Comparison of ecological quality indices based on benthic macrofauna and sediment profile images: A case study along an organic enrichment gradient off the Rhône River

Several studies have shown the occurrence of major discrepancies between the most commonly used benthic biotic indices within the EU Water Framework Directive. Such studies have however failed to assess, which indices perform best because of the lack of an independent assessment of benthic habitat ecological quality. The present study aimed at filling this gap by comparing the use of several biotic indices derived from benthic macrofauna composition and one index derived from sediment profile imagery, based on the sampling of 16 stations along a gradient of sedimentary organic carbon concentrations (OC) in front of the mouth of the Rhône River. Five biotic benthic macrofauna indices (BQI, BQIES, AMBI, M-AMBI, and W) were computed together with the benthic habitat quality (BHQ) index, which is derived from sediment profile images (SPI). All sampled stations but one belonged to the same community as affected by a gradient of organic enrichment. BQI, BQIES and BHQ correlated significantly between each other's and with OC. Conversely, AMBI and W did not correlate with either BHQ or OC. As organic enrichment is known to constitute a major source of disturbance, this suggests that BQI and BQIES performed better than AMBI, M-AMBI and W in assessing the ecological quality of soft sediments in the studied area. Discrepancies between BQI and AMBI derived indices are discussed based on differences in the sensitivity/tolerance levels attributed to most dominant species by these two indices.