The study of ecology in freshwater (freshwater communities) is usually divided into two main categories, lentic (still) and lotic (running) water. These two bodies of water also have a bearing on which organisms are likely to occupy the area.
An attempt is made here to discuss the nature of these two categories of fresh water, and also, investigate how they affect the life which lives in them.
This article examines the still water communities while the next lesson will be devoted for the running water communities. A third category of fresh water ecology, known as wetlands, is also identified.
It relates to areas where the soil is saturated or inundated for at least a substantial part of the year. Freshwater ecosystems cover 0.80% of the Earth’s surface and inhabit 0.009% of its total water. They generate nearly 3% of its net primary production, and contain 41% of the world’s known fish species.
1. Freshwater Communities and Lentic Waters
Lentic (still water) communities can vary greatly in appearance and size, from a small temporary puddle to a large lake, and is capable of supporting life to some extent.
The type of life which is supported will depend greatly on the biotic and abiotic components of the freshwater ecosystem explained earlier.
The creation of many of today’s long standing freshwater lentic environments are a result of geological changes over a long period of time, notably human activities such as mining and road construction, glacial movement, erosion, and volcanic activity.
The consequence of these actions results in troughs in the landscape where water can accumulate and be sustained over time.
The size and depth of a still body of water are major factors in determining the characteristics of that ecosystem, and will continually be altered by some of the causes mentioned above over a long period of time.
One of the important elements of a still water environment is the overall effect that temperature has on it. The heat from the sun takes longer to heat up a body of water as opposed to heating up dry land.
This means that temperature changes in the water are more gradual, particularly so in more extensive areas of water. When this freshwater ecosystem is habitable, many factors will come into play, and eventually determine the overall characteristics of the environment which organisms will have to adapt to.
As with osmosis, temperature will even out across a particular substance over time, and this applies to a still body of water.
Sunlight striking the water will heat up the surface, and over time will create a temperature difference between the surface and basin in the body of water.
This temperature difference will vary depending on the overall surface area of the water and its depth.
With time, two distinctly different layers of water become established, separated by a large temperature difference and providing unique ecological niches for organisms.
This process is called stratification, where the difference in temperature between surface and water bed are so clear that they can easily be distinguished apart.
The surface area is referred to as the epilimnion, which is warmed water as a result of direct contact with sunlight.
The lower layer on the other hand is known as the hypolimnion, is found below the water surface and due to increased depth, receives less heat from the sun and therefore results in the colder water underneath.
Previous sections have elaborated on the importance of light to the freshwater community. Some factors can affect the amount of light received by autotrophic organisms (organisms that perform photosynthesis) can affect their level of photosynthesis and respiration, hence affect their abundance and therefore, subsequent species that rely on them.
Man has continuously polluted water sources, especially since the industrial revolution. Litter for example, and especially non-biodegradable litter, will block out light for light dependant organisms.
An oil spillage will also have the same effect, perhaps more extreme as the oil will situate itself on the surface of the water and block out light.
Organic material and sediment can enter the still water environment via dead organisms in the area, and water flowing into the area from hills and streams. Buoyant material will also block out light required by the primary producers of the ecosystem.
The friction caused by moving water against the water bed and its banks will result in disturbing loose sediment.
Depending on the weight of this sediment, heavier particles will slowly sink back to the bottom of the body of water while lighter materials will remain suspended in the water.
The lightest material will rise to the surface, resulting in less light available to organisms underneath the surface.
Naturally, the consequences of the above will result in less light for organisms that rely on photosynthesis as a means of food, and subsequently, organisms that feed on these autotrophic organisms will soon find that their food source is less freely available.
Another major factor affecting still water communities is the oxygen concentration of the surrounding area. Oxygen concentration is primarily affected by three factors
The surface area which the body of water is exposed to the open air environment The circulation of water, mainly due to temperature variations in different areas of the water body (convection currents)
Oxygen created as a result of respiration, consumption, and the oxygen consumed by animals and bacteria.
As mentioned earlier, temperature can also affect the concentration of oxygen available, which in turn, means that the depth of the water will therefore also have an effect.
In turn, carbon dioxide levels, which are closely related to the oxygen levels available, will be required by organisms undergoing photosynthesis.
The availability of these will affect the organisms in the ecosystem. Their relationships with temperature will also affect their availability.
Evidently, some of these factors vary through different conditions, and changes in one of the factors usually results in changes in the others. This is also the case of pH, for example, as an increase in carbon dioxide results in a drop in the value of pH.
Read Also : Life in Aquatic Ecosystems
With this information, we can now understand how organisms survive in these habitats in relation to these conditions described.
2. Still Water Animals
Through millions of years of evolution, animals living in an aquatic environment have diversified to occupy the ecological niches available in the ecosystem.
In studying the habitats of these particular organisms, three main areas of the freshwater environment can be distinctly classified.
The Profundal Region – This refers to an area of still water that receives no sunlight, and therefore lacks autotrophic creatures. The animals in this zone rely on organic material as a means of food, which is sourced from the more energy rich areas that are above this region.
The Pelagic Region – The pelagic region can be found below the surface water, and is defined by the light that is available to it. The pelagic region does not include areas near the shore or sea bed.
The Benthic Region – The benthic region incorporates the entire freshwater environment in contact with land, barring the shallow shore areas.
The benthic region is capable of hosting a large volume of organisms, as nutrient and mineral rich sediments are available as a food source while part of the benthic region can occupy the euphotic zone, the area of water where light is available.
This will allow an ecological niche for autotrophic organisms which in turn can be a food source for herbivores.
Another distinctive niche for the animal community is that above (epineuston) and below (hyponeuston) the water surface.
Epineustic animals receive food from the surrounding hydrosere vegetation, where small animals fall into the water from vegetation and are preyed upon by these epineustic animals.
Below these surface dwelling animals are a collection of animals called the nekton, which live in the pelagic and profundal regions, though rise to the pelagic regions to feed upon these epineustic animals.
Fish are included in this nekton community, which play a vital cog in these freshwater communities. Some of these fish are only temporary members of the community, as they move between fresh and salt water.
Anadromous fish spawn in freshwater, but live much of their lives in salt water. Catadromous fish are the opposite of this, and spend much of their lives in the freshwater community. Each way, the fish present in the environment at any time form the link between the upper and lower layers of the freshwater community.
Previous sections have described how plants are the primary producers of the freshwater community, harnessing new energy from the sun into the environment.
The next page looks at some of the animals that rely on these plants in the community, and animals that survive in the depths of the water and along the water shore and bed.
3. Freshwater Lentic Communities and Animals
Plants that live partially or completely submerged in water are deemed hydrophytes. A form of symbiosis occurs with these hydrophyte plants, which provide means for algae and other organisms to survive in the surrounding environment.
This is because the hydrophytes provide the conditions for the likes of algae and bacteria to survive in the environment. In return, herbivorous animals tend to feed on this rich blanket of algae as opposed to the plants themselves, thereby protecting them from being consumed.
Animals in this environment feed on these algae, and also upon the detritus matter (the rich organic material found on the water bed).
It is an area of abundant organic material because the plants that survive in this area provide a source of food, and also a source of shelter which can provide protection from predators or a location to hatch offspring in a closed protected area.
This energy rich environment and suitable conditions allows a wide range of aquatic animals to successfully breed and survive in the area .Particularly, herbivores thrive in these niches of the community, as there is a rich sources of food (plants) growing from the nutrient rich soil.
4. Still Freshwater and Plants
Plants in the freshwater community provide a means of food for herbivores, and harness new energy into the community as a whole via photosynthesis from available sunlight.
Plants are usually the pioneers of a new ecosystem, and therefore a bustling freshwater environment will have an abundance of plants.
The ecological niche alongside the still water banks is occupied by plants called hydroseres, which are partially or totally submerged by water along the banks.
Some of these hydroseres are rooted in the water, though some of their leaves penetrate the water surface, while others float on the surface, one side in contact with the water, the other side in contact with the open air environment.
In essence, hydroseres possess evolutionary adaptations and dithering respiration rates from land plants that have allowed them to adapt in live in such an environment. Such evolutionary adaptations in plants have resulted in changes in their physical structure to suit the environment, and therefore making freshwater plants distinctly unique in appearance.
An example of these adaptations is the lack of rigid structures in freshwater plants. This is due to the density of the water (much higher than that of an open air environment), which ‘pushes’ against the plant in its daily life. This allows such plants to be more flexible against oncoming water tides, and prevents damage to the plant.
As plants require a minimum concentration of gases in their diet such as carbon dioxide, they also require a degree of buoyancy so that contact can be made with the open air environment. Adaptations may include;
Air Spaces – Air spaces in the plant will decrease density and increase buoyancy.
Broad Leaves – Broader leaves will spread their weight more evenly across the water surface allowing them to float.
Waxy Cuticle – On the upper half to allow water to run off the surface to prevent the weight of the water dragging the leaves under the surface.
As these plants are either partially or totally submerged in water, their transpiration rate is very different from that of land plants. Such adaptations allow the freshwater community plants to cope with these conditions and thrive.
However, alterations to the transpiration rate of these plants have proved essential, as without these adaptations they would not be able to maintain their water balance.
5. Still Water Community Plants
5a. Freshwater Plants and Water
As mentioned about still water plants, the method of transpiration as a whole is altered in freshwater plants, due to the abundance of water in their external environment, or in the case of some, uptake of water from a wet environment, but loss of water via their leaves in the open air environment.
An example of transpiration problems for such plants is as follows;
The plant lives in a marshy environment, where roots uptake water from soaked ground, allowing plenty of water to be up taken and transported up and across the plant.
The difference in water concentration between the plants’ leaves and the open air environment is so great that much of the water absorbed is lost to the external environment, meaning the plant loses water rapidly.
Such a problem is solved by evolutionary adaptations as described in the plant water regulation page. These adaptations essentially address the issue of re-balancing the critical deviations between the water that is absorbed and lost in a plant.
5b. Freshwater Plants and Nutrients
In addition to the need for plants to maintain a suitable water concentration in plant cells, they also require various nutrients which are found in the nutrient rich soil and the surrounding waters.
In addition to the carbon, hydrogen and oxygen required for photosynthesis, plants require a range of macro-elements, notably magnesium (Mg), nitrogen (N), phosphorous (P) and potassium (K).
Some of these elements, notably the gases, are readily available in the atmosphere, while carbon dioxide is produced from decomposing organic matter.
Other elements are readily available in the soil, with nutrients becoming available from decomposing matter adding to the fertility of the surrounding soil.
Oxygen becomes available from the photosynthetic activities of plants, which provide the link between oxygen and carbon dioxide concentrations in the area.
Availability of such elements will affect the productivity of the plants in the freshwater ecosystem, and the combined productivity of the ecosystem as a whole. Evidently, the environmental factors of the freshwater ecosystem has great bearing on how plants survive in the community.
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