Seasonal Patterns of Mercury Species in Water and Plankton from Softwater Lakes in Northeastern Minnesota

• Published in: Biogeochemistry 1998-03, Vol.40 (2/3), p.147-162
• Main Authors: Monson, Bruce A., Brezonik, Patrick L.
• Format: Article
• Language: English
• Subjects: Bioaccumulation; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Freshwater; Freshwater fishes; Geochemistry; Growing season; Hydrology; Hydrology. Hydrogeology; Lake water; Lakes; Mercury; Mineralogy; Natural springs; Plankton; Pollution, environment geology; Seasonal variations; Silicates; Soil water; Spring; Statistical analysis; Summer; Water chemistry; Water geochemistry; Water samples; Zooplankton
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1023/a:1005967413585

Twelve softwater lakes in NE Minnesota were sampled in spring, summer, and fall of 1992 and 1993 for labile (unextracted) methyl-Hg, total (extracted) methyl-Hg, and total Hg in lake water and net plankton (≥300 μm). The lakes are small (5.6-56 ha), low productivity, headwater drainage or seepage lakes. They are acid-sensitive (ANC ≤200 μeq/L) but not low pH lakes (average pH 6.6). The lakes ranged in color from 8.5 to 70 PCU. Statistical analysis of the water chemistry variables and mercury species support the conclusion that these were a homogeneous set of lakes; therefore, seasonality of mercury forms was analyzed on combined (mean) data from the 12 lakes. Methyl-Hg in water declined throughout the growing season. HgT also declined sharply from spring to summer but increased again in the fall. In contrast to the methyl-Hg and Hg in water, concentrations in plankton were at the lowest levels in spring and rose to higher levels in summer. The mass of mercury in plankton increased from spring to fall, as did the methyl-Hg fraction, which increased from 20% of HgT in spring to 52% in autumn. Bioaccumulation factors (BAF) for methyl-Hg in net plankton increased over the growing season. Overall, log BAF for HgT in net plankton (wet wt.) was 4.45. Log BAF for methyl-Hg in plankton was 4.90 to 5.43 depending or the analytical form of methyl-Hg in water (labile or total). Seasonal patterns of methyl-Hg and HgT did not covary in water, but did covary in plankton. These results support the conclusion that measurement of Hg in water is not adequate in itself to determine the amount of bioavailable Hg (i.e., methyl-Hg) in a lake. Labile (unextracted) methyl-Hg could be a useful measurement of bioavailable Hg. Labile methyl-Hg exhibits the same seasonal patterns as total methyl-Hg, but does not require the extraction steps necessary for measuring total methyl-Hg.