Sources and biogeochemical cycling of particulate selenium in the San Francisco Bay estuary

As part of a study of estuarine selenium cycling, we measured the concentration, chemical form (speciation), and distribution of particulate selenium under various river flow conditions in the North San Francisco Bay (from the Golden Gate to the Sacramento and San Joaquin Rivers). We also conducted laboratory studies on the accumulation of selenium by phytoplankton, the critical first step in the transformation of dissolved to particulate selenium. Total particulate selenium concentration in the North SF Bay was relatively constant between high and low flow periods, ranging spatially from 0.05 to 0.35 nmol l(-1) and comprising between 5 and 12% of the total water column selenium inventory. Mean concentrations were generally highest in the Carquinez Strait-Suisun Bay region (salinity 0-17) and lowest in Central Bay. However, selenium content of suspended particles varied with river flow, with higher content during low flow (9.76 +/- 4.17 nmol g(-1); mean sd; n = 67) compared to high flow (7.10 +/- 4.24 nmol g(-1): n = 39). Speciation analyses showed that most particulate selenium is organic selenide (45 +/- 27%), with a smaller proportion (typically < 30%) of adsorbed selenite + selenate and a varying proportion (35 +/- 28%) of elemental selenium. Based on the amount of elemental selenium in the seston (total suspended material), we calculate that resuspension of estuarine sediments could contribute 29-100% of particulate selenium in the water column. While selenium content of SF Bay seston (> 0.4 mu m) is relatively unenriched compared to phytoplankton (13.6-155 nmol g(-1) dry weight) on a mass basis, when normalized to carbon or nitrogen, seston contains a similar selenium concentration to SF Bay sediments or phytoplankton cultures. SF Bay seston is thus comprised of selenium-rich phytoplankton and phyto-detritus, but also inorganic clay mineral particles that effectively dilute total particulate selenium. Selenium concentrations in algal cultures (11 species) exposed to 90 nmol l(-1) selenite show relatively large differences in selenium accumulation, with the diatoms, chlorophytes and cryptophytes generally having lower selenium cell content (3.8 +/- 2.7 x 10(-9) nmol selenium cell(-1)) compared to the dinoflaggellates (193 +/- 73 x 10(-9) nmol selenium cell(-1)). Because phytoplankton are such a rich (but variable) source of selenium, their dynamics could have a profound effect on the particulate selenium inventory in the North SF Bay. (c) 2006 Elsevier Ltd. All rights reserved.