Abstract:
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 StraiteSuisun Bay region (salinity 0e17) 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 þ 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 mm) 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 dinoflagellates (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.
