EVPP 550
Waterscape Ecology and
Management
Professor
R. Christian
Jones
Fall 2007
Water Chemistry – CO2, alk, pH
• Global carbon
cycle includes:
– Photosynthesis
– Respiration
– Fossil Fuel
combustion
– Ocean
interactions
– Rock
interactions
(over long
term)
Water Chemistry –
CO2, alk, pH
• 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 carbonatebicarbonate buffering
system that controls pH
in most freshwaters
Ice core data
Direct
Measurements
Water Chemistry – CO2, alk, pH
• 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
• 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
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 – 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 – CO2, alk, pH
• 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
CO2 + H20 ↔ H2CO3 ↔ HCO3- + H+ ↔ CO3-2 + H+
• 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
• 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
• 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
• 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
• 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.
Water Chemistry - Nitrogen
• Forms
– Equilibrium
between
ammonia
nitrogen
forms is a
function of
temperature
and pH
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 - 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 - 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 - 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 - 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 - Nitrogen
Water
Chemistry Nitrogen
Water Chemistry - Nitrogen
Water Chemistry - Nitrogen
Water
Chemistry Nitrogen
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 phosphatecontaining sedimentary
rocks
– Guano beds, bone
skeletons
– Human and animal waste,
detergents
Water Chemistry - Phosphorus
• 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
• 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
• P cycle in lakes
Water Chemistry - Phosphorus
• P cycle in lakes
Water Chemistry - Phosphorus
• 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
Water Chemistry - Iron
• 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
• 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
• However, when
anoxic conditions set
in, the Fe(OH)3
dissolves and PO4-3
can be rapidly
released fueling algal
growth
Water Chemistry - Iron
• However, when
anoxic conditions set
in, the Fe(OH)3
dissolves and PO4-3
can be rapidly
released fueling algal
growth
Water Chemistry
- Iron
• 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