Oil and the seagrass ecosystem of the red sea

Abstract

There are many sources of oil pollution in the Red Sea and, therefore, a considerable potential for damage to coastal systems where spilled oil tends to accumulate. Seagrasses are very common in the shallow, sheltered areas along the shorelines, and the seagrass beds can thus be classified as highly vulnerable. Depending on the degree of oiling, short-term effects on the seagrass plants can be expected, particularly when the above-ground plant parts (the leaves and leaf sheaths) are in direct contact with floating oil. However, there is no evidence of significant long-term or persistent effects, unless the beds are completely covered with oil or below-ground plant parts are affected by oil penetration into the sediment.

As a consequence of the sensitivity of specific algal and faunistic components of the seagrass system to acute and long-term exposures to oil, adverse population changes may persist for long periods of time. The ultimate effects on the seagrass system largely depend on its complexity and the vulnerability of the habitat. The complexity of the system is determined by the number of vertically arranged vegetation layers, each characterized by its own specific floral and faunal assemblages. The number and characteristics of these layers are generally related to the seagrass growth form, rather than to the seagrass species. In the intertidal zone the complexity of the system increases with percentage water coverage and in the sublittoral with increasing depth; a maximum usually occurs a few metres below extreme low water level. Thus, the most complex and susceptible part of the system tends to be situated at depths where the likelihood of serious long-term exposure to spilled oil and subsequent damage is small.

Seagrass in the intertidal area forms a definite buffer between floating oil and the community components under the leaf canopy. Acute exposure incidents will lead to a simplification of the community structure. Chronic exposure will lead to a gradual modification of the structure and basic processes. However, as long as the frame of the community, i.e. the seagrass itself, is not seriously affected, the system is able to regain stability more easily than other, unvegetated parts of the coast. Recovery times are estimated to be one to a few years. Where the seagrass itself is damaged recovery may last several decades.

Proper selection of spill-combat methods may effectively prevent oil from reaching the vulnerable shorelines. Usage of dispersants offshore and mechanical clean-up on- and near-shore are discussed in view of their potential for additional damage to the seagrass system.