An Introduction to Environmental Chemistry

The Chemistry of Continental Waters 173

Atmosphere

Water column Sediments

Predatory

fish

Smallfish

Zoo-

plankton

Phyto-plankton

Wet

deposition

Dry

deposition

Methylation

Source

Oxidation

Hg

2+

Hg

0

Hg

0

Oxidation

Hg

2+

Hg

0

Hg

2+

CH

3

Hg

+

Hg

0

Hg (CH)

3

(Methylation)

S

S

CH

3

Hg

+

ORG

V

V

V

Fig. 5.15

Cycling of mercury in lakes. Ionic mercury (Hg

2 +) is readily methylated by bacteria, especially sulphate-reducing

bacteria in reducing sediments. Methyl mercury (CH

Hg 3

+) enters the food chain through phytoplankton and is biomagnified in

fish. Elemental mercury (Hg

0 ) is quite insoluble except when complexed by dissolved organic ligands (Hg

0 ∫

org), making it more

mobile and susceptible to oxidation in the water column. Pathways labelled S represent sedimentation, pathways labelled Vrepresent volatilization.