The highest profile of all potential impacts on rocky shores is
achieved by those short-term environmental disasters with highly visible and
media-friendly effects. Such factors may not necessarily be those most important in the
long term but where impacts are dramatic, shores may take years to recover.
Red tides are blooms of toxic dinoflagellates and other algae which can cause the
widespread mortality of marine and littoral animals. Although red tides occur naturally,
there is increasing evidence that they can also result from human activities.
Eutrophication of coastal waters may have caused the severe blooms of Gyrodinium
aureolum, Chrysochromulina polyepis and Craetium spp. which devastated North
Sea shores in 1988 (Smayda, 1989, 1990; Lindahl, 1993). The main rocky shore animals
affected by these events were barnacles, dogwhelks, mussels and limpets which, as space
occupiers, grazers and predators, have a strong effect on the community ecology (e.g.
Southgate et al., 1984).
In recent years, British shores have been affected by two major oil spills. The Braer
spilled 85,000 tons of oil near Shetland in 1995. The Sea Empress ran aground off
South Wales in February 1996, releasing 72,000 tons of oil which had disastrous effects on
ecosystems along more than 100km of coastline. A more severe impact was caused by the Torrey
Canyon oil spill in March 1967. Of the 119,000 tons of Kuwait crude on board, 100,000
tons were spilt and 40,000 tons came ashore. 14,000 tons, borne on the highest spring
tides for half a century, were stranded on the Cornish coastline and 10,000 tons of
dispersant were used in the clean-up operation. The effects of this dispersant on marine
life were little understood at the time. However, it later became apparent that the
effects of the dispersant were more severe than those of the oil itself. The Torrey
Canyon oil spill provides an excellent case study of the effects of acute pollution on
the rocky littoral thanks to the long term observations on affected shores which are
reported in Southward and Southward (1978, Hawkins et al. (1983) and Hawkins and
Southward (1992), and summarised in Raffaelli and Hawkins (1996).
Impact and Recovery from the Torrey Canyon spill
The first-generation dispersants available in 1967 proved highly toxic
to marine life. Subsequent laboratory studies showed that the concentration required to
kill 50% of intertidal organisms in 24 hours of exposure was between 5 and 100 ppm, with
limpets in the genus Patella being highly susceptible. These lethal concentrations
were much lower than those needed to disperse the oil, and consequently all animals and
many algae were killed in areas of the shore close to dispersant spraying.
The effects on rocky shores of the removal of much of the biota were
profound and long-lasting. The loss of Patella had a special significance in this
respect, since this grazer is a key species in the north-east Atlantic, responsible for
structuring midshore communities on moderately exposed and exposed rocky shores.
The time course of recolonization of rocky shores in Cornwall,
expressed in years from the date of the Torrey Canyon disaster, March 1967
(extracted from Hawkins and Southward, 1992).
(w. of harbour)
|Relative exposure to waves
|Amount of oil stranded
|Persistence oil/oil-dispersant mix
|Maximum Fucus cover
|Minimum of barnacles
|Maximum numbers of Patella
|Fucus vesiculosus starts to decline
|Fucus vesiculosus all gone
|Increase in barnacles
|Numbers of Patella reduced
|Normal richness of species regained
Patterns of Recolonization
The table above, based on Southward and Southward (1978), summarizes
information on the patterns of recolonization of various rocky shores between 1967 and
1977. The similarity of the overall pattern allows a generalized account of the course of
recolonization on the midshore region. Following the death of grazing animals, a dense
flush of ephemeral green algae (Enteromorpha, Blidingia, Ulva) appeared which
lasted up to one year. After six months or so, large brown fucoid algae (mainly Fucus
vesiculosus and F. serratus) began to colonize the shores. Very few animals
were present under these dense growths of algae. Any surviving barnacles were overgrown
and eventually died, whilst the dense canopy prevented subsequent recruitment of barnacles
by the sweeping action of the Fucus fronds and larval barrier effects. This was
probably reinforced by the dense population of the predatory dogwhelk Nucella which
built up under the canopy so that barnacles declined to a minimum on most shores between
1969 and 1971.
Reappearance of grazers
The limpet Patella vulgata first recolonized the shores during
the early winter of 1967-8 and survived well in the damp conditions under the extensive
Fucus canopy, preventing subsequent recruitment of Fucus by their grazing
activities. As the plants aged, grazing of the holdfasts reduced Fucus further and
between 1971 and 1975 the shore became very bare with even fewer algae than before the
With the disappearance of Fucus, the abnormally dense population
of limpets abandoned their normal homing habit and migrated in fronts across the shore.
Barnacle numbers increased on all shores once the fucoids declined. Following the bare
period between 1974 and 1978, the shore went through a phase of increased Fucus
cover, although overall cover never exceeded 40%. Limpet density increased with some
fluctuations from 1975 before dropping in the early 1980s and then rising again, probably
reflecting normal levels of spatial and temporal variation typical of limpet populations.
During the period of dominance by the initial colonizing cohorts recruitment of limpets
was low (e.g., 1969 - 1972) - presumably due to intense inter-age-class competition.
Subsequently, recruitment improved, and after 1988 the population generally had 60-70%
juvenile limpets under the length of 15mm.
Recovery of barnacle populations
After the initial decline following the spill, the barnacle population
slowly built up at all shore levels. At a shore level equivalent to high water of neap
tides, all counts from 1979 onwards were within one standard deviation of the pre-spill
mean, although only one value (in 1990) exceeded the mean. At mid-tide level, a similarly
irregular pattern was observed, with exceptionally high counts just after the bare phase
in 1976. Lower on the shore, there was much greater fluctuation which probably reflected
the greater influence of biological interactions, including predation and competition with
spasmodic settlements of Mytilus, and this may account for the drop observed in the
Reference to natural conditions
Before recovery can be assessed, the unaffected condition must be
considered. The eulittoral of exposed shores of Devon and Cornwall is normally
characterised by small-scale spatial and temporal fluctuations in the major components of
fucoids, barnacles and limpets. Isolated patches of Fucus occur, but they are never
more than clumps of a few plants and total cover rarely exceeds 10 to 20%. The patchiness
and fluctuations are partly generated by variation in recruitment and small-scale
differences in microhabitat, predation and physical disturbance. Therefore,
"recovery" can be defined as a return to levels of spatial and temporal
variation seen on unaffected shores.
Subsequent damped oscillation
After the initial massive increase in Fucus, and a similar but
aphasic increase in the key herbivore Patella, subsequent fluctuations have been
much smaller. Fucus cover was clearly abnormal for the first 11 years, and was
perhaps slightly elevated in the early 1980's, before fluctuating around normal levels
after 15 years or so. The abundance and population structure of Patella vulgata
were clearly abnormal for at least 10 years, probably 13.
Time scale for recovery
The time scale for recovery at these sites seems to be at least 10
years. If limpet population structure and barnacle densities are used as criteria then 15
years may be more realistic. These time scales are not surprising when the long life spans
of the main organisms are considered: Fucus 4-5 years, Patella up to 20
years, but usually < 10 years, and Chthamalus at least 5 years and possibly 20.
If we estimate that the average life span of a limpet is 7-10 years it is highly likely
that population structure will take 15 years or so to stabilize.
The time scale for recovery is clearly much longer than was thought by
many ecologists in the early 1970s. Dense growths of seaweeds were seen as a sign of
recovery, rather than of a highly disturbed community, and there were suggestions that the
system had returned to normal within two years. Even the more pessimistic considered that
only a few more years were needed for a complete return to normal, but Southward and
Southward (1978) rightly dismissed optimistic forecasts and the myth of rapid recovery. At
that time, they could only assert that some shores heavily treated with dispersants had
not returned to normal after 10 years, whilst many had taken at least 5-8 years. It is now
clear that it may take 15 years or so for the worst affected shores to recover. In
contrast, recovery at the only shore where oil was substantially untreated because of its
proximity to a seal breeding area (Godrevy) was rapid and almost complete within three
Toxic effects of dispersants
It was very quickly learnt from this incident that large-scale use of
dispersants causes acute toxic effects. In the few weeks taken for the oil to cross the
English Channel, a very different approach was adopted by the French and the Channel
Islanders - manual removal or the use of suction devices with dispersants applied
sparingly. These lessons were absorbed by those in charge of responding to the Santa
Barbara blow-out in 1969 and subsequent spills such as the Amoco Cadiz in 1978,
the Braer in 1995 and the Sea Empress in 1996. Impetus was also given to the
development of less toxic dispersants (NAS 1989) and to physical dispersal and mechanical
collection. In most instances, manual methods (whether removal of oiled plants or use of
absorbent materials) seem to cause less disturbance than that associated with the
trampling and movement of equipment, vehicles and vessels during mechanized operations.
During recent spills, there have been trials of methods which enhance
the microbial breakdown of oil (see Swannell et al., 1996 and Mohammed et al.,
1996 for reviews). These methods have proved successful although operational limits on
their use have not been fully defined. Considerable hope has been raised by the
possibility of enhancing natural biodegradation "bioremediation" by adding
nutrients which limit bacterial growth to the oil, particularly in oleophilic media.
Laboratory tests and field trials have been encouraging and we should be in a better
position to judge once their effectiveness during the Exxon Valdez and their
limited use in the 1991 Gulf War clean-ups has been fully evaluated in the long term (see
Hoff, 1993). Current opinion on the effectiveness of bioremediation varies from over
optimism to extreme scepticism. The results that are emerging from the applications in
Alaska after the Exxon Valdez spill are often conflicting. Concerns have also been
expressed about eutrophication caused by bioremediation and toxic side effects of some
preparations. However, with further field trials and experience of appropriate use
bioremediation still has considerable promise for the future. Evidence has accumulated
from other coastal systems of the length of time needed for recovery from oil spills. Work
in Panama in a deep mud associated with fringing mangroves (Burns et al, 1993) has
suggested that 20 years or more is required due to the long term persistence of oil
trapped in anoxic sediments and subsequent release into the water column.
Clearly, the Torrey Canyon incident was an early example of a
major ecological disaster made much worse by an inappropriate response. A more considered
approach to spills has emerged and more sensible procedures have evolved which were
implemented during the Exxon Valdez spill. The least expensive and most
ecologically sound option for restoring exposed and moderately exposed shores covered by
oil is probably to do nothing, but, as Foster et al. (1990) point out, during an
environmental crisis social pressure to "do something", and the political need
to be seen to be doing something, often outweigh ecological considerations.