Mercury in the San Francisco Estuary - Reviews of Environmental Contamination and Toxicology - page 42

V Bioaccumulation and Biomagnification

Although some organisms in San Francisco Estuary have relatively high and poten-
tially toxic concentrations, their routes of exposure are not well known, because
few investigations have focused on trophic transfer of mercury in the estuary (e.g.,
Pickhardt et al. 2006). It is assumed that the predominant form in biota is MMHg,
which is readily bioaccumulated at the base of food chains and then biomagnified
in higher trophic levels in the estuary, as elsewhere (Wiener et al. 2003).

A Bioaccumulation

Measurements of mercury uptake by phytoplankton demonstrate the importance of
mercury speciation in the estuary. Luengen (2007) reported a biodepletion of dissolved
MMHg, but not the inorganic form, during a phytoplankton bloom in the southern
reach of the estuary. This apparently selective uptake or scavenging of MMHg is con-
sistent with mesocosm and lake studies showing an inverse correlation between algal
abundance and MMHg concentrations in zooplankton and fish (Chen and Folt 2005;
Pickhardt et al. 2002). Subsequent bloom decay in the estuary also appears to acceler-
ate the formation of MMHg in suboxic benthic sediments (Luengen 2007).
In a phytoplankton culture experiment using two waters from the freshwater
delta region of the estuary containing different concentrations of dissolved organic
carbon (DOC), Pickhardt and Fisher (2007) showed greater bioaccumulation of
added MMHg, which tended to accumulate in cytoplasm, than of added Hg(II),
which accumulated in cell walls and membranes. Additional work using live and
heat-killed cells suggested active uptake of MMHg in some phytoplankton.
Pickhardt and Fisher also noted that bioaccumulation of MMHg was greater in
high-DOC water, citing the possibilities of relatively higher neutral MMHg species
in the higher-DOC water, or that the higher-DOC water enhanced phytoplankton
membrane permeability to MMHg.
In contrast to this active uptake of MMHg by phytoplankton, some invertebrates in
the estuary have been found to show relatively low bioaccumulation. Gunther et al.
(1999) measured median accumulation factors of unity (0.9-1.3) for mercury in three
species of filter-feeding bivalves ( Mytilus californianus , Crassostrea gigas , and Corbicula
fluminea ) transplanted from relatively pristine sites in central California to
contaminated sites within the estuary. This lack of bioaccumulation indicates that much
of the mercury at the lower end of food chains in the estuary is not bioavailable (e.g., in
inorganic forms) or is relatively dilute because of a bloom dilution effect. Similarly,
concentrations measured in the tissues of a resident bivalve, Macoma petalum , at a tidal
mudflat location in South San Francisco Bay are comparable to local sediment concen-
trations (Moon et al. 2005), demonstrating a lack of bioaccumulation.
Studies on trophic transfer of mercury and the effects of food web characteristics
on its accumulation in organisms from San Francisco Estuary are few. Pickhardt et al.