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Limnology

What is limnology?

Introduction to limnology

The term limnology was used by F.A

Forel (1892) during his scientific studies

on Lake Geneva.

It is combination of two Greek words, “Limne”

means lake and “logos” means study.


• Limnology is the study of inland water.

• Study of the functional relationships and

productivity of freshwater communities, as

they are regulated by the dynamics of their

physical, chemical, and biotic environments.


• Limnology includes standing water (lentic

habitats) as well as running water (lotic

habitats).

• This includes the study of lakes and ponds,

rivers, springs, streams and wetlands.


Limnology covers the biological, chemical,

physical, geological and other attributes of all

inland waters (running and standing waters,

fresh and saline, natural or man-made).


A more recent sub-discipline of limnology,

termed landscape limnology, studies, manages,

and conserves these aquatic ecosystems using

a landscape perspective.


Limnology is closely related to aquatic

ecology and hydrobiology, which study aquatic

organisms in particular regard to their

hydrological environment.

Although limnology is sometimes equated

with freshwater science, this is erroneous since

limnology also comprises the study of inland

salt lakes.


Significant progress in limnology was made in

20th century:

Phytoplankton succession and primary

forcing functions.

Transfer of energy (food network) Hydro-

geochemical and geochemical studies on

sediments.

Importance of limnology as a science


The study of limnology is, like other sciences

basically a search of principles. Those

principles that are involved in certain

processes and operating mechanisms in the

field of limnology.


For example, When comparing the

hydrodynamics of river, lakes and reservoirs,

certain basic functional aspects are immediately

understood that affect the life cycle and

distribution and biomass of aquatic organisms.


In addition to scientific interest and basic

knowledge, limnology can provide some

important applications,

Planktonic successions

Study of evolution of lakes and reservoirs

Geomorphology


Hydrodynamic and its effects.

The ration of allochthonous (externally

derived) to autochthonous (internally

derived) material.

Comparison between lakes and reservoirs.

Importance of

Water

Importance of Water

Water is medium of life. Without water life is

impossible.

About 71% of earth surface is covered with

water. 97% of this water is saltwater present in

oceans while only 3% is available as freshwater.

Importance of Water

The overall

distribution

of water on

the earth

surface is

depicted

by this diagram.

Importance of Water

Water importance

Photosynthesis

Irrigation

Domestic use

Industrial use for power generation

Recreational purpose

Importance of Water

Drinking

Transport of nutrients throughout body

Removal of waste from body

Lubricate body tissues

Maintains the temperature of body

Essential for proper functioning of body

Maintenance of pH

Physical properties

of water

Physical properties of water

Some physical properties of water are,

o Molecular shape

o Polarity

o Surface tension

o Specific heat capacity


o Density

o Viscosity

o Solubility

o Dissolved Oxygen

o Cohesion

o Adhesion


Molecular shape

One molecule of water (H2O)

is made up of 2 hydrogen atoms

bonded with 1 oxygen atom.


Polarity

Water is a polar molecule.

The positive hydrogen

ends of 1 molecule are

attracted to the negative

end of the oxygen of

another molecule.


Surface tension

The property of the surface of a liquid that

allows it to resist an external force. It enables

some objects (e.g. water striders) to float on

surface of water.


Specific heat capacity

Amount of energy needed to increase

temperature of one gram of water to one degree

Celsius.

Water has a specific heat of 4.18 J.


Density

Mass a substance per unit volume.

Liquid is more dense than ice, that’s why ice

floats on liquid water.

The density of liquid water is maximum at 4°C.


Viscosity

The quantity of internal resistance in the fluid or

simply the thickness of a fluid.

Honey is more viscous than water because it is

more thick and hence offer more resistance in

flow than water.


Solubility

Universal solvent

On earth, water exists in 3 physical states

Solid – ice

Liquid – water

Gas – vapor


Dissolved Oxygen

Oxygen saturation in water medium.

Dissolved oxygen range for healthy biological

system is, 5-12 mg/liter.


Cohesion

The force of attraction between water molecules.

Water molecules like to stick to each other.

Adhesion

The force of attraction between the molecules of

water and other substances.

Chemical properties

of water

Chemical properties of water

pH

Alkalinity

Salinity

Total dissolve solids

Hardness


pH

Hydrogen ion concentration of a medium.

A measure of acidity or alkalinity of a medium.

The pH of pure water is 7.


Alkalinity

Ability of water to absorb hydrogen ions when

acid is added to it.

Also called as acid neutralizing capacity (ANC).


Salinity

A measure of total concentration of ions

dissolved in water.

Fresh water: 0-0.5 ppt

Brackish water: 0.5-25 ppt

Saltwater: 25-35 ppt


Total dissolved solids

A measure of total amount of inorganic (calcium,

magnesium, potassium, sodium, bicarbonates,

chlorides, and sulfates) and organic salts (organic

matter) dissolved in water.


Hardness

A measure of amount of dissolved calcium and

magnesium ions in the water.

Water with high concentration of dissolved calcium

and magnesium carbonates is regarded as hard water

while water with less concentration of dissolved

calcium and magnesium ions is called as soft water.

Freshwater

ecosystem

Freshwater ecosystem

Freshwater Division

Freshwater ecosystem includes

1. Lentic water bodies or systems

2. Lotic water bodies or systems

o Cover almost about 3% of the earth’s surface.

o They provide a number of important ecological

and economic services to humans.

o Possess diverse flora and fauna.


Lotic water bodies

Lotic water bodies

The study of freshwater is known as limnology.

Those water bodies that hold running water are

termed as lotic water bodies.

High dissolved oxygen due to continuous flow of

water

Lotic water bodies

Examples of lotic water bodies are

1. Rivers

2. Streams

3. Springs

Lotic water bodies

Lotic water bodies receive water supply from

• Precipitation

• Snow melt

• Springs

Lotic water bodies

Rivers

A deep fast flowing freshwater body with high dissolved oxygen content.

Lotic water bodies

Streams

A shallow fast flowing water body smaller than rivers.

Lotic water bodies

Springs

A freshwater body

in which water

flows out of the

ground.

Streams

Streams

A small, shallow and fast moving water body

that supports variety of aquatic flora and fauna.

A watershed, or drainage basin, is the land

area that delivers runoff, sediment, and dissolved

substances to a stream.

Streams

In many areas, streams begin in mountainous or

hilly areas that collect and release water falling

to the earth’s surface as rain or snow melts

during warm seasons.

Streams have low depth and narrow banks.

Streams

Streams receive nutrients from

Falling leaves

Animal feces

Insects

Biomass washed into streams during heavy rainstorms or by

melting snow.

Thus, the levels and types of nutrients in a stream depend on

what is happening in the stream’s watershed.

Streams

Streams receive their water from

o Precipitation

o Melting snow

o Groundwater

They lose water by

• Evaporation

• Sinking into the ground

• Discharge at their terminus or mouth.

Streams

The velocity of a stream depends on following

factors

i. Volume of water

ii. Stream channel width and depth

iii. Slope or gradient of stream

Streams

Streams habitat

A typical stream contains three major types of

habitats:

Riffle

Pool

Run

Streams

A riffle is an area of shallow and fast moving,

water.

A run is an area of smoothly moving water.

A pool is an area of deep and stagnant water.

Each habitat type supports different types of aquatic

life, so a stream with a greater diversity of habitat

types will generally support a greater diversity of

organisms.

Stream order

Stream order

The concept of stream order classifies streams in

relation to tributaries, drainage area, total length

and age of water. . It is actually a measure of

relative size of streams.

Stream order

A stream order assigns numerical designations

that indicate where in a watershed drainage

system a certain stream segment lies.

First order stream

Second order stream

Third order stream and so on……….

Stream order

Perennial streams without tributaries are

termed as first-order streams.

Two first-order streams combine to form a

second-order stream.

While two second-order streams combine to

form a third-order stream and so on.

Stream order

A comprehensive

model of stream

order depicting

various orders of

steams.

Stream order

The Mississippi River is a 10th-order stream

when it empties into the Gulf of Mexico.

The Amazon River in Brazil is the world’s

largest river because it carries more water into

the ocean than any other. It is a 12th-order

stream when it reaches the Atlantic Ocean.

Rivers

Rivers

Small streams join to form rivers, and rivers flow

downhill to the ocean. Rivers and streams are

powerful transporting agents as they roll and

push material on their bed down the channel. The

suspended matter give river water a muddy look.

Rivers

The downward flow of surface water and

groundwater from mountain to the sea typically

takes place in three zones characterized by different

environmental conditions:

Source zone

Transition zone

Floodplain zone

Rivers

Three zones in the downhill flow of water: – Source zone containing mountain (headwater) streams

– Transition zone containing wider, lower-elevation streams

– Floodplain zone containing rivers, which empty into the ocean

Drainage Basin

Drainage Basin

The area of landscape that contributes to the

water supply of river/ stream is called as

Drainage basin or catchment basin.

The area or boundary between two drainage

basins is known as a drainage divide or

watershed.

Drainage Basin

Drainage divide or watershed is the boundary

line that makes a differentiation between one

drainage basin to other.

Small drainage basins generally contribute to

streams, while the water from larger drainage

basins come together to form large rivers.

Drainage Basin

The drainage basin includes both the streams and

rivers that convey the water as well as the land

surfaces from which water drains into those

channels.

The drainage basin acts like a funnel, collecting

all the water within the area covered by the basin

and channeling it into a waterway.

Drainage Basin

Morphology and flow

in river ecosystems

Morphology and flow

in river ecosystems

The terms river morphology is used to describe

the shapes of river channels and how they

change in shape and direction over time.

The study of river morphology is accomplished

in the field of fluvial geomorphology, the

scientific term.

Morphology and flow

in river ecosystems

Fluvial geomorphology is the study of the form

and function of rivers/streams and the interaction

between rivers/streams and the landscape around

them.

The fluvial system of the river valley can be

divided into three main zones.

Morphology and flow

in river ecosystems

Fluvial geomorphology

An erosional zone

A transport zone

A depositional zone

Morphology and flow

in river ecosystems

Erosional zone

In the first or upper zone the erosional process

predominates and the stream and riverbeds are

generally degraded. The streams join together

and their slopes are generally steep. The bed

material is characteristically composed of

gravels.

Morphology and flow

in river ecosystems

Transport zone

The second (middle) zone is considered as near

equilibrium condition between the inflow and outflow

of water and sediment. The bed elevation in this

equilibrium zone is fairly constant and the river

generally flows in a single channel.

The sediment material generally composes gravels and

sands of various sizes.

Morphology and flow

in river ecosystems

Depositional zone

The lower zone is characterized by net

sedimentation and river bed aggradations. There

is branching of the river into channels and the

slope of these channels is rather flat. The bed

material generally composes of fine sand to silt

and clay.

Morphology and flow

in river ecosystems

The river's mouth is classified into two types:

1) Delta

2) Estuary

Delta:

Land formed by the material accumulated by the

river into contact with the sea.

Morphology and flow

in river ecosystems

Estuary:

The place where the river comes into contact

with the sea, where sea water enters into the

mouth of the river. The mix water is now called

estuarine water.

Channel morphology

Channel morphology

The basin or valley trough containing the

flowing water is the stream or river channel.

The channel is described physically in terms of

length, width, depth, cross-sectional area, slope

etc.

Channel morphology

The channel is usually bordered on one or both

sides by a flat area called the flood plain.

Much of the soil of the flood plain is connected

hydrologically to the water of the channel.

Channel morphology

The forms of river channels are,

o Straight

o Meandering

o Braided

Channel morphology

Straight channel

Straight stream channels are rare and are found

in the most tectonically incised/active areas.

Even in straight channel segments water flows in

a sinuous fashion.

Channel morphology

Meandering channel

A meandering channel is one that takes twists

and turns over its length.

Meanders are bends in

a river that form as a

river’s sinuosity increases.

Channel morphology

Sinuosity of a river

a measurement of how much a river varies from

a straight line.

Sinuosity = Channel Length / Displacement

A sinuosity of 1 means that the channel is

perfectly straight. A sinuosity greater than 1

means that the river meanders.

Channel morphology

Braided channel

Develops when a stream channel is divided into

several smaller ones by the accumulation of in-

channel deposits.

Sand or gravel bars

accumulate

subdividing the flow of water into many smaller

channels.

River continuum

concept

River continuum concept

The river continuum concept, first proposed by

Vannote and some others in 1980, provides a

model of changes that might take place as water

travels from headwater streams to larger rivers.


The RCC proposes a progressive shift in

following parameters from headwaters to down

water,

Physical gradients and energy inputs

Trophic organization

Biological communities.


An overview of RCC

Headstream water:

Headwater has steep gradient with riffles,

rapids and falls.

Limited photosynthesis because of limited

sunlight due to overhanging trees.


Leaves and woody material falling into the stream

are a main source of energy.

Aquatic insects break down and digest the

terrestrial organic matter.

Water is cooled by springs and often supports

trout.

The fauna is dominated by detritivores and filter

feeders.


Downstream water:

The river grows having less gradient with few

riffles and rapids.

Phytoplankton and zooplankton contribute to the

food base along with dissolved organic matter.

Supports a greater diversity of invertebrates and

fish.


Photosynthesis increases due to increase in

light initially at start but later decreases along

the course of downstream due to increase of

sediments that ultimately make water turbid.

Colonization of algae and plants contribute

significant energy to the community.

Stream communities

Stream communities

Life in First-and second-order streams

o In the headwaters of a stream the water is

shallow, the stream bottom is often rocky, and

there are few aquatic plants. A lack of food

limits the number of animals that can live

there.

Stream communities

o Benthos, is a key part of the food web. These

include benthic macro-invertebrates, such as

mussels, aquatic insects, and other

invertebrates.

o Animals at the bottom of the food web depend

on the leaves, stems, and animals that may fall

into the stream from the land.

Stream communities

o Aquatic insects, such as stonefly nymphs,

chew and tear leaves and stems into tiny bits.

They are called shredders.

o Small pieces not eaten by shredders are eaten

by filtering and gathering collectors.

Stream communities

o Grazers (snails, for example) appear further

downstream as the channel widens and feed on

the algae.

o Most fish that live in headwater streams are

small predators such as darters or minnows

that feed on smaller animals, such as aquatic

insect nymphs and larvae.

Stream communities

Life in Third- through fifth-order streams

Rooted and floating aquatic plants and algae

Grazers such as snails and water pennies eat

the growing number of plants. Collectors

increase with the varied plant life.

Stream communities

Plant diversity increases, shredders begin to

decrease.

A large variety of fish species live in the

deeper and more varied mid-level streams.

Stream communities

Life in High-order streams

• Few rooted plants may grow because the water

is too deep and turbid (cloudy).

• More collectors than shredders. One major

group of collectors in big rivers is mussels

living in the river’s benthic zone.

Stream communities

• Fish and turtles and reptiles in large rivers are

also an important part of the food web.

Predators such as sunfish may specialize in

eating insects, while others such as the Catfish

and Spotted bass consume smaller fish.

Predators range in size from

tiny zooplankton to 300 pound alligator gar.

Stream communities

• Otters are small

carnivorous mammals

that are often the top

predators in streams

and feeds on mussels,

fish, turtles, frogs, etc.

Types of streams

Types of streams

Streams and rivers are primarily characterized by

flow. There are three main types of streams,

Perennial

Intermittent

Ephemeral

Types of streams

Perennial stream:

Perennial streams and rivers are those that flow

year-round.

These streams flow all year around because their

channels are in constant contact with the

groundwater table.

Types of streams

Most well developed streams in New England are

perennial.

Types of streams

Intermittent streams:

Intermittent streams and rivers are those that

become dry for a period of a week or longer each

year.

They flow continuously only during certain

seasons when the groundwater table is high.

Types of streams

These water bodies are generally associated with arid or

semiarid areas or areas of average rainfall that are

experiencing especially dry conditions.

Types of streams

Ephemeral stream

Intermittent streams that regularly exist for only a short

period of time are classified as ephemeral streams.

Types of streams

Ephemeral streams are best illustrated by the dry

stream beds called arroyos, that flow only

following rainfall and cease to flow soon after.

Types of flow

Types of flow

There are in usually three types of fluid flow

• Laminar flow

• Turbulent flow

• Transient flow

Types of flow

Laminar flow

The more basic type of stream flow is referred to

as laminar.

A mode of flow in which the fluid moves in

layers along continuous, well-defined lines

known as streamlines.

Types of flow

A flow with regular, predictable motion is

called laminar flow. Laminar flow is when water

is organized in parallel layers and moves in an

orderly manner.

It is possible because there are minimal rocks or

other physical barriers in these types of flows.

Types of flow

Turbulent flow

The more complex type of steam flow is referred

to as turbulent. A flow with irregular,

unpredictable motion is called turbulent flow.

Turbulent stream flow is when water does not

remain within parallel layers and does not move

in an orderly manner.

Types of flow

Laminar vs Turbulent

flow

The term laminar refers to

streamlined flow in which a fluid

glides along in layers that do not

mix (fig a).

The more the flow increases the

more turbulent the water flow

becomes (fig b).

Types of flow

Transient flow

Transitional flow is a mixture of laminar and

turbulent flow, with turbulence in the centre of

the channel, and laminar flow near the edges.

Transient flow is a condition where the velocity

and pressure of a fluid flow change over time

due to changes in system status.

Lentic water bodies

Lentic water bodies

Standing bodies of freshwater that supports

variety of life.

Wind action is very vital in these water bodies

because it increases the content of dissolve

oxygen that is necessary for the aquatic life in

these water bodies.

Lentic water bodies

Examples of such water systems are

Ponds

Lakes

Wetlands

Lentic water bodies

These still water bodies possess following zones,

–Littoral zone

–Limnetic zone

–Profundal zone

–Benthic zone

Lentic water bodies

Overturn

The mixing of water from the bottom of the lake

with the water close to the surface of lake.

This occurs during spring and fall. Lake overturn

enhances the dissolve oxygen content of lake

that is necessary for survival of life present in the

lake water.

Lakes

Lakes are large natural bodies of standing

freshwater formed when precipitation, runoff, or

groundwater seepage fills depressions in the

earth’s surface.

Lakes

Lake formation

Factors that are vital in lake formation are

• Glaciation

• Crustal displacement

• Volcanic activity

Freshwater lakes vary tremendously in

• Size

• Depth

• Nutrient content

Lakes

Lakes

Deep lakes normally consist of four distinct

zones

1. Littoral

2. Limnetic

3. Profundal

4. Benthic

Lakes

Littoral zone:

A shallow water area near the shore where

maximum light can penetrate.

Littoral zone possess

• High biological diversity

• Ample sunlight

• Inputs of nutrients

Lakes

Limnetic zone:

Open water area away from shore where still

enough light can penetrate.

Main photosynthetic body

Produces food and oxygen for consumers

Most abundant organisms are microscopic

phytoplankton and zooplankton

Lakes

Profundal zone

A deep zone of standing body of water where

light can not penetrate.

No photosynthesis.

Oxygen levels are often low

Cool and dark water with fish species is found

Lakes

Benthic zone

The bottom of the lake Containing organisms

called benthos.

Decomposers

Detritus feeders

Invertebrates

Some fishes

The origin of lakes

The origin of lakes

The formation and life history of lakes is an

important part of landscape ecology of lakes.

The study of geomorphology contributes

significantly to understanding the origin of lakes

and the dynamics of the formative processes of

lake ecosystems.

The origin of lakes

The influx of nutrients, stratification, thermal

de-stratification and retention time all depend

on the lake’s geomorphology.

The origin of lakes

Morphology, the study of lake shapes, is related

to the origins of each system.

Morphometry deals with the quantification of

these forms and elements. Lake morphology and

morphometry basically depend on the processes

from which lakes originated.

The origin of lakes

Lakes that were formed from specific

geomorphological events in certain geographical

areas have similar characteristics and are

therefore grouped into lake districts.

The origin of lakes

Although these characteristics are similar, there

are differences in the morphometry,

productivity and chemical composition of the

water. Comparative studies of lakes in the same

district and between different lake districts allow

for regional classification.

The origin of lakes

Lakes are formed by the action of following

process,

o Glaciation

o Tectonic activity

o Volcanic activity

o Biological activity

Lake formation by

Glaciation

Lake formation by Glaciation

Glaciation is a major geological process that

forms lakes.

Glaciers are formed by accumulation of snow

during cool climate conditions.

Accumulated snow compacts to form ice and the

mass slowly spreads out.


Glaciers create basins for lakes by dropping

sediments that dam rivers and valleys leaving

iceberg in sediments.

Continental glaciers have been occurring for

about last 1.5 million years.


Alpine Glaciation

Alpine glaciers often dig out basin at the base of

a steep cliff.

If the basin is filled with water it is called a

Tarn.


Glacial till or Glacial drift

Glacial sediments that dam valleys and rivers are

called as Glacial till or Glacial drift. These

include, clays, sand, gravel etc.

Piles of glacial till is known as Moraines.


Kettle pond

They are formed among moraines.

As the glaciers melted, they dumped

accumulated till mixed with large pieces of ice.

Blocks of ice melted, sometimes producing a

basin within the glacial till.


Alluvial Dam

When two rivers meet, the one that is flowing

faster will dump sediments into other slower

stream/river forming an alluvial dam.

Sediments carried by river/Stream currents are

known as Alluvium.


Plunge basin

A depression that is created by the action of

falling water from waterfalls.

As the glaciers melt they released water that falls

and creates a basin.

Lake formation by

Tectonic activity

Lake formation by Tectonic activity

The lake is formed by movements of the Earth’s

crust, such as faults that result in depressions.

They are often formed in rift valleys (Graben).

The basin created by broken earth crust is known

as Graben and it can host a lake.


Tectonic movements can occur through the

emergence or subsiding (lifting or sinking) of

areas with shifts in sea level. The formation of

lakes then begins with isolation from the ocean.

Some lakes, which were ancient fjords, formed

when their connection with the sea was closed.


Well-known examples are,

Lake Baikal (Russia)

Lake Tanganyika and Lake Victoria (Africa)


Lake Baikal

Lake Baikal is the largest freshwater lake in the

world (by volume) and the world's deepest lake

found in Russia.

It is 1741-meter deep and a host to 2000 species

of plants and animals.

Lake formation by

Volcanic activity

Lake formation by Volcanic activity

During a volcanic explosion the top of the cone

may be blown off leaving behind a natural

hollow called a crater (A cup shaped depression).

This may be enlarged by subsidence into a

caldera (A large crater).


After the eruption of magma has ceased, the

crater frequently turns into a lake at a later time.

This lake is called a ‘caldera’.

Examples:

• Lonar lake in Maharashtra

• Krakatao lake in Indonesia


Volcanic Landforms

Volcanic landforms are divided into extrusive

and intrusive landforms based on weather

magma that cools within the crust or above the

crust.

Rocks formed by cooling of magma within the

crust are called ‘Plutonic rocks’.


• Rocks formed by cooling of lava above the

surface are called ‘Igneous rocks’.

• In general, the term ‘Igneous rocks’ is used to

refer all rocks of volcanic origin


Intrusive Volcanic Landforms

Intrusive landforms are formed when magma

cools within the crust (Plutonic rocks or intrusive

igneous rock).

The intrusive activity of volcanoes gives rise to

various forms.


Extrusive Volcanic Landforms

Extrusive landforms are formed from material

thrown out during volcanic activity.

The materials thrown out during volcanic activity

includes lava flows, pyroclastic debris, volcanic

bombs, ash and dust and gases such as nitrogen

compounds, sulphur compounds and minor

amounts of chlorine, hydrogen and argon.

Lake formation by

Biological activity

Lake formed by Biological activity

Several species of relatively large animals can

make lakes. Mainly following biological

activities are involved in the formation of water

reservoir.

1) Beaver ponds

2) Wallows

3) Human activity


Beaver ponds:

Beavers are water animals and exist on a diet

of bark, twigs and buds of trees. They are

excellent swimmers and wood cutters. They

use trees not only for food but also for building

lodges and dams.


Beavers are another good example of a foundation species. Acting as

“ecological engineers,” they build dams in streams to create ponds

and other wetlands used by other species.


Beaver ponds create important habitat for

many other species, including juvenile Coho

salmon. Some migratory birds also prefer

landing on beaver ponds instead of more open

bodies of water.


Wallows:

A wallow is a natural depression in the prairie

that holds rain water.

The wallows serve as temporary watering holes

and they are a spot favored by the buffalo to

"wallow in" to cool off and drink from on hot

days.


Human activity:

Besides the natural lakes, man has now created

artificial lakes by erecting a concrete dam across

a river valley so that the river water can be kept

back to form reservoirs. An example is Lake

Mead above the Hoover Dam on the Colorado

River, U.S.A.

General features of

lakes

General features of lakes

The chemical and biological characteristics of

the lake depend on the following:

• Lake formation

• Basin size and shape

• Topography and chemistry


• Regional climate

• Biological community

• Anthropogenic activity


Lakes are extremely variable in their physical,

chemical and biological characteristics.

Physically they vary in terms of

Level of light

Temperature

Water currents


Chemically they vary in form of

Nutrients

Major ions and contaminants

Biologically in terms of

Biomass

Population numbers and growth

Zonation in lakes

Zonation in lakes

Deep lakes normally consist of four distinct

zones

Littoral zone

Limnetic zone

Profundal zone

Benthic zone

Zonation in lakes

Littoral zone:

A shallow water area near the shore where

maximum light can penetrate.

The aquatic weed and phytoplankton are the

basis of the aquatic food chain which supports

the most diverse life forms ranging from aquatic

plant life, zooplankton, crustacean and fish.

Zonation in lakes

Littoral zone possess

• Algae

• Aquatic macrophytes

• Animals may include tiny crustaceans,

flatworms, insect larvae, snails, frogs, fish, and

turtles.

Zonation in lakes

Limnetic zone:

Open water area away from shore where still

enough light can penetrate.

The part of the limnetic zone that gets sunlight is

called the Euphotic zone.

Zonation in lakes

Aphotic zone is just below the photic zone

where light cannot penetrate.

Limnetic zone possess

Main photosynthetic body

Produces food and oxygen for consumers

Most abundant organisms are microscopic

phytoplankton and zooplankton

Zonation in lakes

Profundal zone:

A deep zone of standing body of water where

light can not penetrate.

No photosynthesis

Oxygen levels are often low

Cool and dark water with fish species is found

Inhabited by fish adapted to cool dark waters

Zonation in lakes

Benthic zone:

The bottom of the lake containing organisms called

benthos.

o Decomposers

o Detritus feeders

o Invertebrates

o Some fishes

Zonation in lakes

Physical processes in

lakes, reservoirs and

rivers

Physical processes in lakes,

reservoirs and rivers

The principal mechanisms and functions of

physical force that affect the vertical and

horizontal structure of lakes and reservoirs

include the following:

1. External mechanisms

2. Internal mechanisms



External mechanisms:

• Wind

• Barometric pressure

• Heat transfer

• Intrusion



• Downstream flow

• Coriolis effect (from the rotation of the Earth)

• Discharges on the surface



Internal mechanisms

• Stratification

• Vertical mixing

• Selective removal or selective loss

downstream

• Density currents



Mixing and vertical stratification are dynamic

phenomenon for the structure and organization

of chemical and biological processes in lakes,

reservoirs, rivers and estuaries.



Wind exerts an action of turbulence stress on

the water surface. As a consequence, the

following phenomena occur:

• Surface currents

• Accumulation of water on the surface, in the

direction of wind, and an oscillation in the

stratified interface.



Wind-produced kinetic energy, therefore,

generates currents, waves, turbulence and

transient situations that promote mixing and

dissipation.

Penetration of solar

energy in water

Penetration of solar energy in water

The term Light is generally used to refer to the

portion of the electromagnetic spectrum to which

the human eye is sensitive (i.e., the region of the

spectrum considered visible – in the range of

390–740 nm).The aesthetic value of a body of

water is related to its color and hence to the

quality of the water.


Life essentially depends on the amount and

quality of solar energy that is available on the

surface and distributed in the water column. In

any medium, light is related to its color, and this

in turn, to water quality.


The solar radiation reaching the top of the

atmosphere (in unit area and unit time) is called

the solar constant. The intensity and quality of

solar radiation changes, and its quantity is

substantially reduced during its passage through

the atmosphere.


Light plays an important role in lake ecology and

determines the potential rate of photosynthesis,

which supplies dissolved oxygen and food in the

water. Nearly all energy that controls the

metabolism of lakes and streams is derived

directly from the solar energy utilized in

photosynthesis.


Light intensity varies with seasons and depth.

Deeper the light penetration, higher is the rate of

photosynthesis.

The rate at which the penetration of light

decreases with depth depends on the amount of

suspended particles in water.


When light passes down through a column of

water, the amount of light available at any

particular depth decreases exponentially, as most

of the light is absorbed in the surface waters. The

depth of light penetration depends very much on

the color and turbidity of the water.


A Secchi disk is an 8-

inch (20 cm) black and

white colored wooden

disk used for

measuring light

penetration in water.


It is lowered into the water of a lake until it can

no longer be seen by the observer. This depth of

disappearance, called the Secchi depth, is a

measure of the transparency of the water.

Density

Density

Water differs from other compounds as it is less

dense as solid than as liquid (most dense at 4°C

and less dense at both higher and lower

temperatures). When the surface water of the

lake warms up, heat takes a long time to

penetrate down through the water.

Density

During summer, temperature differences

between the upper and lower layers become

more distinct. Deep lakes generally become

stratified into three identifiable layers known as

Epilimnion

Metalimnion

Hypolimnion

Density

Change in density at the metalimnion acts as a

physical barrier preventing the mixing of

epilimnion and hypolimnion. Thermocline is

actually the plane or surface of maximum rate of

decrease of temperature with respect to depth.

Thus, thermocline is the point of maximum

temperature change within the metalimnion.

Density

Thermal stratification and heat exchange depend

on solar radiation and wind. In addition to

climate, the size and wind exposure of a lake and

water inflow are the major factors that determine

the type of circulation

Density

Circulation pattern in lakes:

• Amictic

• Meromictic

• Holomictic

• Monomictic

• Dimictic

• Polymictic

Density

Amictic:

Never mix as they are permanently frozen.

Meromictic:

mix only partially, the deeper layers never mix

either because of high water density caused by

dissolved substances or because the lake is

protected from wind effects.

Density

Holomictic:

Mix completely

Monomictic:

Mix only once each year, either in winter or

summer.

Dimictic:

Mix twice a year

Density

Polymictic:

Mixing frequently, they are shallow tropical

lakes with great wind exposure.

pH

The pH is a measure of the acid-base balance of

a solution and is defined as the negative of the

logarithm to the base 10 of the hydrogen ion

concentration.

The pH scale runs from 0 to 14 (i.e., very acidic

to very alkaline) with pH 7 representing a neutral

solution

pH

At a given temperature pH indicates the acidic or

basic character of the solution and is controlled

by chemical and biochemical processes in the

solution.

Diurnal variations in pH can take place due to

photosynthesis and respiration cycles of algae in

eutrophic waters.

pH

Acid-base balance of a water body can be

influenced by

• Inflow of industrial effluents

• Domestic sewage

• Atmospheric deposition of acid-forming

substances

pH

Acidity of water is controlled by

o Strong mineral acids

o Weak acids such as carbonic, humic and fulvic

acids

o Hydrolysing salts of metals such as iron and

aluminium

pH

The three main processes that affect lake pH are,

Photosynthesis

Respiration

Nitrogen assimilation

Nutrients in lake

water

Nutrients in lake water

Nutrients are the basic requirements of plants for

their growth along with water and sunlight.

The two most important nutrients present in

sufficient concentrations in fresh waters to

maintain a healthy ecosystem are,

Nitrogen

phosphorous


Aquatic plants and algae respond to even small

changes in the amount of nutrients present in the

water. The identification of the watershed areas

and land use activities that contribute to these

nutrients in the lake water is essential.


Phosphorous and nitrogen enter the lake in the

form of,

• Inorganic ions

• Inorganic polymers

• Organic compounds,

• Living micro organisms

• Detritus


Only a few of these forms are readily available

for plant and algal growth. A nutrient-poor lake

may have only about 1mg/L of phosphorous or

50 mg/L of nitrogen, while the most fertile lake

may have up to a milligram of phosphorous or

20-30 mg/L of nitrogen.

Nitrogen compounds

Nitrogen compounds

Plants and microorganisms convert inorganic

nitrogen to organic nitrogen.

The inorganic compounds include

• Nitrite

• Nitrate

• Ammonium ions

• Molecular nitrogen

Nitrogen compounds

These undergo biological and non-biological

transformations in the environment.

The major non-biological transformations

include

o Sorption (absorption and adsorption)

o Volatilization

o Sedimentation

Nitrogen compounds

Biological transformations include:

Assimilation of inorganic ions (ammonia and

nitrate) by plants and microorganisms to form

organic nitrogen (amino acids).

Reduction of nitrogen gas to organic nitrogen

and ammonia by microorganisms.

Nitrogen compounds

Oxidation of ammonia to nitrite and nitrate

(Nitrification).

Conversion of organic compounds to ammonia

during the decomposition of organic matter.

Bacterial reduction of nitrate to nitrous oxide and

molecular nitrogen under anoxic conditions

(Denitrification).

Nitrogen compounds

In lakes where the concentration of nitrogen

compounds is extremely low, plants can take up

additional inorganic nitrogen immediately. The

discharge of sewage into the water body causes

large growth of algae.

Nitrogen compounds

Nitrogen compounds

Ammonia:

Ammonia occurs naturally in water due to

• The breakdown of nitrogenous organic and

inorganic matter in soil and water

• Excretion by biota

Nitrogen compounds

• Reduction of nitrogen gas in water by micro-

organisms

• Gas-exchange in the atmosphere.

At certain pH levels, high concentrations of

ammonia are toxic to aquatic life and are

detrimental to the ecological balance of water

bodies.

Nitrogen compounds

In aqueous solution, un-ionized ammonia exists

in equilibrium with ammonium ion. Total

ammonia is the sum of these two forms, also

forming complexes with several metal ions, and

may be adsorbed into colloidal particles, and

suspended and settled sediments.

Nitrogen compounds

The concentrations of un-ionized ammonia

depend on,

o pH

o Temperature

o Total ammonia concentration

Nitrogen compounds

Nitrate and nitrite:

Nitrate ions are commonly found in natural

waters. It may be reduced to nitrite by

denitrification process (usually under anaerobic

conditions).

Nitrogen compounds

The nitrite ions rapidly oxidize to nitrate, which

is an essential constituent of aquatic plants,

although seasonal fluctuations can occur due to

plant growth and decay.

Determination of nitrite and nitrate in surface

waters gives a general indication of the nutrient

status and level of organic pollution.

Nitrogen compounds

Organic nitrogen:

Organic nitrogen consists of protein substances

and the product of their transformations. It is

subject to seasonal fluctuations of the biological

community, formed in water by phytoplankton

and bacteria, and recycled within the food chain.

Phosphorous

Phosphorous

Phosphorous is an essential nutrient for living

organisms and exists in water bodies as

dissolved and particulate matter. In natural

waters, it occurs mostly as dissolved

orthophosphates and polyphosphates and

organically bound phosphates.

Phosphorous

Phosphorous is rarely found in high

concentrations in freshwaters and ranges from

0.005 to 0.020 mg/L. High concentrations of

phosphates can indicate the presence of pollution

and are responsible for eutrophic conditions.

Phosphorous

Phosphorous is much more readily lost from an

ecosystem than nitrogen and carbon as it reacts

with mud and chemicals in water in ways that

make it unavailable for plants. Plants can absorb

phosphorous only as dissolved inorganic

phosphorous, which is rapidly taken up by algae

and macrophytes.

Phosphorous

Bacteria in the sediments at the bottom of the

lake break down organic content of dead plants

and animals and phosphate is released into the

water in the spaces between the sediment

particles. This process is rapid in sediments

devoid of oxygen.

Phosphorous

Aquatic plants and algae absorb the released

phosphate in the water and their population’s

increase. This enhances the death and

decomposition of more phosphate containing

materials in the water, which in turn reduces the

oxygen levels and speeds up the release of more

phosphate.

Organic matter

Organic matter

Organic matter in freshwater arises from living

material and as a constituent of many waste

materials and effluents. The total organic matter

can be a useful indication of pollution. In surface

waters, concentration of total organic carbon is

less than 10 mg/L.

Organic matter

Chemical Oxygen Demand (COD):

Chemical oxygen demand is the amount of

oxygen that is required to oxidize the organic

matter present in water.

COD testing is used to determine the amount of

organic matter in a water sample as well as

amount of inorganic chemicals in a sample.

Organic matter

Biochemical Oxygen Demand (BOD):

Biochemical Oxygen Demand or Biological

Oxygen Demand, is a measurement of the

amount of dissolved oxygen (DO) that is

required by microorganisms to decompose

organic matter in water.

Organic matter

Chemical Oxygen Demand (COD) is similar to

Biochemical Oxygen Demand (BOD) in that

they are both used to calculate the oxygen

demand for oxidization of organic and inorganic

matter.

Organic matter

The difference between the two is that Chemical

Oxygen Demand (COD) measures amount of

oxygen that can oxidize both organic matter as

well as inorganic matter, whereas Biochemical

Oxygen Demand (BOD) only measures the

oxygen demanded by organisms to oxidize

organic matter in water.

Organic matter

Humic and Fulvic acids:

Organic matter from flora and fauna makes a

major contribution to the natural quality of

surface water, the composition of which is

extremely diverse. Natural organic matter is not

toxic but exerts major influence on biochemical

processes in the water body.

Organic matter

Humus is formed by biochemical and chemical

decomposition of vegetative residues and

microorganism activity. It enters directly from

the soil or is a result of biochemical

transformations within the lake. Humus is

divided into humic and fulvic acids.

Major ions in lake

water

Major ions in lake water

The chemical composition of a lake is a function

of its,

o Climate

o Hydrology

o Basin geology


Each lake has three major anions and four major

cations which are in ionic balance.

Ion balance means the sum of negative ions

equals the sum of positive ions when expressed

in equivalents. These ions are expressed as mg/L

(ppm).


Anions Percent Cations Percent

HCO3 73% Ca 2+ 63%

SO4 16% Mg2+ 17%

Cl 10% Na+ 15%

K+ 4%

Other <1% Other < 1%

Ion balance for fresh water


Hard water lakes

Lakes with high concentrations of calcium and

magnesium are called hard water lakes.

Soft water lakes

Those lakes with low concentrations of these

ions are called soft water lakes.


Importance of major ions in lake water:

Calcium is essential for all the cell processes

of plants and animals and serves as a structural

component for the shells of invertebrate

animals like molluscs.

Magnesium is important for photosynthesis.


Iron is a major component of the red blood

pigment haemoglobin (found in all

invertebrates) and also for certain cell

processes.

Silica is essential for the growth of diatoms as

their outer cast (frustule) is entirely made up of

silica.

Reynolds Number

Reynolds Number

In 1883, a British scientist named Osborne

Reynolds (1842-1912) classified flows as either

laminar or turbulent from a series of experiments

known as the Reynolds’ experiment.

Reynolds Number

Flows were

visualized by

pouring a stream

of ink into a pipe

in which water

flows.

Reynolds Number

Laminar flow

The result showed that, when the water

velocity was low, the ink moved downstream in

a continuous straight line. In this case, the flow

was laminar.

Reynolds Number

Turbulent flow

However, when the water velocity was high, the

ink started in a straight line but began oscillating

and quickly dispersed throughout the pipe. This

flow was turbulent.

Reynolds Number

In the experiment, Reynolds discovered

a dimensionless number could be used to

classify flows as either laminar or turbulent. This

number is called the Reynolds number.

Reynolds Number

Reynolds number formula is given by:

ρ is the density of the fluid

V is the velocity of the fluid

μ is the viscosity of fluid

L is the length or diameter of the fluid.

It is dimensionless

Reynolds Number

Reynolds number formula is used to find the

followings of a fluid

Velocity (V),

Density (ρ),

Viscosity (μ)

Diameter (L)

Reynolds Number

The Kind of flow depends on value of Re

• If Re < 2000 the flow is Laminar

• If Re > 4000 the flow is turbulent

• If 2000 < Re < 4000 it is called transition flow.

Thermal stratification


The thermal stratification of lakes refers to a

change in the temperature at different depths in

the lake, and is due to the change in water's

density with temperature.

This profile changes from one season to the next

and creates a cyclical pattern that is repeated

from year to year.


Lake stratification is the

separation of lakes into three

layers:

Epilimnion - Top of the lake.

Metalimnion- Middle layer

that may change depth

throughout the day.

Hypolimnion - The bottom

layer.


Lake stratification in summer

As air temperatures rise in late spring, heat from

the sun begins to warm the lake.

As the amount of solar radiation absorbed

decreases with depth, the lake heats from the

surface down.


The warm water is less dense than the colder

water below resulting in a layer of warm water

that floats over the cold water. The layer of

warm water at the surface of the lake is called

the Epilimnion.


The cold layer below the epilimnion is called the

Hypolimnion.

These two layers are separated by a layer of

water which rapidly changes temperature with

depth. This is called the Metalimnion.


In the metalimnion, the region in which the

temperature drops at least 1°C per meter is called

the Thermocline.

The three distinct layers of water, each with a

different temperature or range of temperatures, is

an excellent example of thermal stratification

within a lake system


Lake stratification in winter

As the winter approaches, the lake gets colder

until the water attains a uniform temperature of

4°C at which it has maximal density. As the

surface cools below it becomes lighter.

Eventually the surface water may freeze at 0°C.


During the winter season, the ice cover forms on

the surface and in such ice-bound lakes there

exists an inverse stratification of water

temperature, with the coldest water (ice) at the

surface and the warmest water (4°C) on the

bottom.


and mixing

Thermal stratification and mixing

Thermal stratification depends on mainly two

factors

Water density

Water temperature

When water temperature and density become

same in a lake then mixing of lake water occurs.


In this topic we will discuss that how process of

thermal stratification and lake water mixing

occurs with respect to different season like

summer, fall, winter and spring depending upon

the lake water density and temperature.


Thermal stratification and mixing in Summer:

• In summer, the sun heats the top layer of a lake,

the epilimnion, which causes it to become less

dense.

• The bottom layer of the lake, the hypolimnion,

does not receive sunlight and therefore remains

cold.


• Since the epilimnion is less dense, it floats on top

of the hypolimnion and the two do not mix.

• The thermocline is the dividing area between the

top and bottom layers.

• As the epilimnion is the only part of the lake that

sunlight can penetrate, it is where plants and algae

grow.


• Around the shoreline of a lake, the area where

sunlight penetrates and vascular plants grow is

called the littoral zone.

• In the middle of the lake, the epilimnion is

home to algae and zooplankton.


• When algae and zooplankton die, they sink to

the bottom of the lake.

• Invertebrates and microbes living in the

benthos recycle and decompose this dead

material. This recycling process uses up

oxygen.


• Since the lake does not

mix during the summer,

the hypolimnion is

completely cut off from

the epilimnion and does

not receive a fresh

supply of oxygen.


Thermal stratification and mixing in Fall:

o In fall the sunlight is not as strong and the

nights become cooler.

o This change in season allows the epilimnion to

cool off.


o As the water in the epilimnion cools, the

density difference between the epilimnion and

hypolimnion becomes minimum.

o Wind can then mix the layers.

o In addition, when the epilimnion cools it

becomes more dense and sinks to the

hypolimnion, mixing the layers.


o This mixing called

fall overturn allows

oxygen and

nutrients to be

distributed across

the whole water

column.


and mixing


Thermal stratification and mixing in Winter:

In winter season, the lakes are covered with

ice. Under the ice, the water cannot mix because

it is not exposed to wind.

Most of the hypolimnion remains 4 degrees

Celsius. There is a thin layer of water under the

ice that is colder than 4C and therefore less dense.


This thin layer of water floats on top of the

hypolimnion throughout the winter.

This phenomenon is called inverse

stratification because cooler water is sitting on

top of warmer water.


When the hypolimnion becomes anoxic in the

winter it is called winter kill because fish and

other living organisms that need oxygen die.

when the bottom of the lake is anoxic,

chemical processes at the sediment/water

interface cause phosphorus to be released from

the sediments.


When the ice melts in

the spring and the lake

mixes again, this

increased phosphorus

fuels algae growth.


Thermal stratification and mixing in Spring:

In spring season, the ice melts off the lake and

temperature as well as density of both

epilimnion and hypolimnion becomes same.

Now the wind picks up and the lake mixes the

lake water throughout the lake. This is called

spring turnover.


Oxygen and nutrients get

distributed throughout

the water column as the

water mixes.


As the weather becomes warmer, the surface

water warms again and sets up summer

stratification.

Most lakes are considered Dimictic lakes, as

their water is mixed twice a year. Once in

spring and second time in fall season.

Factors affecting

thermal stratification

Factors affecting thermal

stratification

The process of thermal stratification depends on

following factors,

Weather pattern

Lake depth

Wind fetch

Topography

Solutes


stratification

Weather pattern:

Different patterns of annual and daily variations

in temperature, as are found in more tropical and

artic areas, have great effect on thermal

stratification.


stratification

Lake depth:

Shallow lakes tend to be easily stirred by wind,

so stratification is more easily destroyed by wind

storm.

On the other hand deep lakes are thermally well

stratified depending on temperature and weather.


stratification

Wind fetch:

The distance wind travels over water before

meeting an obstacle, like a shore line or reef, is

the fetch of the wind.

The longer the fetch the more likely a wind can

mix the lake.


stratification

Topography:

Topography means surface feature of a specific

area or site.

Lakes and reservoirs form in (sometimes deep)

depressions which naturally reduce the

interaction between surface and bottom water.


stratification

Hence, vertical stratification in lakes and

reservoirs becomes increasingly important as the

lake becomes more laminar, has longer residence

times and deeper bottom water.


stratification

Solutes:

Salts in lake water make the water denser.

Enough solutes will make the water more dense

and it will resist the water mixing in lakes.

Shallow saline lakes such as Great salt lake can

be mixed by strong wind.

Division of lakes on

water mixing

Division of lakes on water mixing

Lakes and reservoirs can be grouped into

categories related to their vertical thermal

pattern and their evolution during the seasonal

cycle of stratification and circulation.


Thus, the systems can be classified as:

o Monomictic lakes

o Dimictic lakes

o Amictic lakes

o Polymictic lakes

o Meromictic lakes


The definitions centered on circulation patterns,

as the roots of the names indicate, and refer to

lakes that are of sufficient depth to form a

hypolimnion.

Monomictic Lakes

Monomictic Lakes

Lakes that mix from top to bottom during one

mixing period each year.

Monomictic lakes may be subdivided into:

1. Cold monomictic lakes

2. Warm monomictic lakes

Monomictic Lakes

Cold monomictic lakes:

• Cold monomictic lakes are lakes that are

covered by ice throughout much of the year.

• Water temperatures never exceed 4°C, and

with only one period of circulation in summer

at or below 4°C.

Monomictic Lakes

• During their brief "summer", the surface

waters remain at, or below, 4 °C.

• The ice prevents these lakes from mixing in

winter.

• These lakes are typical of cold climate regions.

Monomictic Lakes

• During summer, these lakes lack significant

thermal stratification, and they mix thoroughly

from top to bottom.

• Lake Char on Cornawallis island is a good

example of cold monomictic lakes.

Monomictic Lakes

Warm monomictic lakes

Warm monomictic lakes are lakes that never

freeze, and are thermally stratified throughout

much of the year.

These lakes circulate freely once a year in the

winter at or above 4°C and are stably stratified

for the remainder of the year.

Monomictic Lakes

The density difference between the warm

surface waters (the epilimnion) and the colder

bottom waters (the hypolimnion) prevents

these lakes from mixing in summer.

These lakes are not covered with ice.

Monomictic Lakes

During winter, the surface waters cool to a

temperature equal to the bottom waters.

Lacking significant thermal stratification, these

lakes mix thoroughly each winter from top to

bottom.

Lake Oglethorp in Athens is a good example of

warm monomictic lakes.

Dimictic lakes

Dimictic lakes

Lakes are called Dimictic if they circulate freely

twice a year in the spring and fall.

They are directly stratified in summer and

inversely stratified under ice cover in winter.

Dimictic lakes mix from the surface to bottom

twice each year.

Dimictic lakes

Dimictic lakes represent the most common type

of thermal stratification observed in most lakes

of the cool temperate regions of the world.

Such lakes also are found commonly at high

elevations in subtropical latitudes.

Dimictic lakes

Spring Overturn

In the spring the ice melts off the lake. The

epilimnion becomes cool and the temperature

and density of both epilimnion and hypolimnion

becomes same. Now wind causes the mixing of

both layers called as spring overturn

Dimictic lakes

Oxygen and nutrients get distributed throughout

the water column as the water mixes. Then, as

the weather becomes warmer, the surface water

warms again and sets up summer stratification.

Dimictic lakes

Fall Overturn

In the fall months, cooler air temperatures

decrease the surface water temperature of a lake.

The densities of the upper and lower waters

become similar, and the wind mixes the layers of

water together.

Dimictic lakes

When the water temperature in the lake becomes

uniform, this is known as fall turnover.

Oxygen levels are replenished in the deep water

and nutrients are also mixed.

As winter approaches lake again stratified by the

ice cover that prevent mixing.

Dimictic lakes

Example of Dimictic lakes

Lake Mendota, Wisconsin is an good example of

Dimictic lakes in which two turn over; spring

and fall turnover occurs.

Polymictic lakes

Polymictic lakes

Polymictic lakes present many periods of

circulation annually.

Polymictic lakes are influenced by diel (24hour)

fluctuations in temperature, such as superficial

warming and nocturnal cooling.

Polymictic lakes

In general, shallow lakes that experience year-

round wind action present this type of

circulation.

Stratification can occur for a few hours or even

days, but then quickly disappears.

Polymictic lakes

Polymictic lakes are further divided into,

Cold Polymictic lakes

Warm Polymictic lakes

Polymictic lakes

Cold Polymictic lakes

• Cold Polymictic lakes that circulate

continually at temperatures near or slightly

above 4°C.

• Such lakes are found in equatorial regions of

high wind and low humidity where little

seasonal change in air temperatures occurs

Polymictic lakes

• At very high altitudes in equatorial regions,

cold Polymictic lakes gain a significant

amount of heat during the day, but nocturnal

losses are sufficient to permit complete mixing

during them night.

Polymictic lakes

Warm Polymictic lakes

o Warm Polymictic lakes are usually tropical

lakes that exhibit frequent periods of

circulation at temperatures above 4°C.

o Annual temperature variations are small in

equatorial tropics.

Polymictic lakes

o These variations result in repeated periods of

circulation between short intervals of heating

and weak stratification, followed by periods of

rapid cooling.

o Under these circumstances, convectional

circulation is sufficient, in combination with

wind, to disrupt stratification.

Polymictic lakes

Examples include Lake George (Uganda) and

Clear Lake (California).

The reservoir behind the Lobo dam , in the State

of Sao Paulo, is an example of a classic

Polymictic reservoir.

Meromictic lakes

Meromictic lakes

A number of lakes do not undergo complete

circulation, and the primary water mass does not

mix with a lower portion. Such lakes are termed

Meromictic lakes.

The term Meromictic was introduced by

Findenegg (1935).

Meromictic lakes

In Meromictic lakes, the deeper stratum of water

that is perennially isolated is called the

Monimolimnion.

This stratum underlies the upper Mixolimnion,

which periodically circulates.

Meromictic lakes

These two strata are separated by a steep salinity

gradient, the stratum of which is the

Chemolimnion, the plane of density change is

called the Chemocline.

These lakes have high concentrations of

dissolved substances in the lower layer. In such

lakes, added density is the principal stratifying

factor and not temperature.

Meromictic lakes

1. Ectogenic Meromixis

2. Biogenic or endogenic Meromixis

3. Crenogenic Meromixis

Meromictic lakes

Ectogenic Meromixis

The condition that results when some external

event brings salt water into a freshwater lake or

fresh water into a saline lake is called Ectogenic

Meromixis.

Meromictic lakes

• The result in either case is a superficial layer

of less dense, less saline water overlying a

Monimolimnion of saline, more dense water.

• Such situations are found often along marine

coastal regions where catastrophic intrusions

of salt water from storms associated with

unusual tidal activity are fairly common

events.

Meromictic lakes

Biogenic or endogenic Meromixis

Biogenic or endogenic Meromixis results from

an accumulation of salts in the Monimolimnion,

which are usually liberated by means of

decomposition in the sediments and from

sedimenting organic matter.

Meromictic lakes

Biogenic Meromixis is caused by the substantial

contributions of biological material from internal

or external (autochthonous or allochthonous)

sources – such as litter from forests surrounding

the lake.

Meromictic lakes

Crenogenic Meromixis

Crenogenic Meromixis results from submerged

saline springs that deliver dense water to deep

portions of lake basins. The saline water

displaces the water of the Mixolimnion.

Meromictic lakes

Crenogenic Meromixis results from the

intrusion of saltier water from subsurface

sources, establishing steep vertical gradients of

salinity. A classic example is Lake Kivu in

Africa.

Division of lakes on base

of primary productivity

Division of lakes on base of primary

productivity

Productivity is the rate of production for a given

group of organisms, essentially representing the

net growth rate of organisms. Lakes seldom

suffer from shortage of water but are often

unproductive due to lack of nutrients necessary

for plant growth and reproduction.

Division of lakes on base of

primary productivity

Plants comprise the basic food material, directly

or indirectly, for the whole lake ecosystem. Their

abundance can therefore indicate the lake’s

productivity.



Productivity is measured as the new carbon

collected from the air and fixed as organic

compounds by photosynthesis, so that it can be

added to the total food supply in the lake.



Lakes are classified based on productivity as

follows:

Oligotrophic lakes

Eutrophic lakes

Eutrophic lake

A lake with a large supply of nutrients needed by

producers is called a eutrophic lake.

They are also designated as well-nourished lakes

because they receive maximum nutrients.

Eutrophic lake

Eutrophic lake usually have

o Shallow depth

o Murky brown or green water

o High turbidity

o High net primary productivity.

Eutrophic lake

Eutrophic lake

Cultural eutrophication

Human inputs of nutrients from the atmosphere

and from nearby urban and agricultural areas can

enhance the biological activity of lakes by a

process called cultural eutrophication or

simply eutrophication.

Eutrophic lake

Hypertrophic lake

Lake having excessive amounts of dissolved

nutrients.

Mesotrophic lake

Alake having moderate amounts of dissolved

nutrients.

Eutrophic lake

A eutrophic lake having an excess of plant nutrients. Its surface is covered with

mats of algae and cyanobacteria.

Oligotrophic lakes

Oligotrophic lakes

Ecologists classify lakes according to their

1. Nutrient content

2. Primary productivity.

Oligotrophic lakes

Lakes that have a small supply of plant nutrients

are called oligotrophic lakes.

These have low primary productivity, and low

biomass associated with low concentrations of

nitrogen and phosphorous (nutrients).

Oligotrophic lakes are poorly nourished lakes.

Oligotrophic lakes

• Often deep

• Steep banks

• Glaciers and mountain streams supply water

• Little sediment or microscopic life

• Crystal-clear water

• Small populations of phytoplankton and fishes such as

smallmouth bass and trout

• Low net primary productivity.

Over time, sediment, organic material, and

inorganic nutrients wash into most oligotrophic

lakes, and plants grow and decompose to form

bottom sediments.

Oligotrophic lakes

Oligotrophic lakes

Crater Lake in the U.S. state of Oregon is an example of an oligotrophic lake that is

low in nutrients. Because of the low density of plankton, its water is quite clear.

Dissolved Oxygen

Dissolved Oxygen

Dissolved Oxygen (DO)

Oxygen saturation in water medium.

Dissolved oxygen range for healthy biological

system is, 5-12 mg/liter.

Dissolved Oxygen

Most aquatic organisms require oxygen for their

metabolic activities.

The supply of oxygen in water comes from,

The exchange with the atmosphere.

Photosynthesis in flora and cyanobacteria

(blue green algae).

Dissolved Oxygen

Photosynthesis produces organic matter and

releases oxygen, whereas aerobic respiration

consumes organic matter and uses oxygen.

Oxygen production (photosynthesis)

predominates in the light whereas oxygen

consumption takes place in the dark.

Dissolved Oxygen

Based on these processes lakes can be divided

into two zones,

o Trophogenic zone

o Tropholytic zone

Dissolved Oxygen

Trophogenic zone:

Trophogenic zone also called as lighted

trophogenic zone is a zone where maximum light

is available and organic matter is synthesised

along with production of oxygen.

Dissolved Oxygen

The trophogenic zone often corresponds to the

epilimnion. The organic matter produced during

photosynthesis sinks to the bottom but the

oxygen remains in the surface layers

(epilimnion). The entire water mass is

oxygenated only during periods of circulation

due to action of wind.

Dissolved Oxygen

Tropholytic zone:

Tropholytic zone is a zone where light cannot

penetrate and decomposition of organic matter

occurs that leads to lowering of oxygen level.

Dissolved Oxygen

During stagnation periods (absence of wind),

only the epilimnion is oxygenated. The oxygen

required for the decomposition of organic matter

that sinks to the bottom must therefore come

from the supply obtained during periods of

circulation. Thus the oxygen levels in the

hypolimnion are constantly depleted.

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