Characterization of the ecological quality of the coastal Gulf of Lions (NW Mediterranean). A comparative approach based on three biotic indices

The ecological quality of the Gulf of Lions coast was assessed using three biotic indices (H', AMBI and BQI). The three indices correlated positively. The positive correlation between AMBI and BQI was surprising and was mostly due to the fact that the dominant species Ditrupa arietina featured a low ES50(0.05) but was classified in GI by AMBI. Both H' and BQI were efficient in distinguishing impacted from un-impacted sites but AMBI was not. Differences between H' and BQI were mainly due to the scale used to translate indices in terms of EcoQ. The three indices were able to detect the major changes in macrofauna composition, which occurred in the Bay of Banyuls-sur-Mer during the last 40years. However, the interpretations of such changes in terms of EcoQ differed between indices. These results are discussed relative to the characteristics of the tested indices.     

Chemical reaction rates and entrainment within the Endeavour Ridge hydrothermal plume

The aging of the hydrothermal plume over the Endeavour segment of the Juan de Fuca Ridge was estimated by measuring the²²²Rn³He ratio in the plume as it dispersed. Despite uncertainties in the source function of hydrothermal input, it wa determined that the relative sequence of removal from the plume isH₂ > Δc >²²²Rn>CH₄ Mn, whereΔc is a measure of particle concentration and the mean life of²²²Rn is 5.5 days. H₂ is removed from the plume within hours of input while Mn is not removed within the two-week timescale of the radon-helium clock.Entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone. This is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient-like profile in the deep ocean. Thus we found that significant fractions of the Si and²²⁶Ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height.