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

Abstract

Sampling is conducted during 2006 in Lahontan Reservoir, Nevada to investigate seasonal variation of total mercury (THg) and methylmercury (MeHg) partitioning in different phytoplankton size fractions as a function of point source (fluvial) mercury (Hg) loads, reservoir residence time, and algal growth. Carson River Hg inputs into the reservoir are extremely dynamic with spring loads two orders of magnitude larger than summer loads. Chlorophyll a measurements show two periods of algal growth. A small amount of algal growth occurs March to May. A second more substantial bloom occurs in the late summer, which is dominated by large, filamentous algae. THg concentrations (C b ) and partitioning coefficients (K d ) in total suspended particulate matter (SPM) are highest when fluvial inputs of Hg-contaminated sediment are large and are not necessarily associated with living biomass. However, MeHg K d in the small size fraction is indirectly related to fluvial loads and more strongly associated with living biomass in the later portion of the summer when algal growth occurs and reservoir residence times are longer. Data suggest size distinction is important to MeHg partitioning in the reservoir. Lumping all sizes into a single SPM sample will bias the analysis toward low MeHg C b and low MeHg K d in late summer when Aphanizomenon flos-aquae dominates the phytoplankton assemblage.

Effects of phytoplankton cell size and chloride concentration on the bioaccumulation of methylmercury in marine phytoplankton

“…Overall results suggest the predominance of nano‐ and pico‐phytoplankton over micro‐phytoplankton would lead to heavy MeHg accumulation in planktonic and planktivorous communities. While this hypothesis has not been tested with seawater, Carroll et al () reported that particle–water partition coefficients for MeHg and biomass MeHg concentrations were greater for smaller particles than larger ones in a freshwater reservoir, indicating that small and more palatable phytoplankton are important mediators for MeHg transfer to higher trophic organisms. Various environmental factors, such as nutrient input rate, water residence time, mixed layer depth, and light intensity, are known to affect the cell‐size distribution of the phytoplankton community in seawater (Parsons and Takahash,; Thomas et al,).…”

Interactions between mercury and phytoplankton: Speciation, bioavailability, and internal handling

“…High phytoplanktonic Hg content can be found in contaminated areas, notably those impacted by metal mining . In the Carson River–Lahontan Reservoir (NV, USA), classified as a Superfund site, phytoplankton were found to contain from 30 nmol to 254 nmol Hg g −1 dry weight and 1 nmol to 28 nmol CH 3 Hg g −1 dry weight .…”

“…High phytoplanktonic Hg content can be found in contaminated areas, notably those impacted by metal mining [88,89]. In the Carson River-Lahontan Reservoir (NV, USA), classified as a Superfund site, phytoplankton were found to contain from 30 nmol to 254 nmol Hg g À1 dry weight and 1 nmol to 28 nmol CH 3 Hg g À1 dry weight [90]. Although these accumulation values are useful for assessing the extent of Hg contamination in natural waters, they should be interpreted with care.…”

Modeling the Highly Dynamic Loading of Mercury Species in the Carson River and Lahontan Reservoir System, Nevada

“…Additionally, transient events such as pulses of large loads are simulated to propagate through the system with attenuation over time and downstream movement; while dilution occurs near the dam with influxes from the Truckee Canal. Few data exist in the reservoir, with data collected only at FPO (Carroll et al ., ) or LDM by the USGS. All post‐flood reservoir data were collected during the summer of 2006, with data at LDM collected at three discrete depths and vertically volume‐weighted to compare with model output.…”

“…Ongoing work is looking at bioaccumulation within the reservoir and the impacts of both reducing DMeHg loading from the river on predicted response as well as decoupling the timing of DMeHg influx from peak bioenergetics (i.e., consumption) of Hg enriched phytoplankton (Carroll et al, 2011). In addition, we are currently driving the CRLR transport model with hydrologic predictions (USBR, 2011) based on 112 down-scaled climate projections from the World Climate Research Programme Coupled Model Intercomparison Project 3.…”