituents caused by natural sunlight) and solution.
- 4.14 > In the sea, oil is modified and broken down in a variety of ways. Generally, when an oil spill occurs, the oil immediately forms large slicks which float on the water’s surface. A proportion of the oil evaporates or sinks, but other oil constituents are broken down by bacteria or destroyed by solar radiation. Finally, the oil solidifies into clumps (tarballs), which are more resistant to bacterial breakdown.
- Processes such as sedimentation and breakdown by bacteria, on the other hand, may continue for months or even years, although in some cases, under favourable conditions, they may be completed within a matter of days.
The reason for this discrepancy is that, firstly, the various substance groups contained in the oil undergo biological breakdown at different rates. The speed of breakdown depends primarily on the molecular structure of the oil constituents. The more complex the hydrocarbon molecules, the longer it takes for the oil to be broken down by microorganisms. Secondly, the rate at which the various hydrocarbons are broken down is increased by the following factors:
- high temperatures, promoting bacterial activity;
- a large surface area (if necessary, the surface area of the slick can be increased through the use of dispersants, i.e. surface-active agents [surfactants] which promote the formation of dispersions);
- good oxygen supply for the bacteria;
- good nutrient supply for the bacteria;
- low number of predator organisms which would
reduce the number of bacteria.
Some of the above-mentioned processes have a very considerable influence on the extent of oil damage. Water-in-oil emulsions, for example, are a contributory factor in the formation of “chocolate mousse”. This viscous emulsification can increase the original volume of the oil as much as fourfold, rendering the use of chemical dispersants impossible and making it far more difficult to pump the oil off the water surface. >