Freshwater Ecology

Freshwater Producers and Consumers

There are four main constituents of the living environment that form the freshwater ecosystem, they are as follows.

This simple classification of life in the ecosystem indicates a simplified relationship where each pass on energy in the food chain allowing one another to survive.

An ecological pyramid indicates energy passing along from autotrophic organisms to carnivores at the top of the chain. Those at the bottom of the food chain are usually the smallest in size but not always, and are almost inevitably the largest in number. Those who feed off these primary producers are less in number, usually because they are larger and require more than one portion of prey per meal as a means of fulfilling nutritional requirements for a larger organism. This situation continues to the top of the chain, where few secondary consumers are eaten by an even smaller amount of tertiary consumers.

This is typical of a food chain in a freshwater community. Sometimes the pyramid diagram of a food chain can be inversed, usually in the case of parasites and hyper-parasites, where many smaller organisms rely on much larger organisms as a means of food and survival. 

The next page continues looking at the relationship between organisms and how energy is passed on in the food chains, and looks at ecological pyramids.


Freshwater Community Energy Relationships - Producers & Consumers

The previous page on producers and consumers noted the reliance that organisms have on one another to obtain energy to survive and reproduce. Energy simply moves along in a simple chain like this;

Plants attain energy via photosynthesis adding to the overall energy available in the ecosystem.

Herbivores feed on these plants to attain their energy, while omnivores also eat animal organic material as a means of food.

Carnivores eat these herbivores and other carnivores and so forth

Plants > Herbivores > Carnivores

When energy is passed on, there is always a net loss in the energy that is available in the ecosystem. This is because some energy is always lost somewhere along the line due to inefficiencies and waste produced by each of the organisms that contains some of the biological energy that was created inside them. Detrivores will feed on this waste matter, and once again the energy will be re-siphoned back into the food chain. 

Plants, along with other organisms will inevitably die out, and the organic material produced by these dead plants decomposes into the soil, allowing new energy to be stored in the nutrient soil for plants to re-absorb. 

The food chain diagram is a simplified food chain that results from such relationships, where organisms of a species gain energy from a source and pass it on in a continuous chain.

Ecological Pyramids

Food chains that allow a diversity of species to survive are divided into trophic levels, with plants providing the first trophic level as they are the primary producers of most food chains. In almost all freshwater ecosystems, animals will be present, and form part of the many grazing food chains in the area. Other organisms leach the energy from dead organic matter forming detritus food chains. Such relationships allow the free flow of organic energy to be passed along from species to species, and provide an environment where food is available for them to survive.

The most important fact to be taken from this is that no matter what species occupies an area, chances are they will require another species to be in abundance in the area for feeding. The population of a particular species will depend on density dependant and density independent factors, therefore the abundance of food in the ecosystem (a density independent factor), comes into play. The prey of a particular species will also require the existence of a food source for them to survive, and so forth.

Therefore we can see how the complex interrelationships between organisms allows an ecosystem to support such a wide variety of organisms. Plants are the essential constituent for a healthy freshwater ecosystem, being the primary producers and harnessing energy from sunlight, they provide the building blocks and energy to allow the arrival of herbivores, and subsequently omnivores and carnivores into the ecosystem.


Abiotic and Biotic Factors

Abiotic factors are essentially non-living components that effect the living organisms of the freshwater community.

When an ecosystem is barren and unoccupied, new organisms colonising the environment rely on favourable environmental conditions in the area to allow them to successfully live and reproduce.

These environmental factors are abiotic factors. When a variety of species are present in such an ecosystem, the consequent actions of these species can affect the lives of fellow species in the area, these factors are deemed biotic factors.

This page will go into the abiotic factors of the freshwater environment which determine what sort of life would be suited to living (and adapting) to the conditions of the ecosystem.

As described in previous pages, the light from the sun is a major constituent of a freshwater ecosystem, providing light for the primary producers, plants. There are many factors which can affect the intensity and length of time that the ecosystem is exposed to sunlight;

As you can see, many abiotic factors can play a part in determining the end product, which organisms live and succeed in the freshwater ecosystem. The sun provides light for photosynthesis, but also provides heat giving a suitable temperature for organisms to thrive in. The temperature of a freshwater environment can directly affect the environment as a whole and the organisms that occupy it.

Enzymes operate best at an optimum temperature, and any deviation from this temperature 'norm' will result in below optimum respiration in the organism. All aquatic life are ectotherms, meaning their body temperature varies directly with its environments.

Temperature affects the density of substances, and changes in the density of water means more or less resistance for animals who are travelling in the freshwater environment.

The next page will continue to look at how these abiotic factors affect the way in which organisms operate in the freshwater ecosystem. The above examples of abiotic factors involve physical characteristics of the freshwater environment, which are continued, with subsequent information studying how the chemical composition of the freshwater ecosystem also affects which organisms survive in the environment and how they cope in these conditions.


Abiotic Factors - Water Conditions

Evidently, the light and heat from the sun play an important role in providing suitable conditions. However, the water conditions also inevitably have an effect on life in the ecosystem. 

A still body of water will inevitably be disturbed by various factors, which will affect the distribution of organisms in the water. Wind is considered to be the prime factor responsible for disturbing water, though changes in temperature can create convection currents where temperature is evened out across the body of water via this movement.

Naturally, a river will have water movement as water succumbs to gravity and moves downstream. These are relatively constant factors that affect water movement though, for example, human intervention can also cause water movement. The surface tension of the water will also affect the organisms that occupy the area, depending on the cohesion of water at the surface, it can affect the amount of oxygen that reaches organisms living below the water surface.

These factors all affect the way of life for organisms occupying such a freshwater ecosystem. On a more molecular level, the chemical compositions of the water, soil and surrounding air also play a part in determining the face of the ecosystem. 

The oxygen concentration of the water and the surrounding air will have great bearing on which organisms can survive in a particular environment. Oxygen is required for aerobic respiration in animals, and the concentration of oxygen in an area is determined by many factors, including temperature and abundance of organisms for example.

Many chemical reactions and cellular processes rely on the availability of oxygen, therefore the concentration of oxygen in the ecosystem will inevitably alter the ecosystem itself. The same applies to carbon dioxide concentration. CO2 is required for photosynthesis, and can also affect the pH of the water for example. 

As you can see, many factors will affect the overall existence of organisms in an ecosystem. The chemical and physical characteristics to begin with will determine which organisms are most likely to survive in the freshwater ecosystem. In turn, these pioneers entering the environment will actively manipulate these factors and change the schematics of the ecosystem as a whole, meaning that they also play a part in determining which organisms will succeed in a particular environment. 

The study of ecology in freshwater is usually divided into 2 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. The following pages will separate these two categories of freshwater and investigate as to how they affect the life which lives in them.


Freshwater Communities & Lentic Waters

Lentic (still water) communities can vary greatly in appearance, anything from a small temporary puddle to a large lake 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 on previous pages of the tutorial.

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 glacial movement, erosion, volcanic activity, and to an extent, human intervention. 

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 vast areas of water. When this freshwater ecosystem is habitable, many factors will come into play determining the overall make up 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.

Over 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 the difference in temperature between surface and water bed are so different they can easily be distinguished apart. The surface area is deemed the epilimnion, which is warmed water as a result of direct contact with sunlight. The lower layer is deemed the hypolimnion, found below the water surface, and due to increased depth, receives less heat from the sun and therefore results in the colder water underneath.

The next page will continue to look at how heat from the sun affects the water and how other factors also come into play.


Freshwater Communities & Lentic Waters

Previous pages 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 and 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.

When water moves, the friction caused by the 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 means that 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

As mentioned on previous pages, 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 with the others. This is also the case of pH, for example, as an increase in carbon dioxide results in a drop of pH.

With this information, and the previous pages in hand, we can now understand how organisms survive in these habitats in relation to these conditions described.


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. When studying the habitats of these particular organisms, three main areas of the freshwater environment can be distinctly classified.

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 collective 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 pages 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.


Freshwater Lentic Communities & 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,herbivore animals tend to feed on this rich blanket of algae as opposed to the plants themselves, therefore protecting them from being consumed.

Animals in this environment feed on this algae, and also upon the detritus matter, the organic material that is rich 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 an survive in the area.

Particularly, herbivores thrive in these niches of the community, as their is a rich sources of food (plants) growing from the nutrient rich soil. 

Examples of still water animals will appear soon in the up and coming species database...


Still Freshwater & 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 ha meant that their physical structure has changed 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 require a degree of buoyancy so that contact can be made with the open air environment. Adaptations may include;

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. This is continued on the next page, though related information can be found in the plant water regulation page in the adaptation tutorial.


Still Water Community Plants

Freshwater Plants & Water

As mentioned on the previous page 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;

Freshwater Plants & Nutrients

On top of 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. The following page introduces flowing water communities, which bring new and dithering factors into the equation for possible species occupying the area.


Running Water Freshwater Communities

Running water freshwater communities are also known as lotic communities, lotic meaning running water. Lotic communities are formed by water being introduced to the freshwater body from a variety of sources;

Their are also many other, less significant ways water can enter the stream, for example, due to man made interference such as an outlet pipe, or water transpired by plants in the nearby area.

The accumulation of this water from the areas mentioned above also introduces essential minerals and nutrients that are ideal for plant growth, the primary producers of a community. With that in hand, alongside an abundance of water, pioneers can occupy the running water environment.

One of the main differences between lotic and lentic communities is the fact that the water is moving at a particular velocity in lotic communities. This can have great bearing on what organisms occupy the ecosystem and what particular ecological niche they can exist in. Running water can bring many factors into play affecting the lives of the organisms in this particular environment:

Such factors play a vital role in determining the overall chemical, physical and biological face of the running water ecosystem. As with previous pages, we shall look at these physical and chemical differences that make up a running water ecosystem which will inevitable affect the biological make-up of the freshwater ecosystem. This is continued on the next page.


Running Water Freshwater Communities

This page continues from the previous page introducing lotic (running water) communities. The following is some of the physical and chemical factors that provide the framework of a running water community in which organisms in their favoured ecological niches occupy.

Such factors all play a part in the lives of animals and plants occupying the lotic environment. The following pages look at how plants and animals exist in such an ecosystem.


Lotic Communities & Algae

In general the diversity of plant species in a lotic community is small compared to that of a still water (lentic) community although small parts of the lotic community host similar conditions to that of a lentic community. Most plants have went through evolutionary adaptations to cope with the force and different conditions that running water brings. Such adaptations have allowed a number of species to successfully take advantage of the lotic community as their ecological niche.

As these conditions are more harsh for a typical species of plant, more notably larger plants, smaller species have found the conditions of the lotic community more favourable. This is due to the fact that they are more flexible in regards to the physical conditions of the water. Algae can grow in all sorts of different places and surfaces, and therefore are a successful constituent of the running water ecosystem. Most of these algae have developed evolutionary adaptations over time that prevent the water current sweeping them away. 

You may want to learn more about natural selection if you are unfamiliar with evolution and adaptation

There are many species of algae, all of which are capable of growing and reproducing at a quick rate. This consequence results in competition for niches in the freshwater environment, and in light of this, colonies of algae can heavily occupy one area at one moment in time and weeks later they can be succeeded by another species who can succeed in the conditions more favourably.

Algae are also the primary producers of this community, meaning they harness new energy into the ecosystem from the sun which provides the primary consumers with a valuable food source. With this in hand, it is apparent why algae populations and where they can be found in the lotic community is variable on a short-term basis.

More information on algae and specific references to species will be added over time. The next page continues looking at plants in the running water community, but this time looking at larger plants.


A continuation of a tutorial on freshwater ecology looking at plants in the ecosystem.

As mentioned on the previous page, larger plants (thus with larger surface areas) find it difficult to occupy the running water environment in light of the water current that thrashes against them. Of the plants that do occupy the running water environment, some are rooted to the ground while others float.

Examples of lotic community plants will be uploaded soon


Lotic Communities & Animals

As mentioned in previous pages, the running water environment offers numerous microhabitats that simulate favourable conditions for many types of animals to successfully succeed the freshwater lotic community. As with plants, animals in this ecosystem have also underwent ongoing evolutionary adaptations to better suit this running water environment.

As with plants and their rooting structure, animals have also adapted to cope with the current of the stream.

Some of these animals are sessile, meaning they are immobile and fixed to the one place. These animals are usually small, and include the protozoans and some freshwater sponges. These animals either remain attached to the mass of a plant or the water bank surface or rock. They usually obtain their food via tentacles which branch out into the flowing water and form a catchment area that can trap microscopic organisms (such as plankton) that is floating downstream. 

As much as these sessile animals have developed adaptations to prevent being washed downstream, they are not thought to be one of the important pillars of the freshwater community. Over time when biotic and abiotic factors affect the landscape of the ecosystem over time, the location of these animals may not be as favourable as it once was, and they are unable to correct this due to their immobile nature. With this in light some animals have developed adaptations that allow them to travel through the water without being inhibited in same spot.

animals have developed some of the following adaptations over time that helps them cope with the conditions in hand:


Freshwater Communities & Plankton

Plankton are microscopic organisms that live suspended in the water environment, and form a very important part of the freshwater community. They move via convection or wind induced currents. In almost every habitat of a freshwater ecosystem, thousands of these organisms can be found, and due to their small size and simplicity, they are capable of occupying large expanses of water and multiplying at an exponential rate. 

Plankton can be subdivided into two categories.

They are relatively unspecialised as their environment does not resist the large populations that can exist in within their environment. Physiologically, there are many evolutionary adaptations that can be found that assist in the buoyancy of them, and prevent their deaths by allowing themselves to be suspended in the water away from harm. 

They can be found in large amounts in a small area, and as they are consumed in large numbers by herbivores (phytoplankton) and carnivores (zooplankton), they reproduce asexually to keep population numbers up. This is opposed to sexual reproduction with other organisms, which would take longer although it would increase genetic variation within the species.

Many factors can affect the distribution of plankton in an ecosystem, which has a detrimental effect on the rest of the ecosystem, because as mentioned, they form an essential part of the ecosystem. Phytoplankton harness new energy from sunlight and provide many other organisms as a means of food due to this while zooplankton are also an important source of food for many species. In light of this, a knock on effect of starvation would occur if there is a lack of plankton in a particular environment.

Here are some factors that affect the distribution of plankton in the freshwater community;

Phytoplankton are more abundant in areas with a high intensity of light, as they can convert this light energy into chemical energy while higher temperatures increase growth and multiplication of the both phytoplankton and zooplankton. Elementary, the amount of available nutrients in the environment also plays a part in the distribution and density of phytoplankton. 

You may also want to look at the population regulation page in the Regulation of biological systems tutorial to look at factors which affect species in general and their population levels.


Pollution in Freshwater Ecosystems

As with all ecosystems, the existence and operations of human society inevitably have an effect on the way of life in a freshwater community. Particularly in Western society, where a huge amount of resources are harnessed from the land to fund our lifestyle, there is a resulting effect on the ecosystems of our planet.

For each action that man takes in this lifestyle, there is a resultant effect on the ecosystem, and the following looks at some scenarios where human action results in a response from the ecosystem, either physically or chemically, which in the long run affects the lives of organisms that live in these communities;

As you can see, there are many environmental factors that arise due to the usage of water in one way or another by our species. The most important fact to take from this is that when we use water in the above examples, we are upsetting the fine balance of the ecosystem as a whole, which then has a knock on effect on other organisms living in that particular ecosystem.

In light of this, and more consideration towards our environment, conservation measures are used to ensure that their is no detrimental damage caused to these environments, while at the same time man can continue to harness the water as a valuable resource.


http://www.biology-online.org/6/1_producers_consumers.htm