Salt marshes
the upper fringe of soft sediment shores is often dominated by salt marsh vegetation comprising woody perennials, succulent annuals and grasses. Saltmarshes are usually associated with temperate climates but occur throughout the world, from sub-polar regions to the tropics. On tropical shores, salt marshes are often associated whit mangroves, occupying the upper and lower intertidal zone respectively. Species composition is determined to a large extent by salinity. For example, in low salinity or brackish waters found in the upper reaches of estuaries, marsh vegetation gives way to reed beds. Plant detritus carried away from marshes also contributes to food webs in estuaries and near shore waters. Many saltmarshes have been attributed special conservation status under the Ramsar Convention on Wetland of International Importance, due to their importance as habitats for birds, especially migratory species.
The impact of an oil spill on saltmarshes depends on the time of year relative to periods of plant growth. Temperate or cold region marshes are dormant during winter months, while in the Mediterranean growth is slow during high summer temperatures. A single event is unlikely to cause more than temporary effects but longer term damage, possibly over several years, can be inflicted by repeated, chronic oiling or by aggressive clean-up activity, such as trampling, the use of heavy equipment or removal of contaminated substrate. Cleaning of a saltmarsh is difficult without risking additional damage and so it is frequently recommended to leave marshes to clean naturally. However, if burning or cutting vegetation has died back. In general, as long as the roots or bulbs of the plants are not harmed by serious oiling or excessive compaction during the clean –up, seasonal re-growth may be expected to follow.
Mangroves
Mangroves are salt-tolerant trees and shrubs growing at the margins of sheltered tropical and sub-tropical waters. Mangroves stands provide a valuable habitat for crabs, oysters and other invertebrates as well as important nursery areas for fish and shrimp. Inaddition, thus reducing erosion of coastlines and minimizing deposition of terrestrial sediments on adjacent seagrass bads and coral reefs.
Their location means that mangroves are highly vulnerable to oil spills. Mangroves are also considered to be extremely sensitive to contamination by oil, dependent to a large extent on the substrate in which the mangroves are growing. Mangroves typically grow in dense muddy anaerobic sediments and rely on oxygen supplied through small pores (lenticels) on aerial roots (Figure 9). Heavy oil inundation of the root systems may block this oxygen supply and may cause the mangroves to die. However, in open aerated sediments, which allow relatively free seawater exchange, the root systems draw oxygen from seawater and so have a higher tolerance to smothering by oil. In the second mechanism, the toxic components of oil, notably in light refined products, interfere whit the plants, systems for maintaining the salt balance, thereby affecting their ability to tolerate salt water. Experience has indicated that loss of mangroves due to heavy oil smothering appears to be less likely than mortality due to inundation with lighter products, including some cleaning agents, which can result localized loos of tree cover.
Organisms living within the mangrove ecosystem cam be impacted both by direct effects of the oil and also the longer term loss of habitat. Natural recovery of the complex mangrove ecosystem can take a long time and reinstatement measures may have real potential to accelerate the recovery process in such habitats.
Long term damage
An effective clean-up operation usually includes removal of bulk oil contamination, reducing the geographical extent and duration of pollution damage, and allowing natural recovery to commence. However, aggressive clean-up methods can cause additional damage and natural cleaning processes may be preferable. Over time, several factors reduce the toxicity of oil so that the contaminated substrate can support new growth (Figure10). For example, oil can be flushed away by rain and tides and as the oil weathers the volatile fractions evaporate, leaving less toxic residual oil.
As the marine environment has such a strong capacity for natural recovery, the impact of an oil spill is usually localized and transitory and there are few documented examples of long term damage. However, under certain specific circumstances, damage may be persistent and impairment of an ecosystem longer-lasting than might typically be expected. The circumstances that tend to lead to acute long term damage are associated with the persistence of oil, particularly where oil has become trapped within the sediment and is protected from normal weathering processes. Examples include sheltered habitats, such as marshes, shingle shorelines and near shore waters, especially when an oil spill coincides with storm conditions. A storm surge inundating a marsh, with the associated turbulent conditions, is likely to bring fine sediments into suspension and into contact with naturally dispersed oil. Once the storm abates, the oil incorporated within the sediment settles to the marsh floor. Similar circumstances result in oil being incorporated into fine sediments and settling in near shore waters. In both situations, anaerobic conditions slow any degradation of the oil. On shingle shorelines, the weathering of the mixture of oil and shingle can result in the formation of an asphalt pavement, which may persist for some time. Oil products that are more dense than seawater, such as very heavy oils or fire residues, fall to the seabed where they can remain undisturbed for indeterminate periods and may result in localized smothering of benthic organisms.