Organisation trophique de quatre peuplements de substrats rocheux selon un gradient de pollution industrielle (Golfe de Fos, France)

Abstract

This study describes the trophic organization of the fauna of four superficial rock-bottom assemblages localized along a gradient of industrial pollution (Gulf of Fos, Mediterranean Sea). Six stations were studied over a period of 13 or 15 mo between May 1974 and August 1975. The faunal species (188 species) were classified into four major trophic groups: suspension-feeders, detritivores, herbivores and carnivores, which were all present in significant numbers in all four assemblages. A fifth group, the omnivores, was poorly represented. Qualitatively, the herbivores dominated by number of species (mainly Crustacea and Polychaeta). Quantitatively, the trophic groups were distributed in decreasing numbers along an increasing pollution gradient. Statistical analysis revealed a destabilization of the balance existing between the four major trophic groups along an increasing pollution gradient. Abundance of the four trophic groups exhibited spatio-temporal variations that evolved homogeneously during the annual biological cycle. When an environmental disturbance occurs, a particular trophic group will dominate, depending on the type of disturbance. At the reference station, located in relatively unpolluted waters, the three trophic groups suspension-feeders, herbivores and carnivores seemed to be well-balanced, with a slight dominance of the latter; the two other groups were distributed equally. At moderately polluted stations, suspension-feeders dominated. The most polluted stations were dominated by herbivores. Occurrence of detritivores seemed to be negatively related to the degree of thermal pollution, which provoked massive mortality and important detritus production resulting from internal stress caused by rapid temperature variations. At the population-structure level, such temperature variations are accompanied by important recruitment in a limited number of dominant species. Examination of the trophic organization of rock-bottom assemblages for which structural evolution is already well known provided complementary information on the modality of dysfunction in time and space of certain trophic-web levels, as related to progressive alterations in the rock-bottom assemblages. Such dysfunction can appear very early, even before evolution of the assemblage's physiognomical characteristics. Trends in trophic organization reflect the influence of certain edaphic factors such as particle-rich currents, the impact of local thermal pollution, chemical pollution, or low salinity.