Recursos Marinos y Pesquerías

Cover and mortality of rocky intertidal communities after the Prestige oil spill [Presentation]

07/07/05 ,

Elsa Vázquez, Victoriano Urgorri,
Francisco Ramil, Julio Parapar, Javier Cristobo & Juan Freire. Short-term effects of the Prestige oil spill on cover of exposed rocky intertidal fauna and on mortality of the cirriped Chthamalus
montagui. Extended abstract (pdf)

VERTIMAR 2005 – Symposium on marine accidental oil spills. 13 – 16 july 2005, Universidade de Vigo. Vigo (Spain).

ABSTRACT: This study compares
percent cover of more abundant sessile species of upper rocky intertidal community
using imaging analysis techniques in eight localities with different degree of
affectation by the Prestige oil spill. Mortality of the cirriped Chthamalus montagui was also calculated for
the same localities.

1. INTRODUCTION

Galician
coastline is susceptible to oil spills due to intense tanker traffic. In fact,
in the last 20 years several oil spill occurred being the last one the Prestige oil spill (POS). Despite that, there is still a need for well-designed research on the consequences of oil
spills for rocky marine habitats in Galicia mainly due to the lack of
long-term data.

Because of their relatively immobility sessile fauna are very vulnerable to spills and usually
are killed in large numbers. Barnacles are acutely sensitive to oil and often
experience high mortality after an oil spill as it happened during another oil
spill events.

The objectives of the present study were to compare percent cover of more abundant
sessile invertebrates of rocky substrate community using imaging analysis
techniques in eight localities with different degree of affectation. Mortality
of the cirriped Chthamalus montagui was
also calculated since it is very sensitive to HAPs and it inhabits in the level
of shoreline most affected by POS.

2. RESULTS AND DISCUSSION

Although the environmental effects of a spill depend upon such factors as the extent of the
spill, oil type, local weather conditions, season, sediment type, and latitude,
similar patterns of injury were identified for intertidal shorelines in past
spill events. An initial major consequence of the spill and cleanup six months
after POS was the establishment of uncolonized substrate, bare rock and dried
oil, in the upper intertidal of the heaviest oiled localities. On the contrary
the unoiled locality of Segaño presented a high degree of occupation of its
substrate mainly by the species Chthamalus
montagui (Fig. 1).

In the upper levels of the localities from light two heavily oiled a dramatic reduction in
the number of the limpet Patella spp.
was observed. Patella could have died
not only by the lethal effects of the oil or by the reduction of its food
sources but also because being unable to maintain pedal attachment (Dicks
1973). As it was observed in the POS, in Canyon spill there was an almost complete disappearance of Patella (Southward & Southward 1978); even more in absence of these herbivores, diatoms and algae colonized the rock and inhibited the
settlement of Patella larvae and barnacles for 5 years. After the Exxon Valdez oil spill another limpet, Tectura persona, was sparse or absent on
oiled sites (Highsmith et al. 1996) (Fig. 1).

Decrease in percent cover of Chthamalus montagui was
remarkable at spill-disturbed sites. Its percent cover was less than 10% in the
upper levels of the heaviest oiled localities presenting in the unoiled or
lightly oiled localities percent cover greater than 50% (Fig. 1). In some cases
this percent cover obtained using image analysis turned out to be overvalued
since both living individuals and empty exoskeletons were counted as surface
inhabited by the barnacle. But a strong difference in mortality among
localities was found being the most affected localities where the biggest
mortality was observed reaching even 70% (Fig. 2). Other oil spill events
resulting in large kills of barnacles included the Tampico Maru (North et al.
1964), the San Francisco (Chan et al. 1973) the Santa Barbara (Straughan y Abbot 1971), and the Torrey Canyon oil spills (Southward & Southward 1978).

In the heaviest oiled localities the mussel Mytilus
edulis was very sparse or lacking in the sampling levels while in the other
sites even in those spill-disturbed was present (Fig. 1). The reduction in
mussels on oiled sites was probably caused by a combination of events. Upon
initial exposure, mussels covered with oil would no longer be able to feed.
Also byssal thread activity could be impaired which may result in detachment and
subsequent death.

Figure 1. Average of the percentage of cover of the different species from the three
levels defined in the upper intertidal (standard deviation) in each of the
localities.