Role of Phytoplankton in Mercury Cycling in the San Francisco Bay Estuary

To study the role of phytoplankton in mercury cycling, we measured methylmercury (MeHg) and total mercury (HgT) in surface waters during the spring 2003 phytoplankton bloom in San Francisco Bay. Conditions that described the peak of the bloom, the amount of sorbent, and decay of the bloom were summarized by principal component analysis (PCA). Multivariate analyses conducted with the PCA factors demonstrated that the bloom accounted for a significant (p = 0.03) decrease in dissolved (-1 and was unaffected when chlorophyll a concentrations nearly tripled, indicating that bloom dilution could occur as a result of a limited amount of MeHg. The calculated algal MeHg concentration was 3-10 pmol g-1 (dry weight). As the bloom decayed, dissolved MeHg concentrations significantly (p = 0.04) increased, likely due to MeHg remineralization from decaying phytoplankton and/or production in sediments. By creating suboxic conditions in surface sediments and stimulating microbial activity, decomposing phytoplankton could bolster MeHg production, a potential side effect of large blooms. Unlike dissolved MeHg, dissolved HgT concentrations were not measurably altered by the bloom or decay factors. That difference corroborated previous culture studies in which phytoplankton actively accumulated MeHg, but not HgT. As the bloom decayed, HgT Kd values significantly (p = 0.012) increased, possibly because particles (i.e., phytoplankton) with low HgT concentrations were lost from the water column. Based on the relationship between HgT particulate concentrations and percent phytoplankton, the calculated algal HgT concentration was ~0.5 nmol g-1 (dry weight)

Contrasting Biogeochemistry of Six Trace Metals during the Rise and Decay of a Spring Phytoplankton Bloom in San Francisco Bay

The spring 2003 phytoplankton bloom in South San Francisco Bay (South Bay) affected the cycling of Mn, Co, Zn, Ni, and Pb, but not Cu. We followed this diatom bloom for 2 months, capturing a peak in chlorophyll a (Chl a) of \u3e150 µg L-1 and then an increase in dissolved organic carbon of \u3e400 µmol L-1 as phytoplankton decomposed. To determine how the stages of the bloom affected metal concentrations, we used principal component analysis to reduce our 15 water chemistry variables into a bloom factor, a sorbent factor, and a decay factor. Increasing values of the bloom factor, which was a composite of dissolved oxygen, Chl a, and other variables, significantly accounted for reductions in dissolved Mn, Ni, and Pb. We attributed those declines to microbial oxidation, phytoplankton uptake, and sorption onto phytoplankton, respectively. In contrast, dissolved Cu concentrations were not explained by either the bloom or decay factors, consistent with previous studies showing its strong organic complexation and limited bioavailability in South Bay. The decay factor significantly accounted for increases in dissolved Mn, Co, Zn, and Pb. Decomposing bloom material presumably caused suboxic conditions in surface sediments, resulting in release of metals to overlying waters during reductive dissolution of Mn and Fe (hydr)oxides. These alterations in metal cycling during a nutrient-enriched bloom were evidence of eutrophication. Annually, phytoplankton productivity has the potential to affect metal retention in the estuary; in 2003, 75% of Ni discharged into lower South Bay by wastewater treatment plants was cycled through phytoplankton

Porewater Gradients and Diffusive Benthic Fluxes of Co, Ni, Cu, Zn, and Cd in San Francisco Bay

In order to determine the remobilization of Co, Ni, Cu, Zn, and Cd from the sediments, gradients of their dissolved (< 0.45 pm) and particulate concentrations were measured in both relatively pristine and contaminated sediments of San Francisco Bay, as well as relatively pristine adjacent embayments (Tomales Bay and Drakes Estero). Three mechanisms were determined to regulate the diage- netic release of those elements to porewaters: (i) the degradation of organic matter in oxic and suboxic zones; (ii) the reduction of Mn in suboxic zones; and (iii) the formation of soluble metal-sulfide complexes in anoxic zones. While the estimated diffusive benthic fluxes of Ni, Cu, and Cd were relatively small (< 10%) compared to their riverine fluxes, the estimated diffusive benthic fluxes of Co and Zn were similar (« 100%) to their riverine inputs. These initial estimates also indicate that the total (dissolved and particulate) benthic remobilization of Co, Ni, Cu, and Zn is greater than either riverine or point source inputs of those elements to San Francisco Bay. Moreover, those benthic inputs are projected to become more significant as the anthropogenic inputs of those elements, from both point sources and non-point sources, are reduced

Total and Monomethyl Mercury in Fog Water from the Central California Coast

[1] Total mercury (HgT) and monomethyl mercury (MMHg) concentrations in fog collected from 4 locations in and around Monterey Bay, California during June-August of 2011 were 10.7 ± 6.8 and 3.4 ± 3.8 ng L−1respectively. In contrast, mean HgT and MMHg concentrations in rain water from March-June, 2011 were 1.8 ± 0.9 and 0.1 ± 0.04 ng L−1 respectively. Using estimates of fog water deposition from 6 sites in the region using a standard fog water collector (SFC), depositions of HgT and MMHg via fog were found to range from 42–4600 and 14–1500 ng m−2 y−1, which accounted for 7–42% of HgT and 61–99% of MMHg in total atmospheric deposition (fog, rain, and dry deposition), estimated for the coastal area. These initial measurements suggest that fog precipitation may constitute an important but previously overlooked input of MMHg to coastal environments. Preliminary comparisons of these data with associated chemical, meteorological and oceanic data suggest that biotically formed MMHg from coastal upwelling may contribute to the MMHg in fog water