Selenium in San Francisco Bay - 30 Years of Surprises

Abstract:

<span'','serif'; font-size:="font-size:" 12pt;"="12pt;"">The trace element selenium exists in multiple oxidation states (VI, IV, 0, -II) and chemical forms within an oxidation state, and this chemical speciation affects its bio-availability and geochemical cycling. The interactions between the physical circulation and riverine inputs, changing ecosystem components, and industrial inputs to the San Francisco Bay have had profound and surprising influences on the biogeochemical behavior of selenium in this estuary. <span'','serif'; font-size:="font-size:" 12pt;"="12pt;""> <span'','serif'; font-size:="font-size:" 12pt;"="12pt;"">In the mid-1980s dissolved selenium was relatively elevated and enriched in selenite (SeIV) in the mid-estuary, occurrences that were quantitatively linked to inputs from oil refinery effluents. Suspended particulate selenium concentrations were at a level considered problematic for filter feeding clams with high assimilation efficiencies. By 1999 oil refineries had implemented selenium removal processes that dramatically dropped the concentrations of total dissolved selenium and selenite by over 65% in the estuarine water column. Surprisingly, the concentrations of selenium in suspended particles did not drop as dramatically. We suspect that changes in the ecosystem, including the abundance of certain phytoplankton species and changes in benthic grazing affect the abundance of selenium in suspended particles. These and other changes within the San Francisco Bay system have been simulated in numerical models that reveal other surprising aspects of selenium cycling in this estuary. Data and models will be discussed in this presentation, and implications for other trace elements presented.