La Paz Bay and La Paz Lagoon are water bodies of the Gulf of California that are influenced by waste water discharges from the City of La Paz and from activities of the phosphorite mining company "Rofomex". Because seaweeds concentrate elements from the water and are used as effective indicators of contamination by metals, we investigated their usefulness in this region. Concentrations of certain major elements (Ca, Fe, K and Na) and trace elements (As, Ba, Co, Cr, Cs, Hf, Rb, Sb, Sc, Se, Sr, Ta, Th, U, Zn and Zr) were determined in 12 species of seaweeds from La Paz Bay and La Paz Lagoon using instrumental neutron activation analysis. The contents of trace elements of environmental importance (As, Co, Cr, Fe, Sb, Se and Zn) in all studied samples are within the range of typical levels for a pristine environment not subjected to anthropogenic impact. Somewhat higher concentrations of Cr (81 mg kg −1 ), Hf (4 mg kg −1 ), Rb (48 mg kg −1 ), Sc (6.3 mg kg −1 ), Ta (0.95 mg kg −1 ), Th (6.8 mg kg −1 ), U (33 mg kg −1 ) and Zn (90 mg kg −1 ) were found in the green seaweed species Ulva (formerly Enteromorpha) intestinalis, whereas such elements as As (77 mg kg −1 ), Sb (1.4 mg kg −1 ) and Se (1.8 mg kg −1 ) were mainly concentrated in the species Sargassum sinicola, Codium cuneatum and Padina mexicana respectively. Because of their higher abundance and heterogeneity in elemental composition the seaweeds species Ulva intestinalis and Caulerpa sertularioides seem to be more suitable for further biomonitoring of heavy metal pollution of the coastal waters in this zone.
