Factors Affecting the Conc. of Mercury in Fish

Factors That Influence the Concentration of Methyl mercury (CH₃Hg) in Freshwater Species of Fish

Lakes receive mercury from the atmosphere mainly in its inorganic form.. This mercury is scavenged from surface water and deposited in the lake bottom sediment. The mass budget of mercury in a Wisconsin lake is shown below: Annual Atmospheric Input ~ 1.1 gram Deposited in Sediment ~ 1.0 gram Cycled Back to the Atmosphere ~ 0.1 gram Incorporated in the Lake Food Chain ~ .06 grams/year Mercury in the sediment can be transformed into methyl mercury by bacteria. A number of lake-specific factors influence the production of methyl mercury and its subsequent bioaccumulation in the food chain. Some of these factors are summarized below.
1. Rate of Atmospheric Deposition
	The rate of atmospheric deposition of mercury was not the same in the three lakes we studied. The fish in Clopper Lake, which had the greatest deposition of inorganic mercury, did not have the highest concentrations of CH₃Hg in their flesh. This was a contradiction of our hypothesis which stated that the greater the rate of deposition of inorganic mercury in a lake, the more CH₃Hg there would be in the fish. Droplet shown is approximately 1 gram.


2. Rate of Methyl mercury Production
	The rate of CH₃Hg production depends most on the rate of metabolic activity of bacteria living in the bottom sediment in lakes. The consequence of this activity is the transformation of inorganic mercury to CH₃Hg. The production rate is dependent on several factors related to lake temperature and chemistry. Some of these include: a) Water Temperature - As water temperature increases, bacterial activity increases and the quantity of CH₃Hg released into the water increases. Smaller, warmer lakes increase the rate of bacterial activity which increases the production of CH₃Hg. b) Quantity of Organic Matter - As the quantity of organic matter increases, bacterial activity increases and the quantity of CH₃Hg released into the water increases. c) Quantity of Sulfate Present - Since sulfate is used by the bacteria that produce CH₃Hg in lake bottom sediments in their metabolic process, an increase in sulfate stimulates the production of CH₃Hg. There is a connection between the sulfur present in acid rain, and an increase in the production of CH₃Hg in lakes. d) Low pH and Alkalinity - An increase in the acidity of lake water also increases the solubility of inorganic Hg making it even more available to the bacteria that produce CH₃Hg. The buffering capacity of bedrock and soil within a watershed has a strong effect on pH and alkalinity. This will, in turn, have an effect on the concentration of CH₃Hg in fish. e) Quantity of Dissolved Organic Carbon 1. DOC enhances the mobility of mercury and increases the export of CH₃Hg from wetlands in the watershed. .2. DOC tends to decrease the uptake of CH₃Hg by phytoplankton in the water column.
3. Age of the Reservoir
	The concentration of CH₃Hg in newer, man-made lakes tends to be higher because inorganic mercury has had hundreds of years to accumulate in the soil and vegetation. When newly created reservoirs fill with water, bacteria in the soil and vegetation quickly begin to convert it to its organic form. This effect typically lasts for a few decades.

4. Structure of the Ecosystem

Each additional trophic level in a lake ecosystem will increase the magnification of methyl mercury by a factor of 3-5 times. To calculate the magnification of methyl mercury as it moves through the food chain in a lake, it is necessary to know the Bioconcentration Factor (BCF) and the Biomagnification Factor (BMF). If the concentration of methyl mercury in both the water and the phytoplankton living in the water are known, the BCF can be calculated as shown:

BCF = [CH₃Hg] _{in the phytoplankton} / [CH₃Hg] _{in the water}

The increase in the concentration of methyl mercury from the water to the first link in the food chain is typically on the order of 1 x 10⁵. It is, by far, the single largest increase in concentration at any level in a lake ecosystem. The BMF can be calculated when the concentrations of Methyl mercury in organism at different trophic levels in a food chain are known.

BMF = [CH₃Hg] _{in the predator} / [CH₃Hg] _{in the prey}

In a typical lake ecosystem, the concentration of methyl mercury will increase by a factor of 3 to 5 as it moves from one level to the next up the food chain.

The total biomagnification of a lake ecosystem is multiplicative. That is, the magnification of CH3Hg as it moves through an ecosystem is the product of each of the separate magnifications from one level to the next. It can be calculated from the simple formula shown below:

Total System Biomagnification = (BCF)_phytopl x (BMF)_zoopl x (BMF)_{fish 1} x (BMF)_{fish 2}

(1x10⁵) x (5) x (5) x (5)

5. Wetland Area within the Watershed

Wetlands are sites of rapid bacterial activity due to their high organic matter content. The larger the area of wetlands in a watershed, the greater the chance that there will be higher levels of CH₃Hg in that watershed.

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