1. 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?

2. Solubility of O2 as a function of T and salinity
A B
A B
A B A

3. “Free Water” productivity measurements
Temperature (yellow)
Oxygen (blue)
“algae” (green)
“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 L&O Methods 6: 454
Solomon, Bruesewitz et al L&O 58(3): 849

4. 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

5. 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

6. 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

7. Photosynthesis/ Respiration

8. 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

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

11. 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 ( 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.

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

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

14. pe-pH (Latimer) diagrams

17. 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)

18. 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

19. 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

20. Stakeholder Perceptions of Water Quality

21. Great Pond: Extent of anoxia

23. hv hv

24. 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)

25. Great Pond Oxygen 2015

26. 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!

27. 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

29. Great Pond
2016

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

33. What have we learned?

34. Biogeochemical cycles

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

36. Life as a driving force

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

38. Geoengineering: Good idea?