Mercury in surface waters of a macrotidal urban estuary (River Adour, south-west France)

Abstract

Surface waters were collected in the River Adour estuary (south-western France) during different sampling periods from 1998 to 2001 in order to investigate the phase distribution and speciation of methylmercury (MeHg⁺). Although a high variability is observed, significant higher average concentrations of total MeHg⁺ (dissolved and particulate) are measured during the warm seasons, at 4.40±8.18 pM and 3.90±1.87 pM for July 1998 and September 1999, compared with the winter seasons, with concentrations at 0.99±2.85 pM and 1.00±1.75 pM for February 1998 and February 2001, respectively (one-tailed t-test, P=0.01). The seasonal variations are explained with enhanced bacterial activity during summer and sedimentation/resuspension phenomena. Additionally, signi-ficant longitudinal variations of the MeHg⁺ concentrations are observed. The highest levels in both dissolved and particulate fractions are found within the downstream urban estuarine area. This can be explained by the high methylation potential of the sediments, but direct anthropogenic inputs of MeHg⁺ from specific discharge points cannot be neglected. Biogeochemical factors like phytoplankton biomass and salinity also show a relationship with MeHg⁺ partitioning in the surface waters of the Adour estuary.

Keywords:

• Mercury
• Methylmercury
• Surface water
• Adour River estuary

Acknowledgements

T. Stoichev acknowledges the French Government for his Ph.D. grant. This work is supported by the Aquitaine Region and CNRS in the Framework of the projects ECOMAN and GIS-ECOBAG. The Captain and crews of INSU-CNRS research vessels (Ĉote d’Aquitaine, Ĉote de la Manche) and A. Soulier (CQEL 64) are acknowledged for their technical assistance during the sampling campaigns. We also thank H. Etcheber (CNRS—U. Bordeaux 1) for the organic carbon and pigments measurements and T. Church (University of Delaware, USA) for improving the quality of this manuscript.