EVPP 550 Waterscape Ecology and Management Professor R. Christian Jones Fall 2007.

EVPP 550Waterscape Ecology and

Management

Professor R. Christian

JonesFall 2007

Water Chemistry – CO2, alk, pH

• Global carbon cycle includes:– Photosynthesis– Respiration– Fossil Fuel

combustion– Ocean

interactions– Rock

interactions (over long term)


• 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

Ice core data

Direct Measurements


• 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+


• 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• 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


• 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 – 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


• 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


• 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

CO2 + H20 ↔ H2CO3 ↔ HCO3- + H+ ↔ CO3

-2 + H+

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


• 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


• 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)


• 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 - 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.


• Forms– Equilibrium

between ammonia nitrogen forms is a function of temperature and pH


• 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


• 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


• 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


• 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


• 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 -

Nitrogen

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


• 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


• 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


• P cycle in lakes


• P profiles in various lakes

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


• 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)


• 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


• However, when anoxic conditions set in, the Fe(OH)3 dissolves and PO4-3 can be rapidly released fueling algal growth

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

EVPP 550 Waterscape Ecology and Management Professor R. Christian Jones Fall 2007.

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Transcript of EVPP 550 Waterscape Ecology and Management Professor R. Christian Jones Fall 2007.