17.1D: Freshwater Ecosystems - Biology

17.1D: Freshwater Ecosystems - Biology

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Only 3% of the world's water is fresh. The remainder is found in lakes, ponds, rivers, and streams.

The zone close to shore. Here light reaches all the way to the bottom. The producers are plants rooted to the bottom and algae attached to the plants and to any other solid substrate. The consumers include

  • tiny crustaceans
  • flatworms
  • insect larvae
  • snails
  • frogs, fish, and turtles.

Limnetic zone

This is the layer of open water where photosynthesis can occur. As one descends deeper in the limnetic zone, the amount of light decreases until a depth is reached where the rate of photosynthesis becomes equal to the rate of respiration. At this level, net primary production no longer occurs.

The limnetic zone is shallower in turbid water than in clear and is a more prominent feature of lakes than of ponds.

Life in the limnetic zone is dominated by

  • floating microorganisms - called plankton
  • actively swimming animals - called nekton

The producers in this ecosystem are planktonic algae. The primary consumers include such animals as microscopic crustaceans and rotifers - the so-called zooplankton. The secondary (and higher) consumers are swimming insects and fish. These nekton usually move freely between the littoral and limnetic zones.

Profundal zone

Many lakes (but few ponds) are so deep that not enough light reaches here to support net primary productivity. Therefore, this zone depends for its calories on the drifting down of organic matter from the littoral and limnetic zones. The profundal zone is chiefly inhabited by primary consumers that are either attached to or crawl along the sediments at the bottom of the lake. Such bottom-dwelling animals are called the benthos. The sediments underlying the profundal zone also support a large population of bacteria and fungi. These decomposers break down the organic matter reaching them, releasing inorganic nutrients for recycling.

Fall overturn

Where there is a pronounced change of seasons, the warming of the surface of the lake in the summer prevents this water from mixing with deeper water. This is because warm water is less dense than cold.

The surface water becomes enriched in oxygen some from the air above it and the rest - because it is in the limnetic zone - from photosynthesis. But the water in the profundal zone - eing removed from both these sources - becomes stagnant. In the fall, however, as the surface water cools, it becomes denser and sinks to the bottom — carrying oxygen with it.

Spring overturn

A similar phenomenon occurs when the ice melts in the spring.

Rivers and Streams

The habitats available in rivers and streams differ in several ways from those in lakes and ponds.

  • Because of the current, the water is usually more oxygenated.
  • Photosynthesizers play a minor role in the food chains here; a large fraction of the energy available for consumers is brought from the land; e.g., in falling leaves.

Oceans, like lakes, can be described in terms of zones. There are many parallels between the two but unfortunately a separate vocabulary is used for each.

Freshwater Biology

Freshwater Biology is a branch of limnology, and it focuses on studying ecosystems in fresh water environments. It looks to grasp how the ecosystems interact with each other, and how they react to their physical and chemical environments. Freshwater biology is often used in industrial processes, including water purification and sewage treatment.

Freshwater ecosystems are divided into two subsections, lentic ecosystems (ecosystems that exist in still water) and lotic ecosystems (ecosystems that exist in moving water). This means it can include lakes, rivers, springs and even wetlands. This is opposed to marine ecosystems, which are solely salt water systems.

The need to understand freshwater ecosystems stems from the need to preserve human life. This is because in freshwater, there were many threats to humanity, and since we need to drink water to survive, it was important to understand why humans were getting sick. An example of this would be cholera, which can leak into fresh water through sewage contamination.

Freshwater biology has a few techniques to focus on biochemical oxygen demand (how much oxygen is needed in a community) and how the ecosystem is structured. The way that events are monitored is through rates of growth, the rate in which species reproduce, behavioral changes, or the rate in which species die in the ecosystem.

Macroinvertebrates (Invertebrates you can see without a magnifying device) are used in models that focus on the events. This is because they are easy to collect, can easily be provoked with stress tests, and how valuable they are to the ecosystem. The only issue with this is that it's hard to replicate it on a large scale.

Figuring out how a freshwater ecosystem that is healthy should look like is done through use of reference sites. These sites are usually easy to reconstruct in still water as opposed to flowing water. This is because flowing water has a constant unknown variable which makes it extremely unpredictable. Some indicators that a reference ecosystem if correct can be a few things, a couple examples would be insect chitin (skeletons of arthropods) and fish scales.

Chemical stress tests on fresh water ecosystems are one of the procedures that scientists will use to help understand ecosystems. Introducing new formerly alien chemicals to simulations can help improve the way of life in a freshwater ecosystem or show how an ecosystem could crumble.

Freshwater biology is a very focused subject that revolves around life in freshwater ecosystems. It has helped keep humans alive through the years. Freshwater biology has provided information to stop harmful toxins, chemicals, organisms or viruses from infecting fresh water, as well as what scientists can do to prevent it from happening. There are many more ways it has helped, and with increasing information, scientists may be able to preserve more fresh water for humanity.

To link to this Freshwater Biology page, copy the following code to your site:


The section consists of a single research group, Freshwater Ecology, which focuses on research in lakes, streams, wetlands and coastal areas, in Denmark and around the globe.

Our research answers questions related to patterns in biological diversity, adaptations of organisms in relation to their environment and to the structure and ecological processes of whole aquatic ecosystems. Together with our extensive network of external collaborators, both nationally and internationally, we focus on five main research areas (see right menu), all of which are interlinked, and with a timely focus on current challenges related to e.g. climate change, land-use and nature management. Research approach covers field experiments and surveys, and experimental setups in the lab. All staff members are involved to a varying degree in most of the five described research areas.

Research infrastructure includes modern laboratory facilities equipped with extensive hardware for analysis of samples, various gear and vessels for field sampling, a state-of-the-art greenhouse and temperature-controlled rooms.

Research areas

17.1D: Freshwater Ecosystems - Biology


16. Community Interactions

16.5. Major Aquatic Ecosystems

Terrestrial biomes are determined by the amount and kind of precipitation and by temperatures. Other factors, such as soil type and wind, also play a part. Aquatic ecosystems also are shaped by key environmental factors. Several important factors are: how far the sun’s rays penetrate the water, the depth of the water, currents, the nature of the bottom substrate, the water temperature, and the amount of dissolved salts.

An important determiner of the nature of aquatic ecosystems is the amount of salt dissolved in the water. Those that have a high salt content (35 parts per thousand or greater) are called marine ecosystems, and those that have little dissolved salt (less than 0.5 parts per thousand) are called freshwater ecosystems.

Just like terrestrial ecosystems, marine ecosystems are quite diverse. Ecologists recognize several categories of marine ecosystems.

Pelagic Marine Ecosystems

In the open ocean, many kinds of organisms float or swim actively. Shrimp, squid, fish, and whales swim actively as they pursue food. Organisms that are not attached to the bottom are called pelagic organisms, and the ecosystem they are a part of is called a pelagic ecosystem.

The term plankton is used to describe aquatic organisms that are so small and weakly swimming that they are simply carried from place to place by currents. As with all ecosystems, organisms that carry on photosynthesis are the base of the energy pyramid. Phytoplankton are planktonic organisms that carry on photosynthesis. Most phytoplankton are microscopic, single-celled algae and bacteria. The upper layer of the ocean, where the sun’s rays penetrate, is known as the euphotic zone. It is in this euphotic zone where phytoplankton are most common. The thickness of the euphotic zone varies with the degree of clarity of the water but in clear water can be up to 150 meters (500 feet) in depth.

Zooplankton are small, weakly swimming animals of many kinds (crustaceans, jellyfish, and juvenile fish), and several kinds of protozoa, that feed on the phytoplankton by filtering the phytoplankton from the water. Zooplankton are often located at a greater depth in the ocean than the phytoplankton but migrate upward at night and feed on the large population of phytoplankton. The zooplankton are in turn eaten by larger animals such as fish and larger shrimp, which are eaten by larger animals such as salmon, tuna, sharks, squid, whales and seals. (See figure 16.27.)

A major factor that influences the nature of a marine community is the kind and amount of material dissolved in the water.

Coral reef ecosystems are produced by coral animals that build cup-shaped external skeletons around themselves. Corals protrude from their skeletons to capture food and expose themselves to the sun. Exposure to sunlight is important because corals contain single-celled algae within their bodies. These algae carry on photosynthesis and provide both themselves and the coral animals with the nutrients necessary for growth. This mutualistic relationship between algae and coral is the basis for a very productive community of organisms.

The skeletons of the corals provide a surface upon which many other kinds of animals live. Some of these animals feed on corals directly, while others feed on small plankton and bits of algae that establish themselves among the coral organisms. Many kinds of fish, crustaceans, sponges, clams, and snails are members of coral reef ecosystems. Because they require warm water, coral ecosystems are found only near the equator. Coral ecosystems also require shallow, clear water since the algae must have ample sunlight to carry on photosynthesis. Coral reefs are considered one of the most productive ecosystems on Earth (see figure 16.28).

Corals are small sea animals that secrete external skeletons. They have a mutualistic relationship with certain algae, which allows both kinds of organisms to be very successful. The skeletal material serves as a substrate upon which many other kinds of organisms live.

An abyssal ecosystem is a benthic ecosystem that occurs at great depths in the ocean. In such deep regions of the ocean there is no light to support photosynthesis. Therefore, the animals must rely on a continuous rain of organic matter from the euphotic zone above them. Essentially, all of the organisms in this environment are scavengers that feed on whatever drifts their way. Many of the animals are small and generate light that they use for finding or attracting food.

An estuary is a special category of aquatic ecosystem that consists of shallow, partially enclosed areas where freshwater enters the ocean. The saltiness (0.5-30 parts per thousand) of the water in the estuary changes with tides and the flow of water from rivers. The organisms that live here are specially adapted to this set of physical conditions, and the number of species is less than in the ocean or in freshwater.

Estuaries are particularly productive ecosystems because of the large amounts of nutrients introduced into the basin from the rivers that run into them. This is further enhanced by the fact that the shallow water allows light to penetrate to most of the water in the basin. Phytoplankton and attached algae and plants are able to use the sunlight and the nutrients for rapid growth. This photosynthetic activity supports many kinds of organisms in the estuary.

Estuaries are especially important as nursery sites for fish and crustaceans such as flounder and shrimp. The adults enter these productive, sheltered areas to reproduce and then return to the ocean. The young spend their early life in the estuary and eventually leave as they get larger and are more able to survive in the ocean. Estuaries also trap sediment. This activity tends to prevent many kinds of pollutants from reaching the ocean and also results in the gradual filling in of the estuary, which may eventually become a salt marsh and then part of a terrestrial ecosystem.

Human Impact on Marine Ecosystems

Since the oceans cover about 70% of the Earth’s surface, it is hard to imagine that humans can have a major impact on them. However, we use the oceans in a wide variety of ways. The oceans provide a major source of protein in the form of fish, shrimp, and other animals. However, overfishing has destroyed many of the traditional fishing industries of the world such as cod fishing off the east coast of North America. Fish farming in the ocean involves the use of pens to enclose fish. The dense populations in the pens result in pollution of the ocean from the food that is provided to the fish and the waste products the fish produce. These captive populations have also caused diseases to spread from farmed species to wild fish. Estuaries are important fishing areas but are impacted by the flow of fertilizer, animal waste, and pesticides down the rivers that drain farmland and enter estuaries. The use of the oceans as transportation results in oil pollution, and trash regularly floats onto the shore. Coral reefs are altered by fishing and siltation from rivers. Mangrove swamps are destroyed as they are converted to areas for the raising of fish. It is clear that humans have a great impact on marine ecosystems.

Freshwater ecosystems differ from marine ecosystems in several ways. The amount of salt present is much less, the temperature of the water can change greatly, the water is in the process of moving to the ocean, oxygen can often be in short supply, and the organisms that inhabit freshwater systems are different.

Freshwater ecosystems can be divided into two categories: those in which the water is relatively stationary, such as lakes, ponds, and reservoirs, and those in which the water is running downhill, such as streams and rivers.

Large lakes have many of the same characteristics as the ocean. If the lake is deep, there is a euphotic zone at the top, with many kinds of phytoplankton, and zooplankton that feed on the phytoplankton. Small fish feed on the zooplankton and are in turn eaten by larger fish. The species of organisms found in freshwater lakes are different from those found in the ocean, but the roles played are similar, so the same terminology is used.

Along the shore and in the shallower parts of lakes, many kinds of flowering plants are rooted in the bottom. Some have leaves that float on the surface or protrude above the water and are called emergent plants. Cattails, bulrushes, arrowhead plants, and water lilies are examples. Rooted plants that stay submerged below the surface of the water are called submerged plants. Elodea and Chara are examples. This region, with rooted vegetation, is known as the littoral zone, and the portion of the lake that does not have rooted vegetation is called the limnetic zone. (See figure 16.29.)

FIGURE 16.29. Lake Ecosystem

Lakes are similar in structure to oceans except that the species are different because most marine organisms cannot live in freshwater. Insects are common organisms in freshwater lakes, as are many kinds of fish, zooplankton, and phytoplankton.

Many kinds of freshwater algae also grow in the shallow water, where they may appear as mats on the bottom or attached to vegetation and other objects. Associated with the plants and algae are a large number of different kinds of animals. Adult and larval insects are particularly common in freshwater ecosystems along with fish, crayfish, clams, and many birds and mammals.

Although the water molecule (H2O) has oxygen as part of its structure, this oxygen is not available to organisms. The oxygen that they need is dissolved molecular oxygen (O2), which enters water from the air or when it is released as a result of photosynthesis by aquatic plants and other photosynthetic organisms. When water tumbles over rocks in a stream or crashes on the shore as a result of wave action, air and water mix, which allows more oxygen to dissolve in the water. The amount of dissolved oxygen affects the kind of organisms that live in the water.

Streams and rivers are a second category of freshwater ecosystem. Since the water is moving, planktonic organisms are less important than are attached organisms because plankton are swept downstream. Most algae grow attached to rocks and other objects on the bottom. Since the water is shallow, light can penetrate easily to the bottom (except for large or extremely muddy rivers). Even so, it is difficult for photosynthetic organisms to accumulate the nutrients necessary for growth, and most streams are not very productive. As a matter of fact, the major input of nutrients is from organic matter that falls into the stream from terrestrial sources. These are primarily the leaves from trees and other vegetation, as well as the bodies of living and dead insects. Within streams there is a community of organisms that is specifically adapted to use the debris from terrestrial sources as a source of food. Bacteria and fungi colonize the organic matter, and many kinds of insects shred and eat this organic matter along with the fungi and bacteria living on it. The feces (intestinal wastes) of these insects and the tiny particles produced during the eating process become food for other insects that build nets to capture the tiny bits of organic matter that drift their way. These insects are in turn eaten by carnivorous insects and fish.

Organisms in larger rivers and muddy streams, which have less light penetration, rely in large part on the food that drifts their way from the many streams that empty into the river. These larger rivers tend to be warmer and to have slower moving water. Consequently, the amount of oxygen is usually less, and the species of plants and animals change. Any organic matter added to the river system reduces the oxygen in the water as it decays. Plants may becomes established along the river bank and contribute to the ecosystem by carrying on photosynthesis and providing hiding places for animals.

Human Impact on Freshwater Ecosystems

Freshwater resources in lakes and rivers account for about 0.02% of the world’s water. Most freshwater ecosystems have been heavily impacted by human activity. Any activity that takes place on land ultimately affects freshwater because of runoff from the land. Agricultural runoff, sewage, sediment, and trash all find their way to streams and lakes.

11. How do phytoplankton and zooplankton differ?

12. Describe how the producers of benthic and pelagic ecosystems differ.

13. List two ways in which the kinds of organisms present in lakes differ from those in shallow parts of the ocean.

Research into Ecosystem Dynamics

The study of the changes in ecosystem structure caused by changes in the environment (disturbances) or by internal forces is called ecosystem dynamics. Ecosystems are characterized using a variety of research methodologies. Some ecologists study ecosystems using controlled experimental systems, while some study entire ecosystems in their natural state, and others use both approaches.

A holistic ecosystem model attempts to quantify the composition, interaction, and dynamics of entire ecosystems it is the most representative of the ecosystem in its natural state. A food web is an example of a holistic ecosystem model. However, this type of study is limited by time and expense, as well as the fact that it is neither feasible nor ethical to do experiments on large natural ecosystems. To quantify all different species in an ecosystem and the dynamics in their habitat is difficult, especially when studying large habitats such as the Amazon Rainforest, which covers 1.4 billion acres (5.5 million km 2 ) of the Earth’s surface.

For these reasons, scientists study ecosystems under more controlled conditions. Experimental systems usually involve either partitioning a part of a natural ecosystem that can be used for experiments, termed a mesocosm, or by re-creating an ecosystem entirely in an indoor or outdoor laboratory environment, which is referred to as a microcosm. A major limitation to these approaches is that removing individual organisms from their natural ecosystem or altering a natural ecosystem through partitioning may change the dynamics of the ecosystem. These changes are often due to differences in species numbers and diversity and also to environment alterations caused by partitioning (mesocosm) or re-creating (microcosm) the natural habitat. Thus, these types of experiments are not totally predictive of changes that would occur in the ecosystem from which they were gathered.

As both of these approaches have their limitations, some ecologists suggest that results from these experimental systems should be used only in conjunction with holistic ecosystem studies to obtain the most representative data about ecosystem structure, function, and dynamics.

Scientists use the data generated by these experimental studies to develop ecosystem models that demonstrate the structure and dynamics of ecosystems. Three basic types of ecosystem modeling are routinely used in research and ecosystem management: a conceptual model, an analytical model, and a simulation model. A conceptual model is an ecosystem model that consists of flow charts to show interactions of different compartments of the living and nonliving components of the ecosystem. A conceptual model describes ecosystem structure and dynamics and shows how environmental disturbances affect the ecosystem however, its ability to predict the effects of these disturbances is limited. Analytical and simulation models, in contrast, are mathematical methods of describing ecosystems that are indeed capable of predicting the effects of potential environmental changes without direct experimentation, although with some limitations as to accuracy. An analytical model is an ecosystem model that is created using simple mathematical formulas to predict the effects of environmental disturbances on ecosystem structure and dynamics. A simulation model is an ecosystem model that is created using complex computer algorithms to holistically model ecosystems and to predict the effects of environmental disturbances on ecosystem structure and dynamics. Ideally, these models are accurate enough to determine which components of the ecosystem are particularly sensitive to disturbances, and they can serve as a guide to ecosystem managers (such as conservation ecologists or fisheries biologists) in the practical maintenance of ecosystem health.

Conceptual Models

Conceptual models are useful for describing ecosystem structure and dynamics and for demonstrating the relationships between different organisms in a community and their environment. Conceptual models are usually depicted graphically as flow charts. The organisms and their resources are grouped into specific compartments with arrows showing the relationship and transfer of energy or nutrients between them. Thus, these diagrams are sometimes called compartment models.

To model the cycling of mineral nutrients, organic and inorganic nutrients are subdivided into those that are bioavailable (ready to be incorporated into biological macromolecules) and those that are not. For example, in a terrestrial ecosystem near a deposit of coal, carbon will be available to the plants of this ecosystem as carbon dioxide gas in a short-term period, not from the carbon-rich coal itself. However, over a longer period, microorganisms capable of digesting coal will incorporate its carbon or release it as natural gas (methane, CH4), changing this unavailable organic source into an available one. This conversion is greatly accelerated by the combustion of fossil fuels by humans, which releases large amounts of carbon dioxide into the atmosphere. This is thought to be a major factor in the rise of the atmospheric carbon dioxide levels in the industrial age. The carbon dioxide released from burning fossil fuels is produced faster than photosynthetic organisms can use it. This process is intensified by the reduction of photosynthetic trees because of worldwide deforestation. Most scientists agree that high atmospheric carbon dioxide is a major cause of global climate change.

Analytical and Simulation Models

The major limitation of conceptual models is their inability to predict the consequences of changes in ecosystem species and/or environment. Ecosystems are dynamic entities and subject to a variety of abiotic and biotic disturbances caused by natural forces and/or human activity. Ecosystems altered from their initial equilibrium state can often recover from such disturbances and return to a state of equilibrium. As most ecosystems are subject to periodic disturbances and are often in a state of change, they are usually either moving toward or away from their equilibrium state. There are many of these equilibrium states among the various components of an ecosystem, which affects the ecosystem overall. Furthermore, as humans have the ability to greatly and rapidly alter the species content and habitat of an ecosystem, the need for predictive models that enable understanding of how ecosystems respond to these changes becomes more crucial.

Analytical models often use simple, linear components of ecosystems, such as food chains, and are known to be complex mathematically therefore, they require a significant amount of mathematical knowledge and expertise. Although analytical models have great potential, their simplification of complex ecosystems is thought to limit their accuracy. Simulation models that use computer programs are better able to deal with the complexities of ecosystem structure.

A recent development in simulation modeling uses supercomputers to create and run individual-based simulations, which accounts for the behavior of individual organisms and their effects on the ecosystem as a whole. These simulations are considered to be the most accurate and predictive of the complex responses of ecosystems to disturbances.

In Summary: Ecology of Ecosystems

Ecosystems exist on land, at sea, in the air, and underground. Different ways of modeling ecosystems are necessary to understand how environmental disturbances will affect ecosystem structure and dynamics. Conceptual models are useful to show the general relationships between organisms and the flow of materials or energy between them. Analytical models are used to describe linear food chains, and simulation models work best with holistic food webs.

Freshwater Ecosystems

Freshwater ecosystems include streams, rivers, lakes and ponds that have water and are surrounded by land. These waters move as rain and snow come and dry up.

With melting snow, lakes and rivers are supplied. Some springs flow underground, and the water comes from under the ground to keep the springs flowing.

Streams and Rivers

The streams and the rivers are oftentimes called lotic because they have waters that are always flowing.
A pond, for example, is sometimes still, but this is not so in rivers and streams.

The rivers and streams come in different sizes and some of them can be super small to super large.

There are many rivers that are considered major rivers in the world.

What Classifies a Stream and River?

In order for water to be classified as a stream or a river, there have to be certain factors. The factors include chemistry, flow, temperature, and light.


The chemistry of a stream or river has to be what types of soil and nutrients are in the river.

The flow of the water and how much strength it has helps to classify streams and rivers.

How fast and how slow a stream or river flow determines what type of life live in or around the waters.


The temperature of the river effects what type of marine and plant life grow or live around these types of waters.


Light is an important part of streams and rivers because in order for plants to grow, there has to be sunlight.

If there is a dry season, or if there is a season with little light, this will change how plants grow.

Animals in the Streams and Rivers

Most of the animals found in the streams and rivers are crabs, snails, fish, beavers, otters, crocodiles, snakes, and salmon.

The plants depend on the area, but some include river birch and willow trees.

Lakes and Ponds

Lakes and ponds are sometimes called lentic because they have waters that standstill.

There are four different types of lake zones, littoral zone, limnetic zone, euphotic zone, and benthic zone.

The littoral zone is the area that is close to the shore and this is where the plants grow.

The limnetic zone is the water that is not close to the shore.

The euphotic zone is the layer of water right below the surface. There is still enough sunlight to reach this area.

The benthic zone is the bottom of the lake or pond.

The temperature of Lakes and Ponds

The temperature of lakes and ponds changes depending on the weather.

The deeper that a person goes into the water, the cooler the water will be because of the lack of sunshine to warm it.

Animals of the Lakes and Ponds

Most animals that are found in lakes or ponds are fish, frogs, turtles, insects, worms, and more.

An Ever-Changing Biome

Freshwater biomes are always changing and they can also change the landscapes that surround them. A winding river may cut away at the land for hundreds of years, becoming more twisted over time. If enough time passes, the river might form a special lake called an oxbow lake.

Beavers have a huge impact on the structure of certain ecosystems. Click for more detail.

Animals can also create new freshwater ecosystems. Beavers and humans are both very good at building dams in streams and rivers, which can end up creating lakes.

Dams can have large effects on the ecology of freshwater ecosystems. For example, the physical barrier created by a dam may change migration routes of fish species, like Pacific salmon. Dams can also cause flooding, which may be very important to the surrounding ecosystems.

Phoenix, Arizona, might not exist today if it was not for the creative irrigation and flooding techniques used by the early Hohokam people. During their inhabitation of the Phoenix valley, the Hohokam created 135 miles of irrigation canals to make the desert land a flourishing area for agriculture.

Additional images via Wikimedia Commons. Salt crystals by Mark Schellhase.

Why this programme?

Our society is dependent on a number of ecosystem services. For instance, inland waters are used for fisheries, recreation, water supply and energy production. There are also other human activities such as agriculture, forestry and urbanisation. These affect natural ecosystems, which can lead to reduced water flow, eutrophication, pollution, spread of invasive species, and climate change. Solid knowledge of ecosystem structure and function is a key to the sustainable use of natural resources.

In the programme with specialisation in Ecosystems and Aquatic Ecology, you will study the interactions between organisms and their environment. Ecology of inland waters (lakes and rivers) is an important part of the programme, but topics related to other natural and constructed ecosystems will also be included in the programme.

  • learn how ecosystems work, and in which way they are affected by human perturbations
  • get a broad and holistic perspective on nature, by crossing disciplinary boundaries among such as Biology, Chemistry, Physics, and Hydrology however, with a focus on inland waters
  • gain skills to analyse and judge the status of ecosystems and natural resources, by means of practical exercises.

The Master's Programme in Biology, with specialisation in Ecosystems and Aquatic Ecology, is the choice if you aim to work at authorities, consultancies and other organisations dealing with the management or conservation of natural resources, water or other aquatic resources. It also prepares you for an international research career in environmental science, aquatic science, or ecology.

Types of Aquatic Ecosystem

There are different types of aquatic ecosystems:

Freshwater Ecosystem

Freshwater ecosystems include lakes, ponds, wetlands, streams, swamp, rivers, bog, and temporary pools. These cover about 0.8% of the earth surface. These ecosystem provide habitat for 41% of the world`s fish species.

The freshwater ecosystems are of the following three basic types:

Lentic Ecosystems

Lentic ecosystem includes standing water bodies. The examples of lentic ecosystems are ponds, lakes, ditches, seasonal pools, basin marshes etc. Among them, lakes have deep waters which influence by light while ponds support a wide range of water plants due to their more light penetration. Besides, algae, shrimps, crabs, frogs and salamanders are the important biotic factors of lentic ecosystem.

Lotic Ecosystems

These are rapidly flowing water bodies where unidirectional water movements are available. They have faster moving turbulent waters which contain high concentrations of DO (dissolved oxygen). These water bodies support wide range of biodiversity. These ecosystems include creek, rivers, brook, streams, spring, etc.

They provide suitable habitats for numerous species including mayflies, beetles, stoneflies and different species of fishes such as eel, trout, minnow, and different anadromous fish, etc. At present, these ecosystems are degraded through various environmental threats such as over extraction of water, dams, pollution and various introduced species.

There are two main zones of lotic ecosystems such as rapids and pools. The rapid zones have fast water flow with clear bottom while the pools are deeper areas which contain slower currents with silt builds up.


Wetlands are the water bodies which contains large varieties of animals and plants. It is the most well productive natural water bodies that provide habitats for large numbers of animal and plant species. It is dominated by vascular plant species due to its high productivity. In this ecosystem, animal lives include invertebrates such as damselflies, dragonflies, various birds’ species and lots of fishes, mammals, etc.

Wetlands are of main four types which include: marsh, swamps, bog and fen. In many cases, wetlands are converted into dry land that has dykes and drains. It is also used for the purpose of agriculture which provide the cultivation of rice and meet the diet of half the world`s population. It gives the benefit to humanity by filtering water, and also helps in storm protection and flood control.

Marine Aquatic Ecosystem

Marine ecosystem is the largest aquatic ecosystem which covers about 71% of the Earth`s surface. It contains about 97% water of the planet. This ecosystem contains about 85% of the dissolved materials such as sodium and chlorine.

This ecosystem has many zones, among them oceanic zone is the largest open zone which provides habitats for many aquatic animals such as sharks, whales, and various fish species. The benthic zone contains many invertebrates while the intertidal zone contains high and low tides.

Various dinoflagellates, brown algae, cephalopods, corals, echinoderms, shellfish such as crabs,, shrimps, lobsters, snails are also found in marine ecosystem.

Components of Aquatic Ecossytem

A pond is the typical aquatic ecosystem which comprises of four components. These include:

Abiotic Substances

The abiotic substances are both inorganic and organic. The chief inorganic substances are H2O, CO2, O2, N2, Ca, P, etc. These substances in a state of solution or solubility are available for the nutrition of organisms from the environment.


The producers in a pond are mainly of two types such as (a) relatively larger plants that grow in shallow water such plants may be rooted (Vallisneria. Hydrilla, etc) or free floating plants, such as pistia, water hyacinth, etc. (b) Minute floating plants, mainly algae constituting the so called phytoplankton. These are distributed throughout the pond as deep as light penetrates. The phytoplankton is much more important than the rooted plants in the production of basic food for the ecosystem.


The consumers are primary, secondary and tertiary. The primary consumers feed on plants which act as producers. These are of two types: (a) Zooplankton (b) benthos. Amoeba, Daphnia, Cyclops, Diaptomus, Bismina etc constitute the zooplankton while snails, small fishes, chironomus larvae, constitute the benthos. The secondary consumers are carnivorous which feed on the plant eaters such as prawn, some fishes. The tertiary consumers eat primary consumers such as Walloga attu, Channa spp, snakes, etc.


Different types of aquatic bacteria and fungi act as decomposers. They are more abundant in the bottom mud rich in dead decayed plant and animal accumulation. By the action of aquatic microorganism, the dead bodies are rapidly decomposed and much simpler substances released for future use of the autophytic plants.

Plankton and Benthos

These are two major life forms in water. The organisms which are more or less dependent on water currents or wind action for their movements are called plankton. Phytoplankton are free floating plant organisms. Organisms attached with or resting on the bottom or living in the bottom sediments are called benthos or bottom forms.

Functions of Aquatic Ecosystems

Aquatic ecosystems show many beneficial environmental jobs. They make water purification, recycle nutrients, recharge ground water, prevent floods and also offer habitats for aquatic wildlife. It also provides human recreation and use as the tourism industry in coastal regions.

Rising CO2 is causing trouble in freshwaters too, study suggests

As carbon dioxide (CO2) levels in the atmosphere rise, more CO2 gets absorbed into seawater. As a result, the world's oceans have grown more acidic over time, causing a wide range of well-documented problems for marine animals and ecosystems. Now, researchers reporting in Current Biology on January 11 present some of the first evidence that similar things are happening in freshwaters too.

The study found that some freshwater ecosystems have become more acidic with rising pCO2 (partial pressure of CO2). They also show in lab studies that increases in freshwater pCO2 can have detrimental effects on at least one keystone species, a tiny freshwater crustacean, leaving them less able to sense and defend themselves against predators. The findings suggest that increasing CO2 levels may be having widespread effects on freshwater ecosystems.

"Ocean acidification is often called the 'climate change's equally evil twin,' and many current investigations describe tremendous effects of rising CO2 levels on marine ecosystems," says Linda Weiss at Ruhr-University Bochum in Germany. "However, freshwater ecosystems have been largely overlooked. Our data indicate another pCO2 problem: pCO2-dependent freshwater acidification."

Studies on ocean acidification have shown that there are consequences for marine food webs, nutrient cycles, overall productivity, and biodiversity. Yet, the researchers say, surprisingly little has been known about the impact of rising atmospheric CO2 on freshwater systems. While scientists expected that there had been increased pCO2 in freshwater bodies, the data were lacking.

To investigate, Weiss and colleagues looked to four freshwater reservoirs in Germany. Their analysis of data over 35 years, from 1981 to 2015, confirmed a continuous pCO2 increase. As in the ocean, that increase has been associated with a decrease in pH (increasing acidity). In fact, they report a change in pH of about 0.3 within 35 years, suggesting that freshwaters may acidify at a faster rate than the oceans.

But what effect was that increasing pCO2 having on freshwater organisms? To begin to explore that question, the researchers focused their attention on small, freshwater crustaceans called Daphnia, also known as water fleas. Daphnia are a dominant species in many lakes, ponds, and reservoirs, and they are important as a primary food source for many larger animals.

When Daphnia sense that predators are around, they respond by producing helmets and spikes that make them harder to eat. Weiss' team found in the lab that rising pCO2 levels get in the way of the water fleas' ability to produce those protective features.

"High levels of CO2 reduce the Daphnia's ability to detect their predator," Weiss says. "This reduces the expression of morphological defenses, rendering them more vulnerable." She adds that such effects on Daphnia may have broader effects on freshwater communities.

Weiss says they were fortunate to obtain such a long data series on four freshwater reservoirs. It will now be important to gather more data representing freshwater impoundments around the world.

"We now want to know the global degree of this phenomenon," Weiss says. The question is: "Are all freshwater impoundments prone to this kind of acidification?"