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Chemical parameters of
freshwater habitats
Dissolved gases
• Principal gases in the atmosphere
Gas
%
Nitrogen (N2)
Oxygen (O2)
Argon (Ar)
Carbon dioxide (CO2)
78.084
20.946
0.934
0.038
• Oxygen and carbon dioxide are chemically and
biologically important
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Factors that affect solubility of gases in water
• Pressure: Solubility decreases with decreased
pressure, and vice versa
• Temperature: Solubility decreases with increased
temperature
• Salinity: Solubility decreases with increased salinity
Dissolved oxygen
• DO levels in water is one of the most important
biological and chemical variables
• Indicates health of an aquatic system
• Measured in unit (e.gs. mg/L, ppm) or percent
saturation in water
• Can range from 0-18 mg/L
• Most natural water systems require 5-6 mg/L to
support a diverse population
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• DO increases with decreasing temperature
• Organisms have direct oxygen tolerances
• Low oxygen leads to eutrophication
• Anoxic conditions release of chemically bound
toxicants
• Streams and rivers both produce and consume
oxygen
Main sources of DO in water
• Direct diffusion of atmospheric oxygen
• Photosynthetic activity of aquatic plants
• Phytoplankton in lakes
• Macrophytes, benthic algae in rivers and shallow waters
• Periphyton in rivulets and brooks
• Agitations of surface water agitations such as by
turbulent flow, waterfalls
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Exit of DO from inland waters
• Respiration by aquatic plants and animals
• Decomposition through bacterial activity through
chemical oxidation at the sediment-water interface
• Wind agitation in shallow decreases oxygen DO in
water due to the resuspension of sediments and
organic materials
• High mortality of aquatic organism a period of
senescence, producing increased oxygen
consumption, e.g. eutrophication
• High suspended matter reduces DO
Effects of reduced DO
• Decrease productivity
• Eutrophication
• Fish kill
• Change in community composition
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Carbon dioxide
• The most soluble gas
• As the partial pressure of carbon dioxide in air is low,
the amount which remains in solution in water at a
given temperature is also low
Sources of carbon dioxide in freshwater
• Direct dissolution of atmospheric CO2 in surface water
• CO2 dissolves in rainwater
• Inflowing ground water becomes charged with CO2 as
it passes through organic soil
• Decomposing organic matter release CO2
• Respiration of aquatic biota
Reduction of carbon dioxide in freshwaters
• Photosynthesis of aquatic plants
• Production of insoluble carbonates by some
organisms such blue-green algae, diatoms, molluscs
• Agitation release free carbon dioxide from water
• Evaporation of waters containing bicarbonates
• Bubbles from bottom of water bodies rise to the
surface to release free CO2
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Humic substances
• Present in all natural waters as dissolved
molecules, colloidal suspensions or particulate
matter
• Composition by weight: O (35-40%), H (4-5%), N
(2%)
• Dystrophic lakes have high concentrations of
humic substances
• Humic acids soluble in water under low pH
(below 2); fulvic acids soluble in water of all pH
conditions
TOM vs TOC
• TOM refers to all organic materials present in a
humic substance, including carbon, oxygen,
hydrogen and nitrogen: TOM = DOM + POM
• TOC refers to all carbon present in a humic
substance: TOC = DOC + POC
• The values of TOM, DOM, POM are twice as TOC,
DOC and POC alone
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Role of humic substances in natural waters
• Influence the availability of organic and inorganic
nutrients for bacteria, fungi, phytoplankton and
aquatic macrophytes
• Influence the complexation and immobilisation of
contaminants
• Influence absorption and bioavailability of
contaminants
pH
Ionization of water
• 2H2O (l) ⇋ H3O+ (aq) + OH- (aq)
• H2O (l) ⇋ H+ (aq) + OH- (aq)
• pH = - log10 [H3O+]
pH scale
• 1-14
• A pH range between 6.5-8.5 is generally suitable for aquatic
life
• The scale is logarithmic
• Meaning there is a ten-fold change in acidity or alkalinity
per unit change
• E.g. water with a pH of 5 is ten times more acidic than water
with a pH of 6
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pH determines
• The solubility of nutrients
• The solubility of metals
• The bioavailability of nutrients/metals for use by aquatic life
pH affected by
• Geology (old inert sandstones vs igneous with Ca/Mg + HCO3-)
• Organic acids from plant material
• Pollution, esp. mining
• WWTWs
• Algal bloom
• Bacterial activity
• Water turbulence
Dissolved salts
• Total dissolved solids (TDS): The measure of all
dissolved substances in water
• Salinity: The measure of sum of all the dissolved
salts in water
• Electrical conductivity (EC): Measure of the ability
of a sample of water to conduct an electrical
current
• An indicator of the salinity resulting from concentration
of salts, acids and bases
• Soluble ions dissolve in drainage basin, mostly
igneous and sedimentary rocks
• Dissolution of ions increases with temperature,
acidity, water flow and dissolved oxygen
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