Benthic habitats in the northwest Mediterranean characterised by sedimentary organics, benthic macrofauna and sediment profile images

A total of 36 benthic stations were studied in four areas in the oligotrophic Gulf of Lions and in one area of the severely disturbed Berre Lagoon in the northwest Mediterranean in October 2001. The study included analysis of sediment characteristics, particularly organic matter, and analyses of macrofauna and sediment profile images (SPIs). In the Gulf of Lions, values of superficial sediment biochemical characteristics were similar to available literature data, with the important exception of enzymatically hydrolysable amino acids (EHAA) and enzymatically to total amino acid ratios (EHAA/THAA) off the Rhoˆ ne River and the city of Saint- Cyprien. All biochemical concentrations, but not EHAA/THAA ratios, were higher in the Berre Lagoon. A total number of 190 taxa were recorded, generally occurring in low density and biomass. In most SPIs from the Gulf, a deep infaunal bioturbation was observed, i.e. burrows and feeding voids were recorded in 85 and 62% of the images, respectively. This infaunal activity was associated with a mean depth of the apparent redox potential discontinuity (aRPD) in the sediment at 2.1–3.5 cm in the gulf and 0.2 cm in the lagoon. The benthic habitat quality (BHQ) index, summarising surface structures, subsurface structures and the depth of the aRPD, assigned most benthic stations in the Gulf to the benthic successional stages II or III, i.e. fairly undisturbed habitats. Demersal trawling, however, probably was a disturbance to most muddy superficial sediments in the Gulf. Multivariate analysis (MDS) of the macrofauna and the SPIs both separated three groups of stations: muddy, sandy and those that were disturbed from the lagoon, but a few stations showed dissimilarities to this general pattern. Principal component analysis (PCA) resulted in the best correlation between faunal abundance and biomass and enzymatically available amino acids in the sediment. PCA grouped almost all stations into three separate groups: muddy, sandy and severely disturbed habitats, and the muddy habitat group was associated with high aRPDs and BHQ indices. It is concluded that SPI is useful for in situ analysis of infaunal bioturbation activity and redox conditions, and for benthic habitat classification in the oligotrophic northwestern Mediterranean and the disturbed Berre Lagoon.