Mercury, Eutrophication, and Fish Kills in S. Arizona Lakes

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Mercury, Eutrophication, and Fish Kills in S.
Arizona Lakes

• Benjamin Tanner
• 11-02-03

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Arivaca Lake
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Pena Blanca Lake
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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.
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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

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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

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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

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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

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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 ?

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Eutrophication in S. AZ lakes

• Natural component
• green tidegtgtgtgreen lake
• Anthropogenic component
• Grazing
• Agriculture
• Interferes with prize bass fishing
• Cyclic in nature

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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

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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
  (well-nourished), 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 pastboth actions designed to maintain a
  variety of structures and to allay the silting
  and sterility that aging desert lakes all
  ultimately face.

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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

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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

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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

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Questions?