Aquatic Ecology
Freshwater - Part 4
Prof. Dr. N. De Pauw
Laboratory of Environmental Toxicology and Aquatic Ecology
Aquatic Ecology
Course Contents
1.
Place of limnology in natural sciences
2.
Historical development of limnology
3.
The water cycle, distribution, age and genesis of inland waters
4.
Structure and physical properties of water
5.
Physical relationships in natural water bodies
6.
Communities of living organisms in natural waters
7.
Materials budget in natural waters I
(= gases, solid and dissolved substances, importance of sediments)
8.
Materials budget in natural waters II
(= production, consumption, decomposition)
7. Materials budget of natural waters I
Contents (1)
7.1. Introduction
7.2. Dissolved gases and dissolved solids
7.3. Gases dissolved in water
7.3.1. Solubility of gases in water
7.3.2. Oxygen content and oxygen budget
7.3.3. Carbon dioxide, carbonic acid and carbonates
7.3.4. Methane and hydrogen sulphide
7.3.5. Nitrogen
7. Materials budget of natural waters I
Contents (2)
7.4 Solids dissolved in water
7.4.1. Solubility of solids in water
7.4.2. Nitrogen compounds
7.4.3. Phosphorous compounds
7.4.4. Sulphur compounds
7.4.5. Iron and manganese
7.4.6. Silica
7.5. Dissolved organic matter in natural waters
7.6. Sediment and the materials budget
7.7. Materials budget of flowing waters
7. Materials budget of natural waters I
7.1. Introduction
= Sum of materials and energy turnover in an ecosystem
FOUNDATIONS
1. Water as a solvent
2. Dissolved and particulate materials
3. Organisms in water
7. Materials budget of natural waters I
7.1. Introduction
Characterized by the following processes
1. Bio-activity of organisms
• Production
• Consumption
• Organisms in water
2. Chemical and biological transport of material + energy
• Into the sediment
• Release from the sediment
3. Transport of material + energy
• In lakes : seasonal rhythm
• In rivers : unidirectional transport
4. Exchange
• With atmosphere (precipitation)
• In and outflow
• Absorption and desorption (suspended particles)
7. Materials budget of natural waters I
Contents (1)
7.1. Introduction
7.2. Dissolved gases and dissolved solids
7.3. Gases dissolved in water
7.3.1. Solubility of gases in water
7.3.2. Oxygen content and oxygen budget
7.3.3. Carbon dioxide, carbonic acid and carbonates
7.3.4. Methane and hydrogen sulphide
7.3.5. Nitrogen
7.2. Dissolved gases and dissolved solids
Spatial and temporal distribution dependent on :
Hydrological factors
Chemical factors
• Precipitation
• Inflow and outflow
• Solution processes
• Complex formation
Physical factors
Biological factors
• Temperature
• Optical properties
• Movement of water
• Photosynthesis
• Respiration
• Mineralisation
7.2. Dissolved gases and solids
Physico-chemical processes
• Dissolution and precipitation of solids
• Absorption and desorption of gases
• Ion exchange at solid surfaces
Chemical processes
• Redox processes
• Soluble complex formation
• Hydrolytic cleavage
Biochemical processes
• Mineralisation of organic matter
• Photosynthesis
• Respiration
Dissolved substances in fresh and seawater
In freshwater : calcium carbonate + silicates + nitrates
In seawater :
sodium chloride
Besides inorganic materials indefinite number of organic
compounds
LAW OF THE MINIMUM (Liebig):
Yield dependent on whatever growth factor is at a minimum in
proportion to all similar factors (e.g. phosphorous vs nitrogen)
7. Materials budget of natural waters I
Contents (1)
7.1. Introduction
7.2. Dissolved gases and dissolved solids
7.3. Gases dissolved in water
7.3.1. Solubility of gasses in water
7.3.2. Oxygen content and oxygen budget
7.3.3. Carbon dioxide, carbonic acid and carbonates
7.3.4. Methane and hydrogen sulphide
7.3.5. Nitrogen
7.3. Dissolved gases in water
O2 and CO2
Direct indicators of biological activity
N2
Metabolic cycle of specific micro-organisms
H2S and CH4
Present in localised amounts due to baterial activity
7.3.1. Solubility of gases in water
Henry’s law:
Solubility of a gas decreases with :
• Increasing temperature
• Decreasing pressure
Quantity of dissolved gas :
Cs = Ks * Pt
Cs = Saturation concentration of the gas
Ks = Temperature dependent solubility
Pt = Partial pressure of the gas
CO2 has highest solubility
CO2 + H2O  H2CO3 / CaCO3
7.3.1. Solubility of gases in water
Important :
• Saturation of the gas : oversaturation – undersaturation
• O2 and CO2 : produced or consumed by living organisms
• Increasing temperature  decrease of oxygen concentration
 increase in oxygen demand organisms
Compensation in warmer water :
• Water movement in flowing water
• Water movement by animals themselves
7.3.2. Oxygen content and oxygen budget of surface waters
Factors affecting the oxygen balance
INPUT
1. From atmosphere
2. Photosynthesis
LOSSES
1. Respiration
2. Decomposition mineralisation
3. Losses to atmosphere
Oxygen balance less positive if :
• Input decreases
• Losses increase
Deductions :
1. Flowing waters with rapid movements and shallower depth
have a more favourable oxygen balance than still waters
2. Input of organic matter into water body has an adverse effect
on its oxygen balance (greater effect in still than in flowing
water)
Dissolved oxygen in lakes
O2 from atmosphere  water  greater depths by water movements:
During seasonal turnover : O2 rich water  bottom
During summer stagnation phase :
• In epilimnion:
• O2 from atmosphere + photosynthesis
• O2 oversaturation during the day + O2 deficit during the night
• Diurnal fluctuations of pH and CO2
• In hypolimnion:
• Exclusively oxygen depletion processes :
Heaviest oxygen demand imposed by microbial mineralisation
of plant and animal residues deposited in profundal zone
• Quantity of organic matter dependent on :
• Production in epilimnion
• Sinking and degradation rate of dead organisms
• Depth of the water
Classification of lakes in temperate zones
On basis of volume ratio
Epilimnion / Hypolimnion (E / H)
Oligotrophic :
ratio  1

Eutrophic :
ratio > 1
Relationship between production, depth and trophic status
HOLOMICTIC LAKE
• Oligotrophic lake:
Orthograde O2 profile
Hypolimnic oxygen uptake low during stagnation period
• Eutrophic lake :
Clinograde O2 profile
Hypolimnic oxygen maybe completely exhausted
Heterograde O2 profile consequence of:
Metalimnic photosynthesis maximum
or
Intensive decomposition in thermocline
MEROMICTIC LAKE
• Monimolimnion : permanently free of oxygen
In tropical lakes : hypolimnion (> 20 °C) = O2 totally depleted
Oxygen budget of flowing waters
Oxygen budget affected by :
• Degradable organic matter carried along
• Organic effluents
Clues provided to oxygen budget :
• In lakes : Vertical differences in O2 concentration
• In rivers : Diurnal O2 saturation profile
Dissolved oxygen in flowing waters
Different types of waters according to diurnal oxygen profiles :
• Type 1 : Abiotic flowing waters
O2 level temperature dependent
• Type 2 : Unpolluted flowing waters
Oversaturation during day, deficit during night
• Type 3 : Slightly polluted flowing waters
No oversaturation during day, deficit during night,
• Type 4 : Strongly polluted flowing waters
Continuous oxygen deficit
As a result of self-purification capacity of flowing waters 
succesion of types 4-3-2 along the river course
7.3.3. Carbon dioxide, Carbonic acid, Carbonate
Sources :
• Atmosphere
• Precipitation
• Infiltration through soil (groundwater)
• Metabolic activity of the organisms
• Aerobic decomposition : C  CO2
• Anaerobic decomposition : CO2 + CH4
• CO2 + H2O  H2CO3  H + HCO3-  H + CO3-Proportions of CO2, HCO3- and CO3-- : pH dependent
• When adding CO2
CaCO3 + CO2 + H2O
Insoluble
form

Ca(HCO3)2
Soluble form
= C reserve for photosynthesis
Excessive CO2 may dissolve chalk
• When removing CO2
Ca(HCO3)2  CO2 + CaCO3 + H2O
• Chemical decarbonation
Crust of CaCO3 on stones, mosses, leaves (travertine)
• Biogenic decarbonation
Crust of CaCO3 on leaves of submerged plants
Fine cristals of chalk formed by phytoplankton:
Calcium-apatite
By the presence of
calcium carbonate in its
blue-green water, the
Havasu creek in the
Grand Canyon National
park, slowly deposits
stone called travertine.
Tuff formations at Mono
Lake (California). They
were formed by the
interaction of calcareous
groundwater with the
CaCO3 and other minerals
in the lake.
Hardness
Chalk content expressed as temporary hardness
on a scale of German degrees of hardness
1 dH° = 10 mg/L CaO or 18 mg/L CaCO3
1 dH° = 7.1 mg/L MgO or 15 mg/L CO3
< 10 dH° = soft water
20 dH° = hard water
> 30 dH° = not usable anymore as drinking water
Buffering action
Great biological importance attached to pronounced
buffering action of CO2-calciumbicarbonate mixtures 
• Acidic waters with low chalk content: weakly buffered
 may undergo high pH rise > 9
• Calcarous waters : strongly buffered
 normal pH range 7 – 8
CO2 consumption compensated by decomposition of Ca(HCO3)2
 pH increase remains small
Finally CaCO3 + H2O  Ca(OH)2 + CO2
 pH increases up to 11 (CO2 only present as CO3 ions)
Abatement of acidification by means of addition of chalk
Vertical distribution of CO2
In lakes : vertical distribution of CO2 arises from activity of
• Autotrophs :
Epilimnion  uitputting van CO2 (planten)
• Heterotrophs :
Hypolimnion  CO2 generated,
recombines with precipitated CaCO3
in epilimnion
In flowing waters : relationship much simpler :see figure
7.3.4. Methane and hydrogen sulphide
Result of anaerobic decomposition of organic matter
CH4
Released to atmosphere
Oxidized to formaldehyde
H2S
Dissolves readily in water
N2
Certain bacteria (cyanobacteria) can fix N
N2 + 12 ATP + 6 H  2 NH3 + 12 ADP + 12 P
N-fixation at sediment-water interface
7. Materials budget of natural waters I
Contents (2)
7.4 Solids dissolved in water
7.4.1. Solubility of solids in water
7.4.2. Nitrogen compounds
7.4.3. Phosphorous compounds
7.4.4. Sulphur compounds
7.4.5. Iron and manganese
7.4.6. Silica
7.5. Dissolved organic matter in natural waters
7.6. Sediment and the materials budget
7.7. Materials budget of flowing waters
7.4.1. Solubility of solids in water
• Water is a particularly suitable solvent for electrolytes:
- High dielectric constant
- Ability to form hydrates
• Solubility of solid substances dependent on:
- pH
- Eh
• Most substances dissolve either :
- In molecular form
- As ion
- In colloidal form
7.4.2. Nitrogen compounds
Nitrogen occurs in the form of numerous compounds:
Inorganic form
• NO3, NO2, NH4
Organic form
• Intermediate stages of microbial protein
decomposition ; Excretion products
• Amino-acids, Enzymes
NO3 and NH4 = nitrogen sources for photo-autotrophic plants
NH4 = result of decomposition of organic residues
7.4.2. Nitrogen compounds
Important
In lakes
• N2 binding : Blue-green algae, Azotobacter, Clostridium
• N-assimilation : N2, NH4, NO3  organic nitrogen
• Ammonification: organic N  NH4
• Nitrate reduction : NO3  NH4
• Nitrification : NH4  NO2  NO3 (Nitrosomonas & Nitrobacter)
• Denitrification : NO3  N2 (Pseudobacter)
In flowing waters
• Not polluted : NO3 most important N-component
• Polluted : NH4 gradually oxidized to NO3
7.4.3. Phosphorous compounds
• P often only as traces
• P often growth limiting factor
Eutrophication involves primarily increase in PO4 levels.
Different fractions :
• Dissolved inorganic phosphate = orthofosphate + polyphosphates
• Dissolved organic phosphate
• Particular organic phosphate = organisms and detritus
7.4.3. Phosphorous compounds
In trophogenic zone :
• Dissolved Inorganic phosphate
taken up by photo-autotrophic producers
 organic compounds of food chain
• Major part
 released again into epilimnion
• Lesser part
 sediments (adsorption, precipitated as FePO4)
 > 10 % O2 : release of PO4 in water
7.4.4. Sulphur compounds
Inorganic sulphur components in water : SO4 (sulphate)
Of great importance:
Activity of micro-organisms in sulphur cycle (chemo+photoautotrophic production)
• Desulfuricans organisms reduce SO4 tot H2S + sulfiden (sediments)
C6H12O6 + 3 K2SO4  6 KHCO3 + 3 H2S
Microbial decomposition of proteins  H2S
= Facultative chemo-autotrophic anaerobic sulphur bacteria
• Sulfuricans organisms oxidize H2S  S  SO4
2 H2S + O2  2 H2O + 2 S
5 S + 6 KNO3 + 2 H2O  2 N2 + 3 K2SO4 + 2 H2SO4
= Chemo-autotrophic colourless aerobic sulphur bacteria +
thiobacteria
= Photo-autotrophic coloured anaerobic sulphur bacteria
7.4.5. Iron
Iron present in natural waters only in small amounts
Exception : groundwater may contain large quantities of :
• Dissolved iron = bivalent iron (as Fe(HCO3)2)
• Insoluble iron = trivalent iron (as Fe(OH)3)
Bivalent iron remains in solution if :
• O2 < 50 %
• presence of degradable organic matter
• >> free CO2
• pH < 7.5
Fe(HCO3)2 + O2  precipitation of Fe(OH)3 + FeO(OH)
7.4.5. Iron
• Iron bacteria (Thiobacillus) are involved in process of
Fe- precipitation:
oxidize Fe2+  Fe3+ (chemo litho authotrophic bacteria ).
• Fe remains in solution in the hypolimnion of eutrophic lakes
during stagnation period
• In trophogenic zone (epilimnion) small amounts of dissolved iron
quickly used up by producers
7.4.5. Manganese
May be released from sediments when O2 still several mg/L.
7.4.6. Silica (silicic acid)
Dissolved silica = Building material for diatoms
Dissolution of silica from the sediments :
Takes place between interstitial water and free water
Affected by :
• Temperature
• Age of sediments of biogenic origin
• pH
• Bottom dwelling animals
7. Materials budget of natural waters I
Contents (2)
7.4 Solids dissolved in water
7.4.1. Solubility of solids in water
7.4.2. Nitrogen compounds
7.4.3. Phosphorous compounds
7.4.4. Sulphur compounds
7.4.5. Iron and manganese
7.4.6. Silica
7.5. Dissolved organic matter in natural waters
7.6. Sediment and the materials budget
7.7. Materials budget of flowing waters
7.5. Dissolved organic matter in natural waters
Dissolved organic matter >> particulate organic matter
DOM >> POM
Origin of DOM :
• Losses due to photorespiration
• Secretion of products of photosynthesis (algae + plants)
• Excretions by bacteria
• Hydrolysis + decomposition of dead organisms
Important group = HUMIC SUBSTANCES (humic acids + fulvic acids)
Origin :
• Incomplete breakdown of plant residues in water bodies
• Affect the materials budget: complex formation with heavy metals
• Prevents precipitation - ensure availability to primary producers
7. Materials budget of natural waters I
Contents (2)
7.4 Solids dissolved in water
7.4.1. Solubility of solids in water
7.4.2. Nitrogen compounds
7.4.3. Phosphorous compounds
7.4.4. Sulphur compounds
7.4.5. Iron and manganese
7.4.6. Silica
7.5. Dissolved organic matter in natural waters
7.6. Sediment and the materials budget
7.7. Materials budget of flowing waters
7.6. Sediment and the materials budget
Important interactions between water and sediment
In contact zone of sediment surface :
• Precipitation
• Dissolution
• Exchange processes :
• Absorption or release
• Determining factor = redoxpotentiaal
Inorganic phosphate : shift of Fe + P from :
Anaerobic conditions in deeper sediments
 sediment surface
 release of P in water at sediment surface
Organically bound P in sediment: stable fraction
7.6. Sediment and the materials budget
Redox potential in upper sediment layer (several cm)
Reducing and oxidizing conditions change with time as a
consequence of periodic succession of turnover and stagnation phase
and amount of decomposed organic matter
• In oligotrophic water and during turnover in eutrophic lakes:
high oxygen content in deep water : Eh = 0.6 V
• In eutrophic water during stagnation phase :
low oxygen content in deep water:
reducing zone migrates upward from deeper sediment to
sediment-water contact zone : Eh decreases
at Eh = 0.2 V : Fe2+ + PO4 go into solution
at Eh = 0.1 – 0 V : reduction of SO4  H2S + S
7. Materials budget of natural waters I
Contents (2)
7.4 Solids dissolved in water
7.4.1. Solubility of solids in water
7.4.2. Nitrogen compounds
7.4.3. Phosphorous compounds
7.4.4. Sulphur compounds
7.4.5. Iron and manganese
7.4.6. Silica
7.5. Dissolved organic matter in natural waters
7.6. Sediment and the materials budget
7.7. Materials budget of flowing waters
7.7. Materials budget of flowing water
More dependent on ecological structure of catchment (= open system)
Less dependent on internal metabolism (cf. lakes = closed system)
Smaller rivers reflect geochemical situation of their catchments :
Geochemical types :
Bicarbonate type
Catchment area : chalk and dolomite rocks
Ca(HCO3)2 – Mg(HCO3)2 
neutral – alkaline and well buffered
Sulphate type
Catchment area : gipsum deposits
CaSO4 
Chloride type
Catchment area : salt deposits or salination
NaCl  of NaHCO3 
Silicate type
Catchment area : silicate rocks
low in lime, poor in electrolytes
neutral – acid, weakly buffered
7.7. Materials budget of flowing waters
Larger rivers integrate the diverse structures
Main factors controlling the chemistry of watercourse during its
transit time :
• Solution processes
• Evaporation – precipitation
• Adsorption – desorption on suspended solids and sediments
• Internal reactions
• Exchange with atmosphere
Processes in flowing waters limited by relatively short transit time :
average 10 days = major difference with stagnant waters (e.g. lakes)
(not water movement !).