Environmental Engineering Reference
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waters, most silver is associated with chloro-complexes and very little free ion is
present (Miller and Bruland 1995 ). Notwithstanding, silver is rapidly bioaccumu-
lated from solution by some marine invertebrates to potentially toxic levels (Engel
et al. 1981 ; Luoma et al. 1995 ; Wang 2001 ).
The nature of silver's biogeochemical cycle in the oceans is still poorly under-
stood. Its vertical concentration profi les in the oceans exhibit typical nutrient-like
behavior with similarities to that of silicate, suggesting that their biogeochemical
cycles are linked (Martin et al. 1983 ; Flegal et al. 1995 ; Ndung'u et al. 2001 ).
However, there are pronounced deviations between the profi les of silver and silicate,
which renders them less analogous (Zhang et al. 2004 ; Ranville and Flegal 2005 ;
Kramer et al. 2011 ). Moreover, recent studies indicate that the anthropogenic inputs
of silver to the oceans may be relatively substantial, compared to natural inputs, and
in contrast to those of silicate, which are dominated by natural sources. Consequently,
it has been proposed that spatial and temporal gradients of silver concentrations in
marine aerosols and surface waters may be used to distinguish between natural and
industrial inputs of silver and associated industrial contaminants (e.g., selenium) to
the oceans (Ranville et al. 2010 ).
2
Oceanic Silver: Data and Data Gaps
Although data on the content in seawater of most trace metals (e.g., Cd, Cu, Fe, Hg,
Ni, Pb, Zn) are limited (GEOTRACES 2006 ), there are even fewer measurements of
silver in the open ocean (see Table 1 and Fig. 1 ). As previously noted, we found only
a handful of peer-reviewed reports of silver in the oceans, and no reports of silver in
the Indian Ocean. Similarly, we found very few peer-reviewed reports on silver in
marine sediments (Table 2 ).
Some data on silver in oceanic hydrothermal plumes have been published, includ-
ing one in the Mid-Atlantic Ridge (Douville et al. 2002 ). Concentrations of silver,
copper and silicate found in those plumes are summarized in Table 3 . This table
shows anomalously higher concentrations of silver in sulfi dic waters (4-51 nmol/kg)
than in other oceanic waters (0.2-87.7 pmol/kg). From this disparity, we believe
hydrothermal inputs may be relatively important sources in the budget and cycling
of silver within the oceans.
In contrast to the paucity of data for dissolved silver concentrations in oceanic
waters, there is a relative wealth of data on silver in estuarine waters—at least in San
Francisco Bay (Sañudo-Wilhelmy et al. 2004 ). There, silver concentrations in total
dissolved (<0.45 m) and total (unfi ltered) surface waters and sediments have been
systematically measured for more than three decades (Flegal and Sañudo-Wilhelmy
1993 ; Smith and Flegal 1993 ; Sañudo-Wilhelmy et al. 1996 ; Rivera-Duarte and
Flegal 1997 ; Spinelli et al. 2002 ; Squire et al. 2002 ; Flegal et al. 2007 ; Huerta-Diaz
et al. 2007 ). In addition, a few other measures of silver in other estuarine and coastal
waters exist (Sañudo-Wilhelmy and Flegal 1992 ; Wen et al. 1997 ; Buck et al. 2005 ;
Clark et al. 2006 ; Beck and Sañudo-Wilhelmy 2007 ; Cozic et al. 2008 ; Godfrey
et al. 2008 ; Zhang et al. 2008 ; Tappin et al. 2010 ) (Table 4 ).
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