The impact of the “Erika” oil spill on the tidal rock pool community, and
particularly on two species of sea urchin (Paracentrotus lividus and Psammechinus miliaris), was investigated over a
3-year period, at Piriac-sur-Mer (Department of Loire-Atlantique, France,
47°21.6' N; 2°31.7' W). A dramatic increase in the abundance of
two macroalgae Ulva sp. and Grateloupia doryphora occurred following a 100% mortality of sea urchins
observed three weeks after the oil spill. The density of sea urchins and of
other main herbivores, the periwinkle Littorina littorea and the trochid mollusks Gibbula umbilicalis and
Gibbula pennantii, were monitored between January 2000 and March 2003. There was significant
inverse relationship between the overall density of herbivores (sea urchins,
periwinkles and trochid mollusks) and the percent cover of algae in the
tidal pools. The first urchins in the tidal pools were observed two years
after the oil spill and it took three years to reach sea urchin densities
comparable to the reference value of 63 ind.m−2 obtained before the oil
spill.