Aquatic Ecosystem Chemistry
Dissolved Oxygen (DO)
Temperature
Redox Potential (ORP)
Major ions (Hardness; Conductivity; Salinity/TDS)
Carbonate Buffer System (pH & Alkalinity)
Turbidity (Suspended Sediments)
Macronutrients (N, P, Si)
Organic Carbon (Particulate, Dissolved, Color)
The chemistry of natural waters reflects the watershed
surface and subsurface geology; weathering climate; and
biotic and human impacts within the watershed (terrestrial
and aquatic habitats).
Temperature
Greatly influences:
–
–
–
–
DO solubility; hence saturation point & content.
Solubility of other gases and solutes (weathering)
Speed of chemical and biological reactions.
Biological populations (directly and indirectly)
• Measured by:
– standardized thermometer or thermal conductor probe.
– Like DO; must consider time of day and daily range.
• Influenced by:
–
–
–
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Climate / Solar Radiation
Turbulent mixing with other water masses.
Shading (turbidity and adjacent terrestrial canopy)
Watershed dynamics (cover, sheet flow, groundwater)
Redox Potential (ORP)
• Oxidation Reduction Potential (ORP; Eh) reflects the relative
abundance of electrons available for chemical/biochemical
reactions in the environment.
• Measured by an electrochemical probe that compares the
capacity of the environment for donating (reduction) or
accepting (oxidation) electrons relative to that of a standard
platinum electrode for hydrogen (2H+ + 2e-→ H2 ;Eh = 0 mV).
• OIL RIG (Oxidation Is Losing, Reduction Is Gaining). This is
relative to the probes response.
• Oxidation reactions will be inhibited (thermodynamically
unfavorable) if the environment is reducing (electron rich).
Reducing Environment
(Low Redox Potential)
e-
e-
e-
e-
e- e
e- e
e- e e-
Fe+++ + e-
e- - e-ee
e-
e-
(ferric)
eee- - e- - e- e
e -e
e
Fe++
ee- e
(ferrous) - e
e
e- ee e
Oxidizing Environment
(High Redox Potential)
Oxygen Concentration and ORP
• When oxygen is present, redox potential is high:
– Lower numbers of free electrons in solution.
– Average Eh value in freshwater ~ + 500 mV
– As high as + 920 mV
• When oxygen is depleted to low levels, redox potential drops.
– Increased number of free electrons in solution.
– Without oxygen redox can get as low as -240 mV
• In lakes that stratify / mix, this can create dynamic redox
states and ionic concentrations:
Fe+++ + e-  Fe++
At normal redox
conditions, ferrous
oxidizes to ferric ion.
•
Ferric ion forms an
insoluble precipitate,
sinks to benthos.
•
Stratification occurs,
oxygen is lost in
hypolimnion.
•
Redox potential drops,
ferrous ion is reduced
from ferric in sediments.
•
Redox and Biotic Processes
• Oxidation reactions yield energy under oxidizing conditions.
• Reduction reactions yield energy under reducing conditions.
• Reverse reactions require energy inputs.
Yields
Energy
Requires
Energy
Microbe
Mediated Redox
Reactions
reduce to Mn (II)
reduce to Fe (II)
Oxidation and reduction reactions
are “Coupled”; one needs the other!
Many reductions here couple with
the oxidation of organic matter, even
when there is no O2 in the
environment.
The change in EH between arrow
start point between any oxidation
and reduction couple reflects the
relative energy yield of the reaction.
This would be ΔEH = +820 mV for
aerobic respiration; ΔEH = + 640 mV
for nitrate anaerobic respiration;
ΔEH = +220 mV for
methanogenesis.
CO2 reduce to CH4
Chemical Forms and Sources
• Elements and compounds occur in water in either
dissolved or particulate forms.
– A more recent classification is based on the ability of the
“particulate” compounds to settle out of solution:
• Colloidal = does not settle due to gravitational force
• Gravitoidal = does settle out of solution
• As water moves through terrestrial systems (surface,
groundwater, etc) new materials are entrained into
the water by weathering:
– Chemical weathering releases dissolved matter
– Mechanical weathering releases particulate matter
• Amount of runoff correlated to concentration of dissolved
materials;
• greater runoff = less time for dissolving
Chemical Weathering of Limestone
Carbon dioxide from
the atmosphere,
respiration and other
microbial
decomposition
reactions reacts with
water to form carbonic
acid.
Protons react with
calcium carbonate
(limestone) to form the
soluble salt calcium
bicarbonate.
Major Ions of Inland Waters
– Solute concentrations in aquatic
system is linked to abundance
AND solubility of ions in source
area (watershed).
– Biological processes can
incorporate some ions, thereby
reducing their relative
abundance in aquatic systems.
– Inland waters vary greatly!
Hardness:
• The capacity of water to precipitate soap.
• Total sum of Ca+2 and Mg+2 ions.
– Low concentration = “soft”
– High concentration = “hard”
• Measured by titration with EDTA (chelater).
Conductivity
* Ability for current to pass through water (conductivity)
depends on:
- Temperature
- Ion Content
* Current is proportional to ion content in water.
Inland Water Salinity (TDS)
Saline Lake Classification:
Chemistry (Dominant Anion or Cations)
Biological Factors (Salinity Ranges)
Origins of Solutes
Carbonate Buffer System
Carbonate Buffer System
Carbon dioxide reacts with water to form carbonic acid.
Depending on pH, carbonic acid will disassociate to bicarbonate and proton.
At higher pH bicarbonate may also disassociate to carbonate and proton.
More acidic will shift the reaction to carbonic acid (left here).
More basic will shift the reaction to carbonate (right here).
Why is it so soluble?
Is it a gas or is it a salt, or does it react to behave as both?
pH Determines Carbonate System
Proportions in Solution
Alkalinity: The ability to absorb protons (buffering capacity).
Turbidity & Suspended Solids
•
Turbidity: measured optically:
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–
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•
Secchi Depth
Spectrophotometer: (890 nm); linear path.
Nephlometrically: linear and 90º path.
Suspended Solids: measured gravimetrically
1)
2)
3)
4)
Filtration of known volume onto tarred filter.
Oven dry at 105ºC
Weight to 0.01 mg and subtract filter weight.
Divide sample weight by volume filtered (= mg SS/L).
With enough comparisons of both turbidity and SS over different load
regimes for a specific site, a relationship may be established. Thereafter,
SS may be inferred from turbidity, which is by far a simpler more timeefficient method.