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

Abstract

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 5 0.03) decrease in dissolved (,0.45 mm) MeHg. Dissolved MeHg was depleted to 0.026 pmol L 21 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 21 (dry weight). As the bloom decayed, dissolved MeHg concentrations significantly (p 5 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 5 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 ,0.5 nmol g 21 (dry weight).

The Great Salt Lake's monimolimnion and its importance for mercury bioaccumulation in brine shrimp (Artemia franciscana)

“…For example, Pickhardt et al (2002) found a negative correlation between phytoplankton density and Hg concentrations in zooplankton in experimental mesocosms where nutrients were added to some treatments to stimulate algal growth. Others have found that high algal abundance in fresh (Chen and Folt 2005;Chen et al 2005) and estuarine (Luengen and Flegal 2009) waters can dilute Hg concentrations in phytoplankton (Luengen and Flegal 2009) and subsequently in fish (Chen et al 2005;Karimi et al 2007). The deep brine layer in the Great Salt Lake has very high concentrations of dissolved inorganic nutrients (Wurtsbaugh and Berry 1990).…”

“…However, the unusual nature of Great Salt Lake water, particularly the deep brine layer, complicates the interpretation. Both chloride (85 g L 21 ) and DOC (42 mg L 21 ) are very high in the mixed layer, and both of these influence Hg speciation and uptake, but not always in predictable ways (Aiken et al 2003;Pickhardt and Fisher 2007;Luengen et al 2012). Although DOC can help maintain Hg in solution, its reactivity and concentration influence biotic uptake of MeHg (Gorski et al 2006).…”

Conceptual Model of Sedimentation in the Sacramento–San Joaquin Delta

“…Many contaminants are associated with sediment (Turner and Millward 2002;Schoellhamer and others 2007a;Luengen and Flegal 2009). Suspended sediment moving into, within, and out of estuaries provides a pathway for the transport of sedimentassociated contaminants (Turner and others 1999;Turner and Millward 2000;Bergamaschi and others 2001;Le Roux and others 2001).…”

Seasonal Variation of Mercury Associated with Different Phytoplankton Size Fractions in Lahontan Reservoir, Nevada

“…Algal bloom dilution, however, can occur for different reasons: rapid growth such that the growth rate becomes larger than the rate of Hg sequestration in phytoplankton, preferential partitioning of Hg to non-living particles (Laurier et al 2003;Schāfer et al 2006;Luengen and Flegal 2009), or limited supply of dissolved Hg species such that uptake depletes water column concentrations during early portions of a bloom (Luengen and Flegal 2009).…”