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Many chemicals foreign to the marine environment are toxic, whether they are naturally occurring (e.g. crude oil) or man-made (many pesticides). Locally lethal concentrations may have a direct impact on communities by eliminating many or most species. Frequently, toxic compounds become concentrated during progress along a food chain, accumulating in and selectively depleting the top predators; not only are these conspicuous species (such as otters or seals) often of special public or conservation interest, but their removal may have knock-on consequences for the whole structure of a particular ecological community. Only recently has it been widely appreciated that relatively low concentrations of many toxic compounds may affect species through subtle mechanisms leading to, for example, immune system impairment, reduced reproductive success, or developmental aberrations. The toxic effects of oil pollution fall into two categories (Lobban & Harrison, 1994):

• those associated with the coating of the organism
• those due to disruption of metabolism by the uptake of hydrocarbons

Regarding the first of these, the mucilaginous slime covering of kelps is thought to act (serendipitously) as a protective device against coating by oil (O’Brien & Dixon, 1976). Although the oil retained in the canopy of offshore Macrocystis beds, following a spill at Santa Barbara, adhered tightly to the kelp blades reaching the surface, removal of the oil layer revealed healthy tissue beneath it. Observable damage to the kelp plants in the beds was negligible, presumably because they were "saved" by mucus secretions.

The metabolic effects of hydrocarbons on kelp physiology may be serious. The application of three different crude oils to the thallus surface of Laminaria digitata reduced photosynthetic rates during emersion relative to controls (Schramm, 1972). In Macrocystis, 10-100 ppm of unspecified fuel oils in emulsion with sea water reduced photosynthesis by 50% during 4-day exposures (O’Brien & Dixon, 1976). In young blades, the photosynthetic capacity was completely reduced after 3 days exposure to a 1% emulsion of diesel oil in sea water; boiler fuel, containing a higher proportion of toxic components, was even more algicidal. More recently it has been found that reductions in rates of photosynthesis vary with the type of crude oil, its concentration, the length of exposure, the method of preparing the oil-seawater mixture, and the irradiance used during the experiments (Lobban & Harrison, 1994). Although Macrocystis fronds may be exposed directly to floating oil because of their buoyancy, Laminaria hyperborea fronds, being almost exclusively subtidal, would not come into contact with freshly released oil, but only to sinking emulsified oil and oil adsorbed onto particles.

The kelps are relatively insensitive to the dispersants used in oil-spill cleanups, but faunal elements of kelp biotopes may be seriously damaged by oil pollution. Oil is known to interfere with the ability of lobsters to detect the sex pheromone that triggers mating, and with the normal feeding behaviour of sea-anemones (Beveridge et al., 1997).

Early oil-spills in the UK, such as that of the Torrey Canyon on the Scilly Isles, are now not particularly relevant as it is widely believed that much of the ecological damage was due to the use of highly toxic dispersants which have now been replaced by relatively non-toxic chemicals.

Information on the effects of the Milford Haven oil spill (Sea Empress) on kelp biotopes are due for publication in late 1998.

In Norway, Laminaria digitata grown in large concrete basins was continuously exposed to diesel oil for two years. With diesel oil at 130 µg per litre, lengthwise growth was reduced by about 50% (Lobban & Harrison, 1994). At the lower concentration of 30 µg per litre there was no overall inhibition of growth. After two years of continuous exposure, the plants completely recovered during a subsequent oil-free growing season.

The Amoco Cadiz oil spill in Brittany had no significant effects on Laminaria spp. (Lobban & Harrison, 1994). The consequences for other faunal and floral components of the kelp biotopes are not known.

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