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EVPP 550 Waterscape Ecology and Management. Professor R. Christian Jones Fall 2007. Global carbon cycle includes: Photosynthesis Respiration Fossil Fuel combustion Ocean interactions Rock interactions (over long term). Water Chemistry – CO 2 , alk, pH. Water Chemistry – CO 2 , alk, pH.

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evpp 550 waterscape ecology and management

EVPP 550Waterscape Ecology and Management

Professor

R. Christian Jones

Fall 2007

water chemistry co 2 alk ph
Global carbon cycle includes:

Photosynthesis

Respiration

Fossil Fuel combustion

Ocean interactions

Rock interactions (over long term)

Water Chemistry – CO2, alk, pH
water chemistry co 2 alk ph1
Water Chemistry – CO2, alk, pH

Ice core data

  • Earth’s atmosphere contains relatively small amounts of CO2 as compared to O2
  • But the amount has increased greatly over the past several decades
  • As a greenhouse gas, CO2 is a major factor in the warming of Earth surface temperatures
  • CO2 is also intimately involved in the carbonate-bicarbonate buffering system that controls pH in most freshwaters

Direct Measurements

water chemistry co 2 alk ph2
Carbon dioxide dissolves in water to produce carbonic acid

Carbonic acid dissociates to produce bicarbonate and hydrogen ion (1st dissociation of carbonic acid)

Bicarbonate dissociates to produce carbonate and another hydrogen ion (2nd dissociation of carbonic acid)

CO2 + H20 ↔ H2CO3

H2CO3 ↔ HCO3- + H+

HCO3- ↔ CO3-2 + H+

Water Chemistry – CO2, alk, pH
water chemistry co 2 alk ph3
pH = -log [H+]

pH is the negative log of the hydrogen ion concentration

pH = 4 means [H+] = 10-4

pH = 7 means [H+] = 10-7

pH = 10 means [H+] = 10-10

Water Chemistry – CO2, alk, pH
water chemistry co 2 alk ph4
Water Chemistry – CO2, alk, pH
  • The relative amounts of carbonate, bicarbonate, and carbon dioxide-carbonic acid change with pH in a predictable manner based on dissociation equations
  • At high pH, carbonate dominates
  • At intermediate pH, bicarbonate dominates
  • At low pH, carbon dioxide-carbonic acid dominates
water chemistry co 2 alk ph5
Water Chemistry – CO2, alk, pH
  • Alkalinity is the ability of water to resist acidification
  • If the carbonate-bicarbonate system is the major buffer, then pH change can be resisted as long as bicarbonate and carbonate are present since they can absorb hydrogen ions
  • Alkalinity = [HCO3-] + 2 x [CO3-2]
water chemistry co 2 alk ph6
Water Chemistry – CO2, alk, pH
  • pH of rain in equilibrium with atmospheric CO2 is about 5.5
  • Pollutants such as sulfate and NOX decrease it futher
  • The total amount of alkalinity in a given water body is based, not only on the input of CO2 from the atmosphere, but even more so on sources of carbonate and bicarbonate from the watershed
water chemistry co 2 alk ph7
For some purposes we need to know the total amount of dissolved inorganic carbon (DIC) in a water body

This determines the carbon available for photosynthesis and also is needed to calculate the photosynthetic rate using the C-14 method

DIC = [H2CO3] + [HCO3-] + [CO3-2]

Based on equations in handout, if we know pH, alkalinity, and temperature, we can derive total DIC and concentration of all forms of DIC

Water Chemistry – CO2, alk, pH
water chemistry co 2 alk ph8
Effect of photosynthesis on pH and carbonate system

Effect of respiration on pH and carbonate system

Psyn consumes CO2, equilibrium shifts to left resulting in consumption of H+ and increase in pH

Resp releases CO2, equilibrium shift to left resulting in release of H+ and decrease in pH

Water Chemistry – CO2, alk, pH

CO2 + H20 ↔ H2CO3 ↔ HCO3- + H+ ↔ CO3-2 + H+

water chemistry co 2 alk ph9
Water Chemistry – CO2, alk, pH
  • Vertical profiles of pH
water chemistry dissolved ions
Sources

Atmosphere

Soil/rocks

Dissolution

Weathering

Sediments

Measurement

Total Dissolved Solids (TDS)

aka Filterable Residue

Gravimetric procedure

Filter substantial volume of water, then evaporate filtrate until constant weight

Problems: some residues are volatile and some retain water

Water Chemistry – Dissolved Ions
water chemistry dissolved ions1
Range: 1 mg/L to 300,000 mg/L (saturated brine)

Equivalent to 0.001 – 300 ppt

Fresh water: < 1 ppt

Ocean: 35 ppt

Great Salt Lake: 220 ppt

Water Chemistry – Dissolved Ions
water chemistry dissolved ions2
Conductivity

Measures the ability of water to conduct an electrical current

Proportional to the number of ions in solution

Pure water has a very low conductance (<0.1 umho/cm = uS/cm)

Conductance is a rough measure of TDS which can be calibrated more accurately for a given waterbody

Conductivity

Is a function of temperature so values need to be standardized to a given temperature, usually 25oC

Conductivity increases by a factor of about 0.025 per oC

So to get Specific Conductance (Conductivity standardized to 25oC):

Cond(25oC) = Cond (T) x 1.025^(25-T)

Water Chemistry – Dissolved Ions
water chemistry dissolved ions3
Anions

CO3-2 and HCO3- (70-75% by wt)

SO4-2 and Cl- also important

Cations

Ca+2 (60%)

Mg+2 (15-20%)

Na+ (15-20%)

K+ (5-10%)

Alkalinity

[CO3-2] + [HCO3-]

Acid buffering capacity

Hardness

[Ca+2] + [Mg+2]

Reaction to soap

More soap required in hard water because Ca and Mg tie some of it up

Water Chemistry – Dissolved Ions
water chemistry nitrogen
Water Chemistry - Nitrogen
  • Forms
    • N2 = dissolved molecular nitrogen
    • NH4+, NH3, NH4OH = ammonia nitrogen
    • NO2- = nitrite ion
    • NO3- = nitrate ion
    • Organic nitrogen: includes proteins, amino acids, urea, etc.
water chemistry nitrogen1
Water Chemistry - Nitrogen
  • Forms
    • Equilibrium between ammonia nitrogen forms is a function of temperature and pH
water chemistry nitrogen2
Water Chemistry - Nitrogen
  • Transformations
    • Nitrogen fixation
      • N2 → reduced organic N (like amino acid)
      • Three groups of organisms can do this
        • Aerobic and anaerobic heterotrophic bacteria use organic matter as energy substrate/important in sediments
        • Cyanobacteria use light as energy source/important in open water/done in heterocysts/may occur in large blooms in midsummer in enriched lakes
        • Purple photosynthetic bacteria use light as energy source, but need anoxic conditions
water chemistry nitrogen3
Water Chemistry - Nitrogen
  • Transformations
    • Nitrogen fixation
    • Rate of N fixation in water column is increased during N limitation
    • Rate of N limitation is related to light intensity implying that light energy is driving the process
water chemistry nitrogen4
Water Chemistry - Nitrogen
  • Transformations
    • Assimilation of combined nitrogen
      • NH4+ → reduced organic nitrogen (like amino acid)
      • NO3- → reduced organic nitrogen (like amino acid)
      • NH4+ is energetically more favorable as it is already reduced
water chemistry nitrogen5
Water Chemistry - Nitrogen
  • Transformations
    • Proteolysis or ammonification
      • Organic Nitrogen → NH4+
      • Proteolytic bacteria use energy released from this transformation for metabolism
    • Nitrification
      • NH4+ → NO2-
        • Nitrosomonas uses energy released for metabolism
      • NO2- → NO3-
        • Nitrobacter uses energy released for metabolism
        • Reaction occurs quickly so NO2- generally very low
water chemistry nitrogen6
Water Chemistry - Nitrogen
  • Transformations
    • Denitrification
      • NO3- → N2
      • Anaerobic/aerobic interface habitats such as mud-water interface
      • Active in sediments and wetlands, may greatly deplete NO3 in groundwater
water chemistry phosphorus
Importance to organisms

Nucleic acids

Adenosine Triphosphate (high energy PO4 bonds)

Bones and other solid inclusions

Sources

Erosion of igneous rocks

Dissolution of phosphate-containing sedimentary rocks

Guano beds, bone skeletons

Human and animal waste, detergents

Water Chemistry - Phosphorus
water chemistry phosphorus1
Forms of phosphorus

In biological systems and in water, almost all P is in the PO4 form

Can be individual PO4-3 ions or PO4 group can be combined with organic molecules, either dissolved or particulate

Analytic Forms

Phosphate ion aka orthophosphate aka soluble reactive phosphorus

Measured on filtered samples

Total soluble phosphorus

Measured on filtered sample after digestion

Total phosphorus

Measured on whole water samples after digestion

Water Chemistry - Phosphorus
water chemistry phosphorus2
Ortho-P

Only directly utilizable form of inorganic P

May be formed from organic P by enzymatic action

Reacts with other chemicals and adsorps to particles and elements like Fe

Organic P = Total P – Ortho P

Often most P in lakes is tied up in organisms or detritus

Can cycle between ortho-P and organic P

Water Chemistry - Phosphorus
water chemistry iron
Water Chemistry - Iron
  • Iron is a necessary requirement for all living organisms (enzyme systems)
  • Iron has two states
    • Fe+3 = ferric ion
      • Forms insoluble compounds
      • Found under oxic conditions
    • Fe+2 = ferrous ion
      • Is generally soluble
      • Found under anoxic conditions
water chemistry iron1
Even though generally insoluble in oxic epilimnion, Fe can be held there by chelators (compounds that weakly bind it to prevent precipitation, but may give it up to cells)Water Chemistry - Iron
water chemistry iron2
Generally, however, in oxic conditions Fe is found in a precipitated oxide form such as Fe(OH)3

These iron precipitates help to bind PO4 in the sediments and keep it from migrating into the water column

Water Chemistry - Iron
water chemistry iron3
However, when anoxic conditions set in, the Fe(OH)3 dissolves and PO4-3 can be rapidly released fueling algal growthWater Chemistry - Iron
water chemistry iron4
However, when anoxic conditions set in, the Fe(OH)3 dissolves and PO4-3 can be rapidly released fueling algal growthWater Chemistry - Iron
water chemistry iron5
However, when anoxic conditions set in, the Fe(OH)3 dissolves and PO4-3 can be rapidly released fueling algal growthWater Chemistry - Iron
water chemistry silicon
Water Chemistry - Silicon
  • Required for diatioms
  • Removed from the water column during diatom growth and sinking
  • May come to limit diatom growth during the growing season