Lecture 4. Chemistry and Biogeochemical CyclingA. Chemicals Dissolved in WaterIn addition to water (and its component hydrogen and hydroxyl ions) lakes typically contain a variety of dissolved substances, including gases (e.g. carbon dioxide, nitrogen and oxygen) and dissolved salts (e.g. various ions). Most Common Ions Other Ionscalcium, Ca+2sodium, Na+magnesium, Mg+2potassium, K+bicarbonate, HCO3-sulfate, SO4-2chloride, Cl-nitrate, NO3-phosphate, PO4-3silicate, SiO4-2Salinity (mg L∙-1) is defined as the total concentration of ions dissolved in water and is measured as specific conductance (conductivity), the ability of water to conduct electricity. Conductivity increases as the salt content of the water increases. A similar expression, total dissolved solids, includes inorganic and organic salts plus nonionized substances. Finally, hardness refers to the levels of calcium and magnesium in the water.B. Solubility of GasesAtmospheric gases, such as carbon dioxide, nitrogen and oxygen dissolve in water.[T] Bottle of cokeWater can hold more of these gases under pressure and at lower temperatures. At the temperatures commonly experienced in lakes (0-35 C) the oxygen concentration varies by more than a factor of 2.[T] Oxygen = f (temperature); Dodson, Figure 2.4, p. 33Water can hold several times its saturation concentration. Photosynthetic activity when the water column is experiencing quiescent conditions can lead to supersaturation. A condition of supersaturationis ultimately relaxed through the formation of bubbles.[T] Bottle of coke[T] Bubbles on algaeThe solubility of gases also depends on the concentration of solutes, i.e. salts, in the water.[T] Salting out effectThe saturation concentration of oxygen at 4 C is 13.1 mg L∙-1 in freshwater and 8.6 mg L∙-1 in salt water. B. pH, Alkalinity and the Carbonate SystemWater molecules dissociate yielding hydrogen ions (H+) and hydroxyl ions (OH-),2H O H OH+ -� +The ion product (Kw) for water, 141 10 @ 25wK H OH x C+ - -� �� �= =� �� �oand, since the concentration of hydrogen ions must equal the concentration of hydroxyl ions, the concentration of each is 10-7. Remembering that pH is defined as –log[H+], we find that pure water would have a pH of 7 at 25 °C. Depending on the concentrations of other chemicals in water, the hydrogen ion concentration (pH) may vary widely. While the majority of temperate and tropical lakes have a pH between 7 and 9, bog lakes, rich in organic acids, can have pH levels of 2-6 and desert lakes, saturated with carbonates, can reach a pH of 10. pH plays an important role in mediating water chemistry and in establishing the limits of growth for aquatic organisms. Dissolved inorganic carbon (DIC) is a collection of chemical species that includes carbon dioxide, carbonic acid, bicarbonate and carbonate and which is often referred to as the carbonate system. DIC enters lakes through the dissolution of carbon dioxide from the atmosphere and carbonate minerals in the watershed. When carbon dioxide dissolves in water it participates in a series of reactions yielding the various inorganic carbon species –2 2 2 3 3 3CO H O H CO H HCO H CO+ - + =+ � � + � +The position along this reaction continuum depends on the scarcity of hydrogen ions. Initially, carbonic acid is formed. If hydrogen ions are scarce, the carbonic acid will dissociate to form bicarbonate ion. Further scarcity of hydrogen ions leads to the dissociation of bicarbonate ion to form carbonate ion. Thisis an equilibrium reaction and the relative abundance of the various inorganic carbon species varies with pH.[T] Dissolved inorganic carbon = f (pH); Dodson, Figure 10.2, p. 234Because this is an equilibrium reaction, the extraction of carbon dioxide through photosynthesis causes the reaction to move to the left, consuming hydrogen ions and raising the pH. This also increases the carbonate ion concentration, sometimes to the point that the solubility product of calcium carbonate is exceeded, precipitating a white, chalk-like material called marl. In lakes, such precipitation events are termed a whiting. [T] Whiting; Dodson, Figure 10.3, p.236At night (and in the hypolimnion) photosynthesis is absent and respiration leads to a net production of carbon dioxide, lowering the pH. The interplay of photosynthesis and respiration forms what is called a diel cycle, not only in pH, but in oxygen as well (production, photosynthesis; consumption, respiration).Acids enter the water through the dissolution of carbon dioxide and through the deposition of nitric acid (internal combustion engines) and sulfuric acid (coal combustion). The ability to absorb hydrogen ions when acid is added is termed acid neutralizing capacity (ANC) or alkalinity which is defined as –3 3[ 2[ ] [ ] [ ]Alk HCO CO OH H- = - += + + -Waters that are able to absorb significant amount of acids without changing pH are said to be well buffered. The inorganic carbon system is the primary buffering system in lakes, although organic acids may serve this role in bog lakes.Pure water in contact with the atmosphere becomes a weak solution of carbonic acid as it takes up carbon dioxide from the air, yielding a pH of 5.65. Here, some of the carbonic acid has broken down intobicarbonate and hydrogen ions, but at this pH bicarbonate levels are low and carbonate levels negligible.[T] Dissolved inorganic carbon = f (pH); Dodson, Figure 10.2, p. 234This water has a low alkalinity (ANC) and poor buffering capacity. Alkalinity is added to lakes when carbonates are dissolved from limestone (CaCO3) bedrock in the watershed. This increases the ANC of the lake and makes it less vulnerable to the impacts of acid deposition. Watersheds dominated by granitic bedrock provide little additional ANC and it is these that experience the negative effects of acid deposition. Here, the acids in rain titrate the lake water, reducing the lake’s buffering capacity over time until it is exhausted and the pH drops rapidly. D. Organic Carbon In addition to DIC, carbon is present in lakes as organic carbon (DOC). The energy that governs the metabolism of lakes is derived from the solar energy utilized in photosynthesis and fixed in molecules of organic carbon. The organic carbon in lakes may originate from terrestrial vegetation within the watershed (allochthonous sources) or from aquatic vegetation (algae, macrophytes) within the lake