Trace metal concentrations in the surf zone and in coastal waters off Baja California, Mexico

Pronounced spatial gradients were observed in trace element concentrations (Cd, Co, Cu, and Ni) in nearshore surf zone (<100 m offshore) and coastal (5-45 km offshore) waters off Baja California, Mexico. The ranges of metal concentrations in those waters were 0.034-0.26 nM for Cd, 0.022-0.59 nM for Co, 0.93-3.6 nM for Cu, and 3.6-6.4 nM for Ni. Metal concentrations exhibited offshore gradients, with the highest levels nearshore. Metal:salinity, metal:nutrient, and metal:metal relationships indicated that the relatively high nearshore concentrations were caused by upwelling (Cd and Ni) and by benthic particle resuspension and diagenetic remobilization (Co and Cu). The relatively low offshore concentration levels appeared to be caused, in part, by the intrusion of oceanic waters from the North Pacific Central Gyre, as evidenced by similarities in the molar ratios of Ag:Cu in coastal waters off Baja California (9.0 x 10(-3)) and in subtropical oceanic waters off central Mexico (9.6 x 10(-3)). This was also consistent with physical oceanographic measurements, which have traced the surfacing of the northward-flowing California Undercurrent off Baja California. The similarities between metal:nutrient and metal:metal ratios for surface waters off Baja California with those reported for the north Pacific suggested that distributions of Cd, Co, Cu, and Ni in the area of study appear to be dominated by natural physical and biogeochemical processes characteristic of open ocean waters.