Transcript Factors Influencing Concentration of Methyl mercury in Freshwater

Mercury, Eutrophication, and
Fish Kills in S. Arizona Lakes
Benjamin Tanner
11-02-03
Arivaca Lake
Pena Blanca Lake
Don't Eat The Fish!!
By order of the Arizona Department of Environmental
Quality
In 1993, the Arizona Department of Environmental Quality
announced that water quality tests had shown a high level
of mercury in the lake, nearly three times higher than safety
standards set by the federal Environmental Protection
Agency. It is the first such notification ever issued in
Arizona in response to potential mercury health risks. All
traces of mercury are expected to disappear within this
decade and normal conditions will return.
Why is Mercury a problem at
Arivaca and Pena Blanca
Lakes?
• Elevated Hg concentrations in fish from oligotrophic
acidic lakes is a wide spread phenomenon caused by
the atmospheric deposition of Hg and the subsequent
methylation by anaerobic microorganisms in lake
sediments
• Nearby mine tailings may contribute to Hg influx
• Methyl mercury is the most readily bioaccumulated
Hg species in the food chain as a result of its affinity
for protein sulfhydryl groups
• Accumulates in fish
Factors Influencing Concentration of
Methyl mercury in Freshwater Fish
• Rate of Atmospheric Deposition
• Rate of Methyl mercury Production
– Anaerobic sediments
– Water Temperature
– Quantity of Organic Matter
– Quantity of Sulfate Present
– Low pH
– Quantity of Dissolved Organic Carbon
• Age of the Reservoir
• Structure of the Ecosystem
• Wetland Area within the Watershed
Mercury can be degraded by
microorganisms
• Photodegradation
• Microbial reductive demethylation is the
best-characterized demethylation
process. It is mediated by an enzyme,
organomercurial lyase, which breaks the
Hg-C bond to produce Hg(II) and a
reduced organic moiety
Connection to Eutrophication
• Anaerobic environments facilitate
methylation of mercury
• Eutrophic lakes and streams are
anaerobic
• Eutrophication can be anthropogenic in
origin
• Human activities potentially lead to
increased human risk
Eutrophication
• Caused by conversion of allochthonous
C to autocthonous C via algae bloom
• Influenced by nutrients in water/soil
• Algal association
• Fish Kills!
• Thanks to Yang Zhang 
Eutrophication in S. AZ lakes
• Natural component
• “green tide>>>green lake”
• Anthropogenic component
– Grazing
– Agriculture
• Interferes with prize bass fishing
• Cyclic in nature
Arivaca Lake Fish Kill
• Occurred in 1999
• Fish population reduced by 90%
– Recovering
• Attributed to natural cyclic processes
– Storm runoff, silt
• Some anthropogenic activities
implicated
– Heavy grazing
Pena Blanca Fish Kill
• October, 2003
• Attributed to heavy runoff, cloudiness
“Heavy silting from major storm runoff in the last
three decades has shrunk the usable lake size and,
because the waters are so eutrophic (wellnourished), shoreline vegetation proliferates. Habitat
improvements began in 1985 with the sinking of tree
limbs from a nearby pecan grove and weighted
Christmas trees from seasons past—both actions
designed to maintain a variety of structures and to
allay the silting and sterility that aging desert lakes all
ultimately face”.
Research Project
ABSTRACT
Generally, one can culture more bacteria from organic matter
rich environments than from those that are poor in organic
matter. In order to investigate the impact of organic content on
a desert lake, and more specifically to assess the impact of
runoff on eutrophication, concentrations of heterotrophic plate
count bacteria (HPC) were determined at different locations
within Arivaca Lake. Soil and water samples were collected at
various depths and in association with various microhabitats. It
was shown that concentrations of HPC bacteria were greatest in
sediment of the lake, which is notable considering the cultural
techniques employed. High concnetrations of HPC bacteria
were also observed in water closely associated with decaying
organic matter such as leaves. HPC bacteria were found in
lowest concentrations in shallow lake water near concrete.
Thus, massive runoff events, such as occur in Arizona on a
cyclic basis may be responsible for increased microbial activity,
which may eventually lead to eutrophication
Materials and Methods
• Samples were collected by sterile pipet into
1.5 mL collection tubes at 0, 6, 12, and 18
inches water depth from Arivaca Lake, and
assayed in duplicate
• Samples were also collected of sediment and
organic-associated water from a lake tributary
and assayed in duplicate by dilution, rather
than soil extraction
• HPC were cultured on R2A at 25C for 5 days
HPC/mL
1.E+07
1.E+06
1.E+05
1.E+04
1.E+03
1.E+02
1.E+01
1.E+00
0A
0B
6A
6B
12A
12B
18A
18B org1 org2 soil1 soil2
Conclusions
• Bacterial counts in sediment indicate high
concentrations of organic matter
• During fish kill events, mixing of bacteria by
runoff may contribute to toxicity
• Observation: There are far fewer bacteria in
the water of S. Arizona lakes (specifically
Arivaca) than there are bacteria associated
with soil or organic matter
Questions?