Aquatic Ecosystem
Civilizations
have depended on water bodies such as lakes, reservoirs, rivers and wetlands.
Water is essential not only to sustain human life but also to support the activities
that form the basis for thriving economics. Though the water resources are
essential to human societies who could pollute and degrade and limiting their
beneficial uses. Agriculture, mining, urban development and other activities
can pose risks to freshwater bodies and hence steps have to be taken to reduce
these risk factors.
Risk
analysis requires knowledge of how human land use affects physical, chemical
and biological characters of the aquatic systems. One of the critical areas
required to understand how human actions and natural processes affect lakes,
reservoirs, rivers and wetlands is the science called Limnology. It is a
multidisciplinary science that integrates the basic sciences (Biology,
Chemistry, Physics and Geology) in order to study inland waters as complex
ecological systems.
Definition
The
term Limnology is derived from Greek
word; Limne means lake and logos means knowledge. Limnology is often
regarded as a division of ecology or environmental science.
It
is however, defined as “the study of inland waters” (running and standing
waters fresh and some times saline; natural or man made). This includes the
study of lakes, ponds, rivers, reservoirs, swamps, streams, wet lands, bogs,
marshes etc. Hence, it is commonly defined as that branch of science which
deals with biological productivity of inland waters and with all the causal
influences which determine it (Welch, 1963).
Biological
productivity, as used in this definition, includes its qualitative and
quantitative features and its actual and potential aspects. Under the term
inland waters are included all kinds or types of water – running or standing;
fresh, salt or other physicochemical composition which are wholly or almost
completely included within the land masses. Causal influences involve various
factors – physical, chemical, biological, meteorological etc which determine
the character and quantity of biological production.
History
The
term Limnology was coined by Francois-Alphonse Forel (1841 – 1912) who
established the field with his studies on Lake Geneva. Interest in the
discipline rapidly expanded and in 1922 August Thienemann (a German Zoologist)
and Einar Naumann (a Swedish Botanist) co-founded the International Society of
Limnology (SIL, for originally Societas Internalis Limnologiae). Forel’s
original definition of limnology, oceanography of lakes was expanded to
encompass the study of all inland waters.
Welch
(1935) conceived the problem of “Biological productivity” as the central theme
of Limnology. He defined Limnology as that branch of science which deals with
all causal influences which determine it. According to Schwoerbel (1987),
Limnology is the science of inland waters viewed as ecosystems together with
their structures, materials and energy balance.
Kiihnelt
(1960) considered limnology as a sub set of ecology along with “Oceanography”
(which is concerned with marine ecosystem) and “Epheirology” (which deals with
terrestrial habitats).
In short, Limnology is the study of
all aquatic systems including lakes, wetlands, marshes, bogs, ponds,
reservoirs, streams, rivers etc. with regard to their physical chemical and
biological characteristics.
In
addition to the above, certain other terms, like Hydrobiology, Freshwater
Biology, Aquatic Biology, Aquatic Ecology etc, are sometimes loosely used as
synonymous to the word 'Limnology'. But, most of these terms are names under
which a diverse variety of subject matter is included and only a part of it is
limnological in nature.
General characteristics
& faunal adaptations in: • Aquatic (freshwater ecosystem: lotic and lentic)
Fresh water Ecosystem
Freshwater Ecosystem
Characteristics
·
The freshwater ecosystem is a habitat
for various plant and animal species. One of the major reasons is that it is
quite rich in terms of nutrition and minerals.
·
The freshwater ecosystem is less saline,
unlike the marine ecosystem.
·
The temperature in this ecosystem varies
depending on some factors like location, season, and depth from the water
surface.
·
During summers, the temperature of the
freshwater ecosystem generally ranges from 30-71 degrees Fahrenheit. Whereas
during winters, the temperature ranges from 35-45 degrees Fahrenheit.
·
The size and shape of freshwater
ecosystems vary depending on location, an area covered, and depth of water
bodies.
·
The freshwater ecosystem contains
sediments at the bottom. In gentle flowing freshwater bodies or still water
bodies, the sediments remain in place.
·
The freshwater ecosystem provides a
suitable environment for various species of flora and fauna.
The fresh water on
earth is broadly classified as
1.
Lotic water
2.
Lentic water.
Lotic Freshwater
Ecosystem or Running water ecosystem
The lotic water in
simple terms is called running water.
Or
The water bodies moving
in one direction is known as a lotic freshwater ecosystem.
It includes
1.
Rivers
2.
streams
3.
Springs
are
common examples of lotic ecosystems.
There are many rivers
and stream that flows from their origin and ultimately meets with other water
channels or oceans at its mouth. Lotic freshwater travel through different
locations from its source to mouth.
Lentic Freshwater
Ecosystem or Standing water ecosystem
An aquatic ecosystem
within stagnant or standing or still
water like ponds and lakes is known as Lentic Freshwater Ecosystem.
Lentic ecosystem found in various sizes
ranging from a few square meters to thousands of square km.
It includes
1.
Lakes
2.
Ponds
3.
Ditches
4.
Marshes
5.
Bogs
Some ponds last just
for a few months as these are seasonal like sessile pools. On the other hand,
lakes may exist for many years. Lentic ecosystem, i.e. Ponds and lakes, support
a very limited number of species.
Lentic ecosystem divided
into three zones based on their depth
and distance from the shoreline.
1.
Littoral Zone
2.
Limnetic Zone or Photic Zone
3.
Profundal Zone or Aphotic Zone
Littoral Zone
This is the topmost
zone in the area near the shoreline of a pond or lake. The Littoral zone is
characterized by a shallow and warm zone of a lentic ecosystem.
This zone provides
shelter to various species of algae, few species of aquatic plants, clams,
crustaceans, amphibians, snails, and various insects, etc.
Flora and fauna found
in the littoral zone generally serve as food for other creatures like ducks,
turtles, etc.
Limnetic Zone or Photic
Zone
The open water zone
where sunlight supports the photosynthesis process is known as the photic zone.
The photic zone is also termed as a limnetic zone.
It is the zone of a
lentic ecosystem that is generally dominated by planktons (both phytoplankton
and zooplankton). As planktons are the primary producers, hence the limnetic
zone plays an essential role in the food chain of a freshwater ecosystem.
Profundal Zone or
Aphotic Zone
The deepwater zone
where sunlight hardly penetrates is known as a profundal or aphotic zone.
Photosynthesis is not possible in this zone due to the scarcity of sunlight.
The aphotic zone is cold as compared to the other two zones.
The aquatic animals
found in the profundal zone are heterotrophs in nature as they consume dead
organisms.
Lakes
Size of lake
Lakes differ in area from those ranges from a pond to those of great size. Lake Superior, the largest body of freshwater flow has an area of more than 49,600 km2. The Caspian sea with an area of 2,72,000 km2 is sometimes considered as having the quality of lake.
Lake Chad in Africa has 64,000 km2 during wet season, but is reduced to 9,600 km2 in the dry season. Ten of the large lakes in America including Great lakes have an combined area of about 2,03,200 km2. However the number of lakes whose area exceeds more than 8,000 km2 is insignificant when compared to many thousands of lakes of lesser magnitude of 11,000 or more lakes or ponds in Michigan.
Depth of lake
Lakes vary in depth but even the deepest lake will never approaches the depth of ocean. It is important to note that the lake Baikal has a greatest depth contains about 20% of the total volume of freshwater and it is also the deepest known lake with a maximum depth of 1620 m. In North America, Crater lake in Oregon is about 608m. Lake Tahoe is 487m, Lake Chelan (Washington) 457m. Seneka lake 188m, Lake Superior 393m, Lake Michigan 281m, Lake Huron 228m, Lake Ontario 273m, Lake Erie 64m, and the last 5 lakes constitute Great lakes of America.
Thermal Stratification of a Lake
Lakes
in temperate latitudes exhibit marked seasonal temperature changes which may be
described as follows:
Winter:
During
winter the coldest water forms ice at 0°C (32°F) and floats at the surface.The
water at increasing depth below the ice is progressively warmer and denser. The
heaviest water, at the bottom of the lake, has a winter temperature of 4°C and
throughout winter the water remains relatively stable.
Spring:
Following
the ice melt, the surface water gradually warms to 4°C. At this point the water
column is nearly isothermal, i.e., all the water is of uniform temperature and
density. Hence, the strong spring winds cause considerable stirring, which
results in a complete mixing of water, dissolved oxygen, and nutrients from the
lake surface to the lake bottom, a phenomenon known as the spring overturn or
spring turnover As the spring progresses, however, the surface waters naturally
become warmer and lighter than the water at lower levels, as a result, the lake
becomes thermally stratified into the following three zones
Typical
Thermal Stratification of a lake into three strata
(a)
Epilimnion
(b) Hypolimnion:
(c)
Metalimnion:
a. Epilimnion: The upper stratum, which usually
has the highest dissolved oxygen concentration and is characterized by a
temperature gradient of less that 1°C per metre of depth, is the epilimnion.
This
stratum contains more or less uniformly warm, circulating, and fairly turbulent
water.
(b)
Hypolimnion:
The
lower stratum of water characterized by a temperature gradient of less than 1°C
per metre of depth is the hypolimnion (literally the “lake below” or “Bottom
Lake”). This part contains more dense, cooler, and relatively quiet water.
(c)
Metalimnion:
It
is the transitional stratum of marked thermal change between the epilimnion
and hypolimnion. The middle layer of the lake, characterized by a
temperature gradient of more than 1°C per metre of depth is the thermocline.
This zone effectively divides the
lake into two layers, the upper epilimnion, and the lower hypolimnion. This
division is not merely an interesting physical phenomenon but it has
consequential effects on the ecology of the lake.
The
epilimnion is well lit and oxygenated with sufficiently high temperatures to
promote algal productivity and hence to support zooplankton and fish. When
nutrients are in ample supply, algal growth is accelerated and blooms may
occur. By contrast the hypolimnion is cold, dark and becomes progressively
deoxygenated as the decaying remains of organisms sink down from the
epilimnion.
epilimnetic
material provides an energy source for benthic invertebrates. The sinking of
dead algae and zooplankton from the epilimnion not only contributes to the
potential deoxygenation of the hypolimnion but also prevents immediate
recycling of nutrients. Nutrient depletion may become so high that algal growth
is limited.
Lakes
undergoing complete circulation in spring and autumn separated by thermal
summer stratification and winter inverse stratification are called dimictic
lakes . Such
lakes are quite common among temperate lakes of moderate size.
Classification of the lake on the basis of
productivity
Lakes are classified based on productivity as follows:
Oligotrophic lakes: These have low
primary productivity, and low biomass associated with low concentrations of
nitrogen and phosphorous (nutrients). They tend to be saturated with oxygen.
Mesotrophic lakes: These are lakes in
transition from oligotrophic to eutrophic conditions. Some depression of oxygen
concentration occurs in hypolimnion during summer stratification.
Eutrophic lakes: These display high
concentration of nutrients, high biomass productivity and low transparency.
Oxygen concentrations can get very low (as low as 1 mg/L) in the hypolimnion
during summer.
Hypereutrophic lakes: These are lakes at
the extreme end of eutrophication with very high concentration of nutrients and
associated biomass production. Anoxia or complete loss of oxygen takes place in
the hypolimnion during summer.
Dystrophic lakes: These are organic
rich lakes (humic and fulvic acids) fed by external inputs of the lake
(watershed).
Changes, which occur in the lake due to addition of nutrients (on
account of human activities), are called as eutrophication. The additional
nutrients (primarily nitrogen and phosphorous) most usually come from sewage,
industrial effluents or agricultural fertilisers, or all the three.
Bacteria that use up the oxygen in the water first decompose raw
sewage. Sewage always contains a higher concentration of phosphates than
nitrogen due to increased use of phosphate containing detergents. In unpolluted
waters, growth of algae is inhibited by the lack of phosphorous but the
addition of these nutrients (through domestic sewage, other effluents etc.)
increases algal growth in the receiving waters. While algae produce oxygen
during the day due to photosynthesis, they use it up in the night. Thus, dense
algal blooms cause suffocation of fish and other aquatic organisms.
Increased algal production due to eutrophication initiates other
changes in the aquatic ecosystem.
Thermal Classification of lakes
According
to Hutchinson (1957), following are the classification of lakes based on
changes in temperature of surface water.
a. Amictic:
No mixing of bottom and top water; lakes insulated or protected by ice-corer,
there is no effect of weather or external factors.
b. Monomictic:
One mixing of the two waters during the year (most deep lakes of the world).
c. Cold monomictic:
Water here at any depth never exceeds 4°C; they are ice-bound or ice-covered
only in winter; there are inverse thermal stratification top waters 0°C and
bottom waters 4°C (since water at 4°C is heaviest); only one mixing at
temperatures not more than 4°C in spring / summer eg, Polar lakes.
d. Warm monomictic:
Temperature of water never falls below 4°C at any depth. Direct thermal
stratification top waters 10 - 20°C and bottom waters 8 - 4°C; only one mixing
in a year in a winter eg, Most subtropical deep lakes.
e. Ploymictic :
Mixing is continuous, but occurs only at low temperatures.
Ponds
Ponds
are defined as small, shallow, inland standing water bodies, where rooted
plants can grow over most of the bottom. Ponds are mainly of three general
classes, they are :
i.
Those which represent the pond stage in the extinction of previously existing
lakes
ii.
Those whose basins have never been large or deep (not preceded by a lake) but
or for some special reason, have persisted in the pond stage and
iii.
Those whose basins are the results of man’s activities (excavations, quarries,
impoundments, etc.)
Natural
process alone are constantly forming new pond basins (cut-offs from streams
solution basins, beach ponds, and many others), some of which are never more
than temporary ponds from the beginning; others qualifying as permanent ponds
at least for a period in their existence.
Classification of ponds
With
respect to seasonal duration, ponds are divided into two general classes
a. Permanent –
those which contain some water the year round and
b. Temporary –
those in which the basin contains water at certain times or seasons and becomes
dry at others.
Those
which occur for a limited period in spring are called Vernal ponds
Those
which contain water in spring, dry up during summer, and again contain water in
the autumn are called Vernal autumnal
ponds and
Those
which contain some water throughout the open season but freeze to the bottom in
winter have been called Aestival ponds.
Other
classifications of ponds are as follows
Natural ponds
These
are perennial shallow water bodies. When a stream shifts its position it leaves
behind an isolated body of standing water which forms the "Ox-Bow"
pond. In limestone regions where depressions are formed due to the solution of
the underlying strata, the water gets accumulated either by flood water or
rainfall and natural ponds are formed. Sometimes the last remnant of a lake
whose basin has become filled progressively by sedimentation in course of time
is transformed into a pond.
Artificial
pond
Most
of the fish ponds are semi artificial ponds. Some are constructed by erecting
dams across a stream or basin and their water level can be regulated by inflow
and drainage where pisciculture is practiced. Fish pond is a shallow body of
water that can be drained completely. It is often supplied by running water,
but also by spring, ground or rain water.
Pools or Temporary Ponds
They
occur in depressions in the ground either at the margin of glaciers where they
fill with melt water or in the vicinity of river bed, after the floods have
receded. The water thus collected usually is very shallow and measures maximum
to a few feet only. Also prolonged rainfall may form temporary small pools. All
these pools dry up in some part of the year, and as such organisms in these habitats
must be able to survive in a dormant stage during dry periods and be able to
move in and out of the pools.
General Characteristics of ponds
•
Ponds are small, shallow standing water bodies.
•
They have calm water
•
Have more vegetation
•
Growth of plants can also found at the bottom
•
They have outlet streams
•
The movement of water is minimum
•
They have slight wave action
•
The average depth of water is 8 – 10 feet
•
The temperature of the pond more or less changes with that of atmosphere
•
Light penetrates up to the bottom
Running water or
Lotic Ecosystem
Water Streams and Rivers
STREAMS
Streams
are zones where a rapid flow of shallow water produces a shearing stress on the
stream bed, resulting in a rocky or gravel substratum covered by fully
oxygenated water. Streams may vary in size from tiny rivulet to rivers. As time
goes the stream may develop into river or increase its size, whereas the size
of reservoirs decreases as time passes. They are more numerous in regions of
abundant rain fall. They are temporary or permanent. Streams are closely linked
to their watersheds. The productivity of streams is often dependent on
terrestrial bases, grasses and other debris. The allocthonous materials
contribute most of the food and energy to the organisms in the stream. Benthic
invertebrates like insect larvae constitute the invertebrate fauna. True
plankton are almost absent in streams, and are common only in deep slow moving
stretches of rivers. All biota in streams are influenced by the unidirectional
current.
Physical conditions
The
annual change in stream temperature is 10 to 20°C. Although large rivers do not
change in temperature very much on a daily basis, a small unshaded stream may
heat up to 10°C in a few hours on a hot summer’s day and cool by the same amount
at night. The temperature of most streams is lowest in the upland and becomes
gradually warmer in the lower reaches.
The
velocity of stream water varies with the landforms. In plains, streams are slow
and sluggish throughout their length. In mountain stretches the speed of water
may be rapid.
Stream
water has uniform temperature and the difference between the surface and bottom
is virtually negligible. The stream follows air temperatures more closely than
lake waters and the factors responsible are depth of water, current velocity,
bottom material, temperature of entering water, exposure to direct sunlight and
degree of shading etc.
Extreme
of turbidity occur in running water series and streams with rock beds the
turbidity is minimal.
Stream
systems increase their length, width and depth with increasing age. This is in
distinct contrast to the reduction processes characteristic of all standing
water units.
At
any position along the course of a running water system, materials eroded at
that point and all materials suspended or dissolved at the level are
transported downstream with no opportunity to return. Interchanges of materials
are more and have less depth than lakes.
Chemical conditions
The
dissolved oxygen supply in uncontaminated stream is high at all levels often
near saturation. The polluted streams show low dissolved oxygen due to
accumulation of organic wastes. Stream which support more plants show diurnal
variation of dissolved oxygen. The level of dissolved oxygen is controlled by
the slope of channel and mode of flow.
Current
in streams tends to keep the pH in uniform over considerable distances. It
keeps any acidity due to accumulating free CO2 reduced. Streams waters do not
develop the more intense acidities.
The
dissolved solids of streams are affected by their irregular discharges. Most
streams and rivers have maximum discharge during winter rains, particulate
matters, nutrients like phosphate, iron and nitrate are transported to
different parts by the flow of the streams. Streams fed by springs have more
constant nutrients.
RIVER
River
is said to be a natural stream of water usually fresh water flowing towards an
ocean. In some cases river flows into the ground or dries up completely before
reaching another body of water. Usually larger streams are called rivers while
smaller streams are called creeks, brooks, rivulets, rills, and many other
terms.
A
river is a component of the hydrological cycle. The water within a river is
generally collected from precipitation, through surface run off, ground water
recharge and release of stored water in natural reservoirs such as glacier.
Topography
The
water in a river is usually confined to a channel, made up of stream bed
between banks. In larger rivers there is also a wider floodplain shaped by flood
waters over-topping the channel. Flood plains may be very wide in relation to
the length of river channel. This distinction between river channel and
floodplain can be indistinct especially in urban areas where the floodplain of
a river channel can become greatly developed by housing and industry.
Ecology
The
flora and fauna of rivers use the aquatic habitats available, from torrential
waterfalls through to lowland mires although many organisms are restricted to
the freshwaters in rivers eg salmon and Hilsa.
Flooding
Flooding
is a natural part of river cycle. The majority of the erosion of the river
channels and the erosion and deposition on the associated flood plain occur
during flood stage. Human activity, however has upset the natural way flooding
occurs by walling of rivers set straight their courses and by draining of
natural wetland.
Fauna of Freshwater
Ecosystem
The freshwater
ecosystem provides a perfect environment for various animal species. Some
animals of freshwater prefer moving water bodies like rivers, whereas some
others prefer to live in stagnant water like ponds, lakes, wetlands, etc.
•Freshwater
ecosystems contain several types of organisms that are grouped by their
location and by their adaptation.•
·
Three groups of aquatic organisms
include plankton, nekton, and benthos.•
·
Plankton
are the mass of mostly microscopic organisms that float or drift freely in the
water, and can be microscopic animals called zooplankton or microscopic plants
called phytoplankton.•
·
Nekton
are all organisms that swim actively in open water, independent of currents.•
·
Benthos
are bottom-dwelling organisms are often attached to hard surfaces.
·
Decomposers are also aquatic organisms.
Fishes
Fishes are the most
common species of the freshwater ecosystem. Some fishes like salmon, trout, etc.
prefer to live in moving clean water with a high level of oxygen. On the other
hand, small muddy ponds provide an ideal environment for fishes like catfish,
carp, etc.
Some freshwater fishes
like pike and sturgeon require a large area to live as they grow large. Big
lakes are a perfect place for these fishes.
Mammals, amphibians,
and Reptile
Various species of
mammals are also living in a freshwater ecosystem such as beavers, otters, etc.
Most of the mammals live in small water bodies like lakes. This type of
ecosystem is preferable for these mammals because they come to shores to
reproduce, feed and breathe.
Some animals come to
water streams and rivers to feed like bears, whereas some other animals like
muskrat spend their whole lifespan in ponds.
Amphibians like frogs,
salamanders belong to wetlands. The freshwater ecosystem is also a home for
some species of reptiles like an alligator, turtle, snakes, etc. These reptiles
live in a stagnant freshwater ecosystem.
Birds
Some birds are also a
part of the freshwater ecosystem. Ducks, geese, etc. are commonly found in
lakes, rivers, etc. On the other hand, some birds such as swallows belong to
swamps and ponds. The birds like swallows are insect-eating, and swamps are an
ideal shelter for these birds. It provides a good source of food.
