2. Introduction
Classification of freshwater ecosystem
Lentic ecosystem
Lentic system biota
Lotic ecosystem
Lotic system biota
Physical properties of freshwater
Chemical properties of freshwater
Biological properties of freshwater
3. Freshwater ecosystems are a subset of Earth's aquatic
ecosystems. They
include lakes and ponds, rivers, streams, springs,
and wetlands. They can be contrasted with marine
ecosystems, which have a larger salt content.
Freshwater habitats can be classified by different
factors, including temperature, light penetration, and
vegetation.
Freshwater ecosystems are particularly vulnerable to
the different components of climate change.
Freshwater ecosystems can be divided into lentic
ecosystems (still water) and lotic ecosystems (flowing
water).
4. Lentic refers to stationary or relatively still water, from
the Latin lentus, which means sluggish. Lentic waters range
from ponds to lakes to wetlands, and much of this article
applies to lentic ecosystems in general.
Lentic systems are diverse, ranging from a small, temporary
rainwater pool a few inches deep to Lake Baikal, which has
a maximum depth of 1740 m.
In addition, some lakes become seasonally stratified. Ponds
and pools have two regions: the pelagic open water zone,
and the benthic zone, which comprises the bottom and shore
regions.
Since lakes have deep bottom regions not exposed to light,
these systems have an additional zone, the profundal.
These three areas can have very different abiotic conditions
and, hence, host species that are specifically adapted to live
there.
5. Bacteria
Bacteria are present in all regions of lentic waters. Free-living
forms are associated with decomposing organic material, biofilm on
the surfaces of rocks and plants, suspended in the water column,
and in the sediments of the benthic and profundal zones.
Primary producers
Algae, including both phytoplankton and periphyton are the
principle photosynthesizers in ponds and lakes.
Phytoplankton are found drifting in the water column of the
pelagic zone.
Many species have a higher density than water which should make
them sink and end up in the benthos.
To combat this, phytoplankton have developed density changing
mechanisms, by forming vacuoles and gas vesicles or by changing
their shapes to induce drag, slowing their descent.
Invertebrates
Zooplanktons are tiny animals suspended in the water column. Like
phytoplankton, these species have developed mechanisms that keep
them from sinking to deeper waters, including drag-inducing body
forms and the active flicking of appendages such as antennae or
spines.
Fish and other vertebrates
Fish have a range of physiological tolerances that are dependent
upon which species they belong to.
They have different lethal temperatures, dissolved oxygen
requirements, and spawning needs that are based on their activity
levels and behaviors.
Because fish are highly mobile, they are able to deal with
unsuitable abiotic factors in one zone by simply moving to another.
6. Lotic refers to flowing water, from the Latin lotus, washed. Lotic
waters range from springs only a few centimeters wide to
major rivers kilometers in width.
The ecosystem of a river is the river viewed as a system operating in
its natural environment, and includes biotic (living) interactions
amongst plants, animals and micro-organisms, as well
as abiotic (nonliving) physical and chemical interactions.
The following unifying characteristics make the ecology of running
waters unique among aquatic habitats.
Flow is unidirectional.
There is a state of continuous physical change.
There is a high degree of spatial and temporal heterogeneity at all scales
(microhabitats).
Variability between lotic systems is quite high.
The biota is specialized to live with flow conditions.
7. Bacteria
Bacteria are present in large numbers in lotic waters. Free-living
forms are associated with decomposing organic material, biofilm on
the surfaces of rocks and vegetation, in between particles that
compose the substrate, and suspended in the water column.
Primary producers
Algae, consisting of phytoplankton and periphyton, are the most
significant sources of primary production in most streams and
rivers.
Phytoplankton float freely in the water column and thus are unable
to maintain populations in fast flowing streams. They can, however,
develop sizable populations in slow moving rivers and backwaters.
Insects and other invertebrates
Up to 90% of invertebrates in some lotic systems are insects.
These species exhibit tremendous diversity and can be found
occupying almost every available habitat, including the surfaces of
stones, deep below the substratum, adrift in the current, and in the
surface film.
Insects have developed several strategies for living in the diverse
flows of lotic systems.
Fish and other vertebrates
Fish are probably the best-known inhabitants of lotic systems. The
ability of a fish species to live in flowing waters depends upon the
speed at which it can swim and the duration that its speed can be
maintained.
This ability can vary greatly between species and is tied to the
habitat in which it can survive.
Continuous swimming expends a tremendous amount of energy
and, therefore, fishes spend only short periods in full current.
8. Physical properties
Physical properties of aquatic ecosystems are determined by a combination of heat, currents, waves and other seasonal
distributions of environmental conditions.
The morphometry of a body of water depends on the type of feature (such as a lake, river, stream, wetland, estuary etc.)
and the structure of the earth surrounding the body of water. Lakes, for instance, are classified by their formation, and
zones of lakes are defined by water depth.
Light interactions
Light zonation is the concept of how the amount of sunlight penetration into water influences the structure of a
body of water. These zones define various levels of productivity within an aquatic ecosystems such as a lake. For
instance, the depth of the water column which sunlight is able to penetrate and where most plant life is able to
grow is known as the photic or euphotic zone. The rest of the water column which is deeper and does not receive
sufficient amounts of sunlight for plant growth is known as the aphotic zone.
Thermal stratification
Similar to light zonation, thermal stratification or thermal zonation is a way of grouping parts of the water body
within an aquatic system based on how each layer has different temperature variations. The less turbid the water,
the more light is able to penetrate, and thus heating a thicker depth of water.
There are 3 main sections which define thermal stratification in a lake. The first is the epilimnion which is closest
to the surface and experiences primarily wind circulation although the water is generally uniformally warm
because of the close proximity to the surface.
The layer below is often called the thermocline and is an area within the water column which tends to experience
a rapid decrease in temperature. Finally, the layer which is the bottom-most within the body of water is
the hypolimnion which has uniformally cold water because of its depth which restricts sunlight from reaching
it. In temperate lakes, fall-season cooling of surface water to 4 °C (the highest density of water) results in turnover
of the water column.
9. The chemical composition of water in a natural environment is influenced mainly by precipitation, type of soil and
bedrock in the watershed, erosion, evaporation and sedimentation. All bodies of water have a certain composition of
both organic and inorganic elements and compounds.
Water quality
There are hundreds of variables which are considered to play a role in water quality however a few have been
determined to be of greater interest regarding the role they play in aquatic ecosystem health. While certain
biological activities affect dissolved gas concentrations, nutrients, etc. human activity is one of the strongest
influences on water quality.
Oxygen
Dissolved oxygen is an element which is necessary for a number of biological and chemical reactions which are
critical to the proper functioning of the ecosystem. Some of the biological processes which alter the
concentrations of dissolved oxygen include photosynthesis and aquatic organism respiration. Oxygen profile is
based on similar principles as thermal stratification and light penetration. Dissolved oxygen levels are generally
lower as you move deeper into the body of water because of the lower availability of light in those parts of the
water.
Carbon dioxide
Dissolve oxygen and dissolved carbon dioxide are often discussed together due the role they both play in aquatic
organism respiration. These organisms absorb dissolved oxygen from the water to use in respiration and expel
carbon dioxide as a byproduct of this process. Carbon dioxide tends to have an inverse diurnal relationship with
oxygen.
Other nutrients
Nitrogen and phosphorus are ecologically significant nutrients in aquatic systems. Nitrogen is generally present
as a gas in aquatic ecosystems Most of these dissolved nitrogen compounds follow a seasonal pattern with
greater concentrations in the fall and winter months compared to the spring and summer. Phosphorus has a
different role in aquatic ecosystems as it is a limiting factor in the growth of phytoplankton because of generally
low concentrations in the water. Dissolved phosphorus is also crucial to all living things, is often very limiting to
primary productivity in freshwater, and has its own distinctive ecosystem cycling.
10. Lake trophic classification
Limnology (study of freshwater ecosystem), classifies
lakes (or other bodies of water) according to
the trophic state index.
An oligotrophic lake is characterised by relatively low
levels of primary production and low levels of
nutrients.
A eutrophic lake has high levels of primary
productivity due to very high nutrient levels.
Eutrophication of a lake can lead to algal blooms.
Dystrophic lakes have high levels of humic matter and
typically have yellow-brown, tea-coloured waters.
