Distribution of particulate, colloidal, and dissolved mercury in San Francisco Bay estuary. 2. Monomethyl mercury

Abstract

The phase speciation and estuarine behavior of monomethyl mercury (MeHg) were determined in surface waters of the San Francisco Bay estuary in September-October 2000 (low flow) and March 2001 (high flow). Colloidally associated MeHg was isolated using a cross-flow ultrafiltration technique with a nominal molecular weight cutoff of 1 kDa. Filter-passing MeHg was 57 Ϯ 18% of the MeHg in unfiltered water in the fall and 39 Ϯ 12% in the spring. Colloidal MeHg averaged 34 Ϯ 11% of the filter-passing MeHg in the fall and 56 Ϯ 15% in the spring. Significantly higher particle-water partition coefficients were observed between colloidal and dissolved MeHg (log K C ϭ 5.6 Ϯ 0.3, n ϭ 21) compared with those between particulate and dissolved MeHg (log K P ϭ 4.9 Ϯ 0.5, n ϭ 21), which suggests that MeHg is preferentially associated with colloidal material. Strong correlations of MeHg with organic carbon content in the filter-passing, colloidal, and dissolved fractions confirmed the importance of organic matter in the MeHg cycle. Both absolute and relative (as a percentage of Hg) MeHg concentrations were highest in the river water end-member under both flow regimes, which suggests that riverborne MeHg is the major source of MeHg introduced to the estuary. A nonconservative estuarine mixing model suggests that significant amounts of colloidal and dissolved MeHg are removed in the estuary under both flow regimes, standing in marked contrast to Hg, which had a source within the estuary under the low flow condition.

Distribution of particulate, colloidal, and dissolved mercury in San Francisco Bay estuary. 1. Total mercury

“…Ultrafiltration was carried out with ϳ2 liters of sample and a concentration factor (CF, the initial volume/retentate volume) of ϳ15 was used throughout the present study. Additional details on membrane cleaning and ultrafiltration protocol are reported in Choe and Gill (2001 Table 2). The average retention coefficients (R C , the ratio of retentate concentration to initial concentration) of rhodamine 6G and vitamin B-12 were 0.22 (n ϭ 2) and 0.81 Ϯ 0.02 (n ϭ 5), respectively.…”

“…The major focus of this work is to characterize the size fractionation, distribution, transportation, and fate of Hg and to examine the importance of natural colloidal material on the speciation of Hg. The related study of MeHg is presented separately in this issue (Choe and Gill 2003).…”

Role of phytoplankton in mercury cycling in the San Francisco Bay estuary

“…Data from the northern reach of San Francisco Bay (Choe and Gill 2003;Choe et al 2003) along with field data from other studies indicate that phytoplankton can affect mercury cycling. For example, Choe et al (2003) found a relationship between chlorophyll a (Chl a) and particulate Hg T in some seasons in North Bay.…”

“…The combined mercury inputs from the Sierra Nevada and the Coast Range contribute up to several hundred kilograms of mercury to the estuary every year, far more than inputs from contemporary sources (e.g., atmospheric deposition, wastewater treatment plants, and surface-water runoff) (Domagalski 2001;Thomas et al 2002;Conaway et al 2003). Although the cycling of this mercury in the estuary has been previously studied (Choe and Gill 2003;Choe et al 2003;Conaway et al 2003), the role of phytoplankton in mercury cycling needs further investigation (Choe et al 2003).…”

“…For example, Choe et al (2003) found a relationship between chlorophyll a (Chl a) and particulate Hg T in some seasons in North Bay. Those variables were likewise correlated in a study of the Kara Sea, Siberia (Coquery et al 1995), which the authors attributed to uptake of Hg T by phytoplankton.…”