Aquatic Ecology Freshwater - Part 4 Aquatic Ecology Laboratory of Environmental Toxicology and Aquatic Ecology Prof. Dr. N. De Pauw

Aquatic Ecology

Freshwater - Part 4

Aquatic Ecology

Laboratory of Environmental Toxicology and Aquatic Ecology

Prof. Dr. N. De Pauw

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)

7.1. 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 (1)

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


Contents (2)


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

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)

Characterized by the following processes

7. Materials budget of natural waters I7.1. Introduction

7.1. Introduction







7.3.5. Nitrogen


Contents (1)


Spatial and temporal distribution dependent on :

Hydrological factors

• Precipitation• Inflow and outflow

Chemical factors

• Solution processes• Complex formation

Physical factors

• Temperature• Optical properties• Movement of water

Biological factors

• Photosynthesis• Respiration• Mineralisation

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

7.2. Dissolved gases and solids

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)

7.1. Introduction



7.3.1. Solubility of gasses in water




7.3.5. Nitrogen


Contents (1)

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. Dissolved gases in water


Henry’s law:

Solubility of a gas decreases with :

• Increasing temperature• Decreasing pressure

Quantity of dissolved gas :

Cs = Saturation concentration of the gas Ks = Temperature dependent solubility Pt = Partial pressure of the gas

CO2 has highest solubility CO2 + H2O H2CO3 / CaCO3

Cs = Ks * Pt

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

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)

INPUT1. From atmosphere 2. Photosynthesis

LOSSES 1. Respiration2. Decomposition -

mineralisation3. Losses to atmosphere

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

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

Relationship between production, depth and trophic status

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

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

Dissolved oxygen in flowing waters

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 Ca(HCO3)2

Insoluble Soluble formform = 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.

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

Hardness

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)

Buffering action

Abatement of acidification by means of addition of chalk

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

Vertical distribution of CO2


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







7.4.6. Silica





Contents (2)

• 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



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

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



• 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

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



Inorganic sulphur components in water : SO4 (sulphate)

Of great importance: Activity of micro-organisms in sulphur cycle (chemo+photo-autotrophic production)

• Desulfuricans organisms reduce SO4 tot H2S + sulfiden (sediments)C6H12O6 + 3 K2SO4 6 KHCO3 + 3 H2SMicrobial decomposition of proteins H2S

= Facultative chemo-autotrophic anaerobic sulphur bacteria • Sulfuricans organisms oxidize H2S S SO4

2 H2S + O2 2 H2O + 2 S5 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)

• 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. Iron

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







7.4.6. Silica





Contents (2)


Dissolved organic matter >> particulate organic matter

DOM >> POMOrigin 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







7.4.6. Silica





Contents (2)


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

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








7.4.6. Silica





Contents (2)

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

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 !).


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Aquatic Ecology Freshwater - Part 4 Aquatic Ecology Laboratory of Environmental Toxicology and Aquatic Ecology Prof. Dr. N. De Pauw