Mercury, Eutrophication, and Fish Kills in S. Arizona

Lakes

Benjamin Tanner11-02-03

                                                                                                   

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 (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 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

ABSTRACTGenerally, 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+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

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?