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 (Hg(T)) 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 (<0.45 mu m) MeHg. Dissolved MeHg was depleted to 0.026 pmol L(-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 Hg(T) 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 Hg(T). As the bloom decayed, Hg(T)K(d) values significantly (p = 0.012) increased, possibly because particles (i.e., phytoplankton) with low Hg(T) concentrations were lost from the water column. Based on the relationship between Hg(T) particulate concentrations and percent phytoplankton, the calculated algal Hg(T) concentration was similar to 0.5 nmol g(-1) (dry weight).