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

Eutrophication In a normal lake, O2 is higher in the hypolimnion in summer In a eutrophied lake, O2 is low in the hypolimnion due to excessive decay of organic matter (BOD)

What controls growth? Limiting nutrients: N and P Biological pump cycles nutrient levels seasonally At surface, nutrients decrease in summer during growth, replenished in fall when lake mixes

What controls algae growth? Limiting nutrients: N and P Expected ratio C:N:P = 106:16:1 N often limiting in estuaries and oceans P usually limiting in lakes Increased inputs from fertilizer and detergent runoff cause increased growth  eutrophication

Stakeholder Perceptions of Water Quality

Great Pond: Extent of anoxia 1983 2015

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HOT (78o F) COLD (50o F) Great Pond Temperature (ºC) – Summer 2015 GP II III match color scale to all plots: breaks=c(0, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 30)) contour(dat.krg,"var1.pred", add=TRUE, nlevels=11, col="#656 COLD (50o F)

Great Pond Oxygen 2015

Great Pond Oxygen Concentration (ppm) – Summer 2015 NORMAL Great Pond Oxygen Concentration (PPM) Blue = less, Red = more “2” indicates 2 ppm boundary on the contour 2 Low – DEAD!

Phosphorous mobilization When lake is oxic, PO43- is bound to metal oxides in sediments Metals are generally more soluble at lower oxidation states (Fe2+ vs. Fe3+) and low pH Anoxia leads to mobilization of P and metals

Great Pond 2016

What do we do about it? A big environmental chemistry experiment – Summer 2018!!!

What have we learned?

Biogeochemical cycles

Timescales of Global Processes biosphere: hundreds of years atmosphere: days to a year lithosphere: millions of years oceans: thousands of years

Life as a driving force

What do we do about it? A big environmental chemistry experiment – Summer 2018!!!

Geoengineering: Good idea?