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The semi-closed, not deep and pristine Bahía Concepcion of the Gulf of California, which is almost inserted to semi-arid eastern Baja California Peninsula, outstands by its high biological productivity, geothermal hot springs, shallow-water gas-hydrothermal vents, and by the absense of permanent fluvial inputs. During a summer season the density stratification frequently causes the anoxic and hypoxic conditions in the water column. Due to all these reasons the Bahía Concepcion is an excellent natural reservoir to study biogeochemical cycles of trace elements (TEs) in coastal zone. We made there a first assessment of the influence of TEs coming up hot springs and shallow hydrothermal vents on adjacent marine environment. The TEs were determined in seawater, suspended particulate matter (SPM), sediments, and brown seaweeds. The dissolved elements Fe, Mn and Cu were analyzed by AAS after preconcentration with Chelex-100 resin, while dissolved Hg and As were analyzed according to standard EPA methods. The abundance of TEs elements in SPM, sediments and seaweeds were measured by ICP-MS and ICP-AES techniques. The geothermal and hydrothermal fluids are distinguished by the highest concentration of the dissolved As (1.2-52.8 ?g l-1), Fe (196-769 ng l-1), Mn (0.04-26.5 ?g l-1) and mainly reactive dissolved Hg (18-171 ng l-1). The sediments of geothermal hot spring are enriched in As (111 mg kg-1), and Hg (25 mg kg-1). Similar trend was found in sediments of hydrothermal vents having high contents of As (207 mg kg-1) and Hg (143 mg kg-1). However, there is significant quick and drastic drop of TE concentration in the surrounding sea water with distance from the termal apertures. Thus, dissolved mercury, occurring predominantly as the non-reactive (48 to 86 %), in the surrounding seawater samples dropped down to 11-34 ng l-1.The Fe is 99% in particulate form in the fluids and seawater, whereas As and Hg occur mainly dissolved in both water sample types (87-100% and near 100% correspondingly). The contents of both elements decreased rapidly in the sediments of the surrounding area. Sargassum sinicola brown seaweed specimens taken just over the hydrothermal site also presented elevated levels of As and Hg with maximum content of 640 mg kg-1 for As but further study is necessary to establish the primary-secondary relationship of those phenomena.

Palabras clave: Bahía Concepción; Trace elements; Paralarval assemblages; GEOTRACES; national marine laboratory

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