Arsenic and Mercury Distribution in an Aquatic Food Chain: Importance of Femtoplankton and Picoplankton Filtration Fractions

Arsenic (As) and mercury (Hg) are toxic elements variably present in aquatic environments. The literature concerning these toxicants is dominated by work in marine environments, with comparatively fewer studies conducted in freshwater systems. In this study, As and Hg were examined in the Yellowstone Lake food chain, focusing on two lake locations: i) Inflated Plain, an area impacted by lake floor hydrothermal vents; and ii) Promontory Point, a location approximately 20 km south in deep waters and where there is no documented vent activity. Sampling spanned from femtoplankton to the main fish species, Yellowstone Cutthroat Trout and the apex predator Lake Trout. Hg bioaccumulated in muscle and liver of both trout species, biomagnifying with age, whereas As decreased in older fish, which indicates differential exposure routes for these metal(oid)s. Hg and As concentrations were higher in all food chain filter fractions (0.1, 0.8, 3.0 um filters) at the vent associated Inflated Plain site, illustrating the impact of localized hydrothermal inputs. Femtoplankton and picoplankton size biomass (0.1 and 0.8 um filters) accounted for 30-70% of total Hg or As at both locations. By contrast, only ~ 4% of total As and <1% of total Hg were found in the 0.1 um filtrate, indicating that comparatively little As or Hg actually exists as ionic forms or as intercalated with humic materials, which is a frequent assumption in aqueous filtration studies examining <3.0 um fresh or marine waters. 18S rRNA gene sequencing of DNA derived from the 0.1, 0.8 and 3.0 um filters showed significant eukaryote biomass in these fractions, providing a novel view of the femtoplankton and picoplankton size biomass, and assists in explaining why these fractions may contain such significant Hg and As. These results infer that femtoplankton and picoplankton metal(loid) loads, representing food chain entry points, need to be accounted for in aquatic food chain studies, and have important implications for better understanding of Hg and As biochemistry in aquatic systems.