METAL UPTAKE BY PHYTOPLANKTON DURING A BLOOM IN SOUTH SAN-FRANCISCO BAY - IMPLICATIONS FOR METAL CYCLING IN ESTUARIES

The 1994 spring phytoplankton bloom in South San Francisco Bay caused substantial reductions in concentrations of dissolved Cd, Ni, and Zn, but not Cu. We estimate that the equivalent of similar to 60% of the t otal annual input of Cd, Ni, and Zn from local waste-water treatment p lants is cycled through the phytoplankton in South Bay. The results su ggest that processes that affect phytoplankton bloom frequency or inte nsity in estuaries (e.g. nutrient enrichment) may also affect metal tr apping. The bloom was characterized by hydrographic surveys conducted at weekly intervals for 9 weeks. Metal samples were collected from the water column on three occasions, timed to bracket the period when the bloom was predicted. Factors that might confound observations of biol ogical influences, such as freshwater inputs, were relatively constant during the study. Before the bloom, concentrations of dissolved Cd we re 0.81 +/- 0.02 nmol kg(-1), Zn concentrations were 19.8 +/- 1.5 nmol kg(-1), Ni were 42 +/- 1.4 nmol kg(-1), and Cu were 37 +/- 1.4 nmol k g(-1). These values are elevated relative to riverine and coastal end- members, reflecting inputs from wastewater and(or) sediments. At the h eight of the bloom, dissolved Zn, Cd, and Ni were reduced to 19, 50, a nd 75% of their prebloom concentrations, respectively. Dissolved Cu co ncentrations increased 20%. The mass of Cd taken up by phytoplankton w as similar to the mass of Cd removed from solution if particle settlin g was considered, and Cd concentrations estimated in phytoplankton wer e higher than concentrations in suspended particulate material (SPM). Particulate concentrations of Zn and Ni during the bloom appeared to b e dominated by the influence of changes in resuspension of Zn- and Ni- rich sediments.