Chemistry of Lakes What physical processes determine the structure of lakes? How does availability of gases, particularly oxygen, vary in lakes with depth and season? What chemical and biological processes affect the availability of oxygen? How do nutrients cycle in lakes, and how does oxygen availability affect this process? What is eutrophication, and how does the availability of oxygen and nutrients drive this process? What can we do to control eutrophication? What factors determine metal solubility in lakes?

Solubility of O2 as a function of T and salinity A1 -173.9894 B1 -0.037362 A2 255.5907 B2 0.016504 A3 146.4813 B3 -0.0020564 A4 -22.2040

“Free Water” productivity measurements Temperature (yellow) Oxygen (blue) “algae” (green) www.colby.edu “Free Water” productivity measurements Light (I) GPP – Gross Primary Productivity (function of light) Light (I) Gas exchange Wind (U) F – Gas Exchange Zmix R – Respiration Hanson et al. 2008 L&O Methods 6: 454 Solomon, Bruesewitz et al. 2013 L&O 58(3): 849

Lake stratification and mixing Lake mixes in spring, T is low, [O2] is high Surface warms in summer, lake becomes stratified (no mixing), [O2] remains high in hypolimnion

Oxygen profiles vs. season In spring, T is low, [O2] is high, lake is well mixed In summer, surface T is high, lake is stratified, surface [O2] is lower, [O2] is higher at depth

Redox reactions as a power source In an electrochemical cell, the two half reactions are separated Ions and electrons flow through the cell to complete the reaction Energy released by this reaction can be captured as a voltage and current (battery) In the environment, half reactions are not separated, but energy from redox reactions can still be harnessed

Photosynthesis/ Respiration

Assigning Oxidation States 1. The oxidation state of an atom in its elemental form is 0. The oxidation state of a monatomic (free) ion is equal to its charge. 2. The sum of the oxidation numbers of the atoms in any uncharged molecule is 0. The sum of the oxidation numbers of the atoms in a charged species (such as a polyatomic ion) is equal to the charge of the species. 3. Within compounds, the following rules apply in order : a.Alkali metals have oxidation number +1 (e.g., NaCl). b.Alkaline earth metals have oxidation number +2 (e.g., BaCl2). c.Hydrogen (H) has oxidation number +1, except in compounds with alkali metals or alkaline earth metals. d.Fluorine (F) has oxidation number –1. e.Oxygen (O) has oxidation number –2, except in compounds with fluorine, peroxides (O22-), superoxides (O2-) or ozonides (O3-). f.The other halogens have oxidation number –1, except in compounds with fluorine or oxygen. g. Nitrogen (N) has oxidation number -3 when bonded to only H and C h. Sulfur (S) has oxidation number -2 when bonded to only H and C

Biological Oxygen Demand (BOD) The number of miligrams of O2 required to carry out the oxidation of organic carbon in 1 L of water.

BOD example What is the BOD of a solution in which 10 mg of sugar (C6H12O6) is dissolved in 1 L of water? How does it compare with the solubility of O2 in water (8.7 mg/L @ 25 C)? BOD = 60 mg C6H12O6 / (180 g/mol) * 6 mol O2 / 1 mol C6H12O6 * 32 g O2 / 1 mol O2 = 10.7 mg/L = 10.7 mg/L This is more than the solubility of O2.

Biologically Mediated reactions pe(w) 13.8 11.3 7.2 5.8 4.4 -3.9 -3.3 -4.1 -8.2

Geochemical redox sequence Sequence of redox reactions in aqueous environments. As each oxidant is depleted, the oxidant with the next lowest E is used.

pe-pH (Latimer) diagrams