The Muddy Fine Sand Abra alba–Melinna palmata Community of the Bay of Morlaix Twenty Years After the Amoco Cadiz Oil Spill

The muddy fine sand Abra alba–Melinna palmata community from the Bay of Morlaix (western English Channel) was strongly polluted by hydrocarbons from the Amoco Cadiz oil spill in April 1978. Long-term changes of this community (1977–1996) showed that it was weakly affected by the spill. This was due to the low number and low abundance of sensitive species present on the community in normal conditions. Polychaetes, such as Chaetozone setosa dominated the community, supporting high levels of organic matter. Only two opportunistic polychaetes Mediomastus fragilis and Tharyx marioni increased in abundance just after the spill. Temporal changes showed two successive periods: (i) 1978–1984 with an establishment of the polychaete Lanice conchilega, and its associate fauna, then (ii) from 1985 to the end of the survey in 1996 with the disappearance of L. conchilega and the establishment of the polychaete Clymenura clypeata and the bivalve Thyasira flexuosa. The soft-bottom Abra alba–Melinna palmata community of the Bay of Morlaix may support a maximal carrying capacity of approximately 20 g m−2 during the establishment of Lanice, and only 8.0 g m−2 during the last 12 years of the survey (mean annual biomass in decalcified dry weight). During the 20 year survey, the overall mean P/B was 1.71. The results were compared with these obtained on the nearby A. alba Pierre Noire community where the spill affected strongly and durably the community.