aquatic ecology freshwater - part 2

Aquatic Ecology

Freshwater - Part 2

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

(= gasses, solid and dissolved substances, importance of sediments)

8. Materials budget in natural waters II

(= production, consumption, decomposition)


Contents

4.1. Properties of water

4.2. Water molecules and aggregate formation

4.3. Density en density anomaly of water

4.4. Adhesion and cohesion

4.5. Surface tension

4.6. Viscosity and kinematic viscosity

4.7. Thermal properties of water

4.1. Properties of water

Water exhibits some unique properties which derive from :

• the structure of the water molecule

• the tendency to form aggregates

The specific properties affect the existence of single organisms and the biocoenoses.

The life cycle of organisms in natural waters is influenced by:

• the density• the density anomaly• the thermal properties of water

4.2. Water molecules and aggregate formation

Water molecules are strong dipoles owing to spatial arrangements of atoms. Without pronounced dipole character water would not be liquid !

Pronounced dipole moment of molecule (= charge x distance)

Water molecules experience strong attraction to each other and aggregate to form :

hyper-molecular linear, areal and spherical clusters

Elektrostatic attraction of 2 molecules leads to hydrogen bridge formation

Bonding energy of hydrogen bridges is smaller than energy needed for the formation of covalent bonds

WATER = ACTIVE CHEMICAL COMPOUND

Water remains liquid at normal temperatures despite its low molecular weight.In normal ice a rigid crystalline structure is formed.

4.3. Density of water

Dependent on :DISSOLVED SOLIDS : increase dissolved solids results in

increase density

• Continental waters : dissolved solids < 1 g/L• Exceptions : mineral waters, salt lakes, deep water of lakes• Chemical density differences in lakes result in stable stratification

PRESSURE : influence small

• Only effect in deep lakes

TEMPERATURE DIFFERENCES : important

• At 1 atm. greatest density of water at 4 °C• Colder and warmer water are lighter: have a distinct bouyancy relative to water at 4°C.• Density differences increase with increasing temperature = important for stability of thermally stratified water bodies

4.3. Density anomaly of water

Density of water is maximal at temperatures higher than freezing point = consequence of aggregate formation of hydrogen bridging of water molecules

Importance of the density anomaly :• Bottom water of deep lakes cannot be colder than water at its density maximum, which is about 4°C

• Layer of ice protects the deeper water from freezing = protection of organisms

Maximum density also dependent on :• Salt content of water Seawater has a maximum density at -3.5 °C, and freezes at -1.91 °C

• Pressure Increase in pressure (e.g. a depth of 100 m) : temperature of maximum density decreases with ca. 0.1 °C temperature bottom water of deep lake << 4 °C

4.4. Adhesion and Cohesion

Behaviour of water molecules relative to solid surface : has far reaching biological consequences

• if cohesion < adhesion

boundery surface becomes hydrophylic = wetted

• if cohesion > adhesion

boundary surface becomes hydrophobic = water repellent

Hydrophobic properties of body surface

Essential for water-based animals taking up atmospheric oxygen at the surface : need dry connection (e.g. many insects, water spider)

• Reduced water uptake in body of many water-living organisms

• Less colonisation by surface attached organisms

• Osmoregulatory function (insects)

Hydrophilic properties of body surface

• For animals breathing under water: gills or entire body surface hydrophilic

• Reduced friction when moving

4.4. Adhesion and Cohesion

4.5. Surface tension

Surface tension of water varies with:

• Temperature• Dissolved solids content

• Temperature dependence biologically unimportant

Presence of tensio-active substances (e.g. detergents) pronounced decrease of surface tension

• Reduction in surface tension ‘oil patches’ = areas of smooth water which form at the surface of a water mass

• Surface tension has to be allowed for by all those organisms which inhabit the boundery zone between water and atmosphere :

NEUSTON = Micro-organisms +

PLEUSTON = Larger organisms

Surface tension : molecules in the surface layer only interact with molecules below and besides them molecules in the liquid phase however interact in all directions, the energy is thus more dispersed.

4.6. Viscosity and kinematic viscosity

Viscosity = resistance of the water during free flow or other changes in physical form

= Force needed to move a mass of 1 kg over 1m in 1 sec. in the medium (Pa s of N/m2,s)

Dependent on : salt content negligible

temperature important

Temperature influences swimming and floating Water of 25 °C has a viscosity only half of that of water at 0 °C

Kinematic viscosity =

Frictional forces + pondering effects dependent on :• Velocity of motion• Shape of body• Viscosity of water

Example : at 25 °C organisms sink 2 x faster then at 0 °C

Biological meaning not fully understood

Viscositydensity

4.7. Thermal properties of water

Specific heat capacity exceptionally high : 4.18 kJ kg-1 °C-1

Specific heat capacity of ice : 2.04 kJ kg-1 °C-1

Specific heat capacity of air : 1.00 kJ kg-1 °C-1

Means that a heated water body stores a large quantity of heat !

Thermal capacity :

= Ratio of supplied heat and the resulting temperature increase

Quantity of heat needed to evaporate 1 kg of water is exceptionally high ! water has a high thermal buffer capacity water has a low thermal conductivity

= amount of heat passing in 1 second between opposite faces of a cube of 1 cm edge when a temperature difference of 1°C is maintained between 2 faces

Heat transport in water bodies takes place as a result of the movement of the water itself.

C

J

dT

dQC

4.8. Dielectric constant of water

The dielectric constant of a substance indicates how much electrostatic energy can be stocked per unit volume of a substance when a tension of 1 volt is applied

Water has a very high dielectric constant (=80,08 at 20°C)

= expression of dissociating action of water on heteropolar bonds in an electrolyte (two bodies with opposite loadings attract each other with a smaller force then in a vacuum. Upon this principle the possibility is based that in water anions and kations can freely exist side by side)

The dielectric constant decreases when the temperature increases

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



6. Communities of living organisms in natural waters

7. Materials budget in natural waters I

(= gasses, solid and dissolved substances, importance of sediments)

8. Materials budget in natural waters II

(= production, consumption, decomposition)


The physical properties of water manifest themselves in ways typical of either stagnant or running water

Water bodies of some depth are characterized by vertical gradients of :

• temperature• pressure• light • chemical substances

Physical factors of basic importance are :

• Radiation intensity• Heat balance• Motion and exchange processes

dictate biological structure

affect materials balance

5.1. Radiation climate in a water body

5.2. Heat budget of water bodies

5.3. Water movement and water exchange in natural waters



Global radiation received on surface of a water body = short wave radiation: 300–3000 nm

Radiation consists of :

• ultra-violet radiation : 300 – 380 nm• visible light : 380 – 750 nm• infra-red radiation : 750 – 3000 nm

Global radiation made up by :

• direct sunlight• diffuse radiation

Light impinging on the water surface subjected to 3 processes :

1. reflection 2. scattering within water mass3. absorption within water mass

• Reflection of light at the water surface (Fresnel’s formula) :

• Amount of reflected light depends on position of the sun and varies according to time of the day and season

Height of sun ° 2 5 10 15 20 30 40 50 60 70 80 90

Direct sunlight % 80 58 35 21 13 5.9 3.4 2.1 2.1 2.1 2.1 2.1

Diffuse radiation %

- 17 15 14 13 11 9.3 8 7.5 7.1 - -

Reflection of light

)²(tan

)²(tan

)²(sin

)²(sin

2

1

ri

ri

ri

riR I = angle of incidence

r = angle of refraction

Light penetrating the water

• Selective scattering • Absorption

Extinction = amount of radiation retained Transmission = amount of radiation which emerges

during passage through water

z

z

zz

zz

I

I

ZZ

II

eIofeII

00

0

ln1lnln

Iz = radiation intensity at depth Z

Io = incident radiation intensity

= extinction coefficient

Expressing radiation intensity

• Einsteins (1 E = 6.02 x 1023 fotonen)• J / cm²• W / m²

• Amount of scattered energy in water depends on number of suspended particles in the water

Example : in lake Lutz 1 %in lake Leopoldstein 9 %

• Uncoloured clean water appears blue in thick layers because short- wave radiation most intenselyscattered

• Scattered radiation biologically important: use of visible light by photo-autotrophic organisms

• Extinction and transmission dependent on wave-length of incident radiation

• Optical properties of water affected by: dissolved inorganic and organic substances and particles of all kinds

• Absorption increases with content of brownish-yellow humic substances

Radiation climate in running waters

Same factors of importance as in lakes

Differences :

• reflection at surface is higher• in a clear mountain stream light penetrates as far as the streambed• in turbid rivers light can be completely absorbed• organic effluents reduce light transmission

Colour of water

Dependent on optical characteristics which are influenced by:

• Selective transmission of light• Content of suspended matter and dissolved substances• Colour of surroundings - reflection

• Blue is the most strongly transmitted colour• Phytoplankton can impart a green, brown, green-blue and even red colour• Blue = colour of ‘desert’ water• Humic acids give a yellow-brown colour • Wastewater can can lead to a grey or black colour • Mud or silt can colour the water yellow, brown or red

The clear, unproductive waters of a high-altitude lake in the Sierra Nevada, California

The Rio Negro flows into the main branch of the Amazone which is heavily loaded with silt. The water of the tributary is darkly coloured by humic substances present in the soil of its forrested catchment area.

‘Red tide’

Snow and ice

• Ice has the same optical properties as distilled water

• Layer of clear ice : no effect on input of radiation energy

• Snow : prevents passage of radiation in water (e.g. 20 cm snow = 99 % reduction)

Ice covered with snow hardly transmits any light

5.2. Heat balance of water bodies

Determined by :

Heat uptake : • Absorption of radiation energy in the upper layers (red + infra-red long wave lenghts)

Heat dissipation (=losses) :• Emitted radiation• Evaporation• Release of warm surface water• Heat transfer to surroundings

Heat distribution:• Redistribution of thermal energy into deeper layers is result of transport of previously warmed water and thus of mechanical energy

Stationary heating process due to absorption of radiation contributes only to 10-12% of the total heat distribution.

5.2.1. Heat budget of lakes

Considered model :

Lake in temperate zone with cold winters and hot summers

Driving force for transport of water heated during spring to greater depths is the wind

Currents redirected at the banks towards deeper layers of water

Wind can drive surface water skin at speed equal to 4.3 % of wind velocity

The thicker the layer set in motion, the smaller the velocity of drift of water relative to the wind velocity

5.2.1. Heat budget of lakes

Velocity and depth of surface circulation depend on:

• Wind speed

• Wind direction

• Temperature of surface water

The greater the wind speed and the higher the temperature, the less the depth to which the wind can drive the warmed water.

Lakes in temperate zones

4 consecutive stages related to circulation and thermalstratification

(1) SUMMER STAGNATION PHASE = stable thermal stratification

(2) AUTUMN TURNOVER PHASE = thermal stratification destroyed

Wind forces surface water to deeper levelsWhen reaching 4 °C the whole water mass caught incirculation

HYPOLIMNIONBelow temperature falls to ca. 4 °C

METALIMNIONBelow steep temperature gradient

EPILIMNIONSurface layer of warm water 3 layers

Lakes in temperate zones

(3) WINTER STAGNATION PHASE = stable thermal stratification

Cold surface water covered with iceSlightly warmer (4 °C) deep water

(4) SPRING TURNOVER PHASE = thermal stratification destroyed

Melting of ice Uniform temperature restored as consequence of springturnover phase

Lakes geographically grouped according to

different circulation patterns

TROPICAL MOUNTAIN LAKES

Almost continuous turnover

COLD POLYMICTIC

TROPICAL LAKES

Frequent turnover by cooling at night

WARM POLYMICTIC

TROPICAL LAKES

Rarely observed turnover

OLIGOMICTIC

SUBTROPICAL LAKES

Circulation only during winter months

WARM MONOMICTIC

TEMPERATE LAKES

Turnover during spring and autumn

DIMICTIC

POLAR AND SUBPOLAR LAKES

Circulate during summer months

COLD MONOMICTIC

POLAR LAKES

Permanent ice cover

AMICTIC

Tropical lakes

Tropical lakes characterized by high temperatures in surface and bottom layers

Hypolimnion > 20 °C – small temperature differences – small seasonal differences

Despite small differences stratification stable : large density differences

HOLOMICTIC lakes versus MEROMICTIC lakes

Turnover process affects entire water mass Mixing does not extend to the bottom

MEROMYXIS

• Morphologically dictated : surface area too small vs relative depth • Chemically dictated : hypolimnion rich in dissolved solids

MONIMOLIMNION = non-mixed bottom water of meromictic lakes

Entire heat content of a lake = volume x temperatuur Example: Ammersee

24 x 1012 kJ in February difference = 47 x 1012 kJ71 x 1012 kJ in September

stored as heat from February till September released from October till January

These enormous amounts of heat may exert an appreciable influence on surroundings

5.2.2. Heat balance of flowing waters

Dependent on:

• geometry of the water mass (depth and width) • density of the water ()• specific heat capacity of the water (c)

Other factors which affect the heat exchange :

• direct sunlight (S)• diffuse radiation (D)• effective back radiation (E = f(T4))• evaporation or condensation heat (V)• heat exchange with the air (L)• heat exchange with the soil (B)

Change in water temperature with time

)(1

BLVEDSchdt

dT

Temperature fluctuations per day are higher in summer

• the lower the source temperature is in comparison to the air temperature

• the lower the water level is or the quantity of water that comes along

The shallower the water, the faster the water mass will warm up

Downstream from the source, the water takes up heat from the surroundings during the middle of the day

During winter the water loses heat to the surroundings

Temperature fluctuations

River classification based on maximum temperature

> 40 °CHot springs and rivers

29 – 40 °CWarm rivers

17 – 29 °CTemperate rivers

(cold in winter)

0 – 17 °CCold rivers

(cold in summer)

Sources with a temperature of 20 °C or more = thermal springs

Temperature in watercourse

Based on the following factors :• temperature of the source (has the most constant temperature)• daily temperature amplitude during the summer

• first temperature increase in downstream direction • then becomes smaller as a result of 2 processes working opposite

(1) large temperature difference between source and air temperature(2) quantity of water increases in downstream direction

• temperature amplitude during the year also increases in downstream direction

• average temperature in summer increases with distance to source • temperature change per km on the contrary becomes smaller with

distance downstream and the reaches with about the same temperature become longer:

dT 1T = To + C log (1 + Z) or --- = K --------

dt 1 + ZZ = distance to source

To = initial temperatureC or K = proportional constant

All these facts are related to temperate climate.

• Tropical rivers have temperatures of 25 – 30 °C with a small amplitude of ca. 1 °C.

• In the hyporheic interstitial zone diurnal and seasonal amplitudes are smaller than in free water. Amplitudes decrease with increasing depth in bottom sediment

Hyporheic interstitial zone = stable medium

Temperature in summer lower, in winter higher than in free flowing water

Temperature in watercourse

Hyporheic interstitial zone


Besides thermally induced circulation also periodic or aperiodic water movements due to :

• action of wind • thermal effects • differences in air pressure

In running waters :movement of water is an inherent characteristic

In stagnant waters :wind effect : travelling waves formed at surface

Example: Lake Geneva : waves of 1 m height – 20 m long

In shallow water oncoming waves are slowed down by friction against the bottom

= Periodic movements of horizontal layers of water (can be compared to tidal phenomenon in lakes)

• visible: changes in water level at the surface

or

• not visible: internal seiches in deeper layers

Seiches are caused by air pressure differences over different areas of the larger lakes due to the effect of strong wind acting over a long period causing the surface layers to be forced in deeper waters at one end of the lake

Seiches

5.3.1. Water movements in lakes

Besides more regular oscillations, isotherms in lake exhibit arhythmic movements such as intensive tilting, up and downward indentations, twisting due to :

• Convection + turbulence• Wind action + currents

creates considerable vertical transport :

• Warm water bottom• Cold water surface uptake O2 + release nutrients

Limnokinetic processes of great importance for horizontal and vertical distribution of plankton

Arhythmic movements

Limnokinetic processes

Fig. 12. Limnokinetics : position of isotherms in Lake Constance on 12 and 17 May 1961.

5.3.2. Water movements in running waters

Laminar flow versus Turbulent flow

Classification depends on :Cross section (d) - Flow velocity (V) - Kinematic viscosity ()

Turbulent flow• When Re (Reynolds’ number = V*d/) > 2300

Laminar flow• Only in case of very low flow rates + narrow channels• Never in free-flowing water in rivers • Does occur in:

• pore space of water-filled sediments (hyporheic interstitial water) • boundary layer at interface with underwater substrata • possibly in interior of aquatic plant stands

Dead water zone between free flowing water and solid substrate = protection zone for inhabitants of flowing waters

Structure of stream bed

Dependent on flow conditions and depth of the water = hydraulic conditions

• Stationary body (stone or pebble) is dragged only by water when resting energy is less than that of impinging water

Force which is exerted on a body at rest is :

K = V² * A * / 2

V = flow velocity A = surface exposed to flow = densityt of water

• Flow + configuration of body important (a plate is less easily set in motion than a sphere of equal weight)• For mobilization of a stationary particle a larger force must be exerted than is necessary simply for the transport of the same particles (cf. diagram Hjulström)

Hjulström’s diagram

Directly above smooth rigid submerged bodies a laminar boundary layer forms at the surface due to friction in which a steep velocity gradient exists.

The thickness of the boundary layer is dependent on:

• flow velocity • kinematic viscosity

In the velocity range of 0.1 to 1 m/s (mountain river) the thickness of the low-flow boundary layer is about a few mm.

Shoulder height of organisms is < 4 mm animals crouch lower onto supporting surface the higher the velocity of flow = organisms conform to the boundary layer as it becomes thinner

= vital protection against the water current !

Laminar boundary layer

aquatic ecology freshwater - part 2

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