Journal of Applied Phycology (2006) 18: 399–408 DOI: 10.1007/s10811-006-9040-z C  Springer 2006 Element concentrations in some species of seaweeds from La Paz Bay and La Paz Lagoon, south-western Baja California, Mexico Ana P. Rodr´ ıguez-Casta˜ neda 1,∗ , Ignacio S´ anchez-Rodr´ ıguez 1 , Evgueni N. Shumilin 1 & Dmitry Sapozhnikov 2 1 Centro Interdisciplinario de Ciencias Marinas (CICIMAR-IPN), Av. IPN s/n Col. Playa Palo de Santa Rita, Apdo Postal 592, La Paz, Baja California Sur, M´ exico. Becarios COFAA-IPN, EDI-IPN; 2 Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia ∗ Author for correspondence: e-mail:aprodri@ipn.mx Key words: elements, trace elements, seaweeds, instrumental neutron activation analysis, La Paz Bay, Baja California Peninsula Abstract 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. Introduction The development of marine ecosystems is strongly con- trolled by the biogeochemical cycles of chemical ele- ments, which depend on their interactions with each other and the geological, climatic, physical, chemical and biological processes that occur in the water column and on the interfaces with sediments and the atmo- sphere (Chester, 2003; De La Lanza & C´ aceres, 1994). In coastal marine areas, it is important to understand the biogeochemical cycles of both the major and trace ele- ments because of the possibility of the changes which may occur in them as a result of either natural or man- made alterations to the environment. Bays, being partly enclosed water bodies, often show strong variations in terms of sediments and chem- ical composition of the water column. In particular, coastal marine sediments are usually made up of both terrigenous and marine biogenic materials, and their composition can vary depending on hydrodynamic and climatic conditions, the type and strength of material inputs, distance from source and the extent of dilution of natural terrigenous or anthropogenic components, usually enriched in many elements, by silica and car- bonates of marine biogenic origin (Chester, 2003). Some elements are mainly present in the dis- solved fraction (ions and molecules) in the sea water, whereas others are incorporated into either colloidal or