Tide-induced transport, diffuse fluxes and river inputs of arsenic, cobalt, chromium, manganese and nickel were studied in an estuarine zone located at the Ria of Ortigueira in the Galician coast of NW Spain to evaluate comparatively the magnitude of trace-element inputs in the estuarine ecosystem. Short-sediment cores and flooding water were collected at the intertidal area of La-Caleira Inlet in spring and summer 2008 during the first 50 min of tidal inundation. High concentrations of dissolved chromium (up to 23 nM) and nickel (256 nM) were found in waters of the Lourido River during 2008. Chromium (up to 795 mg·kg− 1) and nickel (up to 533 mg·kg− 1) in surface sediments of the study area were also high, pointing to natural geological sources (Cape Ortegal Complex). Fluxes associated with molecular diffusion and tide-induced transport across the sediment–water interface were quantified and compared to the river contribution. Mean fluxes from the Lourido River were 0.03 μmol·m− 2·d− 1 for As, 0.02 μmol·m− 2·d− 1 for Co, 0.43 μmol·m− 2·d− 1 for Cr, 0.72 μmol·m− 2·d− 1 for Mn and 5.98 μmol·m− 2·d− 1 for Ni. Diffusive fluxes were negative which indicates the flow of trace elements towards the sediment − 0.47 ± 0.12 μmol·m− 2·d− 1 for As; − 0.007 ± 0.001 μmol·m− 2·d− 1 for Co; − 0.003 ± 0.009 μmol·m− 2·d− 1 for Cr; − 5.44 ± 1.65 μmol·m− 2·d− 1 for Mn; and − 0.02 ± 0.05 μmol·m− 2·d− 1 for Ni. Tide-induced transport of trace metals was up to four orders of magnitude greater than diffusive fluxes. The contributions of trace elements to the estuarine zone from fluvial and benthic origin were quantified, emphasizing the importance of tide-induced transport in estuarine systems

Dr Ospina-Alvarez gratefully acknowledges support by a post-doctoral grant from the AXA Research Fund. The CSIC, under the program JAE-Doc (Junta para la Ampliación de Estudios) co-funded by the Fondo SocialEuropeo(FSE), is greatly acknowledged for the post-doctoral contractto Dr. Santos-Echeandía and pre-doctoral contract to Dr. Ospina-Alvarez. This article is a contribution to the Spanish LOICZ program and was supported by CICYT under the INTERESANTE project (ref.CTM2007-62546-C03/MAR) and the Spanish–Portuguese Action (ref. 2007PT0021) both in cooperation with the project‘ Land–sea exchange of trace metals and its importance for marine phytoplankton in an up- welling coast’(ref. CTM2011-28792-C02)financed by the‘Ministerio de Economía y Competitividad’ of Spain. The authors are also grateful to two anonymous referees for their comments and suggestions, which greatly improved the quality of this paper

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