Mercury Cycling in Stream Ecosystems

Three related U.S. Geological Survey (USGS) articles on mercury transport,
biogeochemical processes, and bioaccumulation in stream ecosystems have been
published in Environmental Science & Technology (ES&T).  The papers were
published electronically on the March 11, 2009 in the "Just Published
(ASAP)" web version of ES&T, and will be published in final form in the
April 15, 2009 print issue.

An ES&T news story (  <http://pubs.acs.org/doi/full/10.1021/es9005916>
http://pubs.acs.org/doi/full/10.1021/es9005916 ) highlights selected
findings from these papers.  

The USGS studied eight streams in Oregon, Wisconsin, and Florida during
2002-2006.  Streams in urban areas (near Portland, Oregon; Milwaukee,
Wisconsin; and Orlando, Florida), and streams in relatively undeveloped
areas in these states were included in the study.  The streams span a range
of environmental settings and watershed characteristics that can affect
biogeochemistry and bioaccumulation of mercury in streams, including
precipitation, mercury deposition rates, degree of urbanization, and wetland
abundance.  Findings from these studies can help decision makers to better
anticipate concentrations of mercury and methylmercury in unstudied streams
in comparable environmental settings.

Although all eight streams receive mercury predominantly via atmospheric
deposition, watershed characteristics primarily determine mercury transport
and bioaccumulation in these streams. Key factors include (1) the abundance
of wetlands, which influence how much of the atmospherically deposited
mercury is converted to methylmercury (the most toxic, bioaccumulative form
of mercury); and (2) runoff of dissolved organic carbon and suspended
sediment, which control how much mercury and methylmercury is delivered to
the streams.

Findings show that the relative amount of methylmercury in streams is
strongly correlated with streamflow and the production of methylmercury in
the watersheds (particularly in wetland areas), which is subsequently
transported in runoff to streams. An unexpected finding was that
methylmercury production in channel sediments appears to be relatively
unimportant for governing within-stream methylmercury levels.

Large differences in total mercury and methylmercury were observed in the
eight streams, varying by as much as 9-fold. Nationally, atmospheric inputs
(or "mass per unit area" of watershed) of mercury vary only about 4-fold.
Findings show, therefore, that some stream ecosystems are more sensitive to
atmospheric deposition of mercury than others.

Once methylmercury enters streams, it is available for uptake at the base of
the food web (algae and periphyton). Concentrations of methylmercury in
invertebrates and fish (forage and predator) were strongly and positively
correlated with concentrations of filtered total mercury and methylmercury
in stream water, as well as concentrations of dissolved organic carbon and
the extent of wetlands within a watershed.

Methylmercury concentrations increase with each trophic level in the food
chain. Findings suggest that mercury contamination in top predator fish in
the studied streams likely is dominated by the amount of methylmercury
available for uptake at the base of the food web, rather than by differences
in the trophic position of top predator fish. Increases within the food
chain appear to be similar in magnitude among areas and sites.

Publications and data from this study can be accessed electronically at
<http://water.usgs.gov/nawqa/mercury/pubs/> .  For questions on individual
papers, please contact the authors: Mark Brigham (mbrig...@usgs.gov,
763-783-3274), Mark Marvin-DiPasquale (mmar...@usgs.gov, 650-329-4442), and
Lia Chasar (lcha...@usgs.gov, 850-553-3649).