Water ecology FAQ frequently asked questions

The question library on water related issues

What is ecology?

Ecology is the study of the interactions between organisms and their environment. Various different species living in the same place, interacting amongst themselves and with their environment together form an ecosystem. Within an ecosystem there are several food webs. A food web is an overview of which species in an environment consume which species (plant, animal or both). A healthy ecosystem has a variety of organisms that play different roles in various food chains. If the ecosystem loses one of its members, it can be crippled. For instance, if owls in the forest food web would die out, rodents might start to multiply at an enormous speed, causing them to overrun the area and finish resources that other animals also use.
Ecologists are people that study the interactions between organisms and their environment within food webs or other ecological relationships. Fieldwork is an essential component of this study. Laboratory experiments are also applied, under field conditions. Most of the time ecologists are involved in studying the natural environment and communities, but some are involved in applied ecology, using ecological knowledge in ecosystems directed by humans, commonly known as agro-ecosystems.

What kind of aquatic environments are there?

There are two kinds of aquatic environments, which can sustain life. These are saltwater life zones and freshwater life zones. The major types of organisms found in aquatic environments are determined by the salinity of the water. Salinity means the amounts of salts dissolved in a volume of water. That is why aquatic life zones are divided up between saltwater life zones and freshwater life zones.
An example of a saltwater or marine life zone is a coral reef. An example of a freshwater life zone is a lake.

What kind of saltwater life zones are there?

The largest saltwater life zones on earth are not very hard to find, as these are in the oceans. Oceans cover about 71% of the earth's surface and are very important for the preservation of all life on earth. Oceans play an important part in the hydrological cycle, because precipitation (rain) consists of evaporated oceanic water and in the regulation of the earth's climate. Oceans also participate in other matter cycles. Oceans are the living environment for about 250,000 species of marine plants and animals. Unfortunately, oceans are also dumps for human waste, because the (polluted) water of all inland water bodies eventually ends up in oceans.
Oceans can be divided up into the coastal zone and the open sea. Here we will give an explanation of the various types of life zones found in the coastal zone and in the open ocean, along with a schematic overview of all these life zones.

Coastal life zones

The coastal zone makes up only 10% of the oceanic environment, but it contains 90% of all marine species. Coastal zones are the most nutrient-rich life zones of the oceans.
Coastal zones can be divided up into several different life zones. One life zone that can be found in a coastal zone is an estuary. Estuaries are enclosed areas of coastal water where seawater mixes with freshwater from inland streams and rivers.
Temperatures and salinity levels of estuaries always depend upon the size of the flow from saltwater oceans and freshwater rivers and streams.
Another life zone found in a coastal zone is a coastal wetland. Wetlands are land that is covered with salt water all or part of the year. Coastal wetlands are the life zones for a number of species and they are popular recreation points. They aid the maintenance of the coastal water quality by filtering and settling out pollutants and nutrients. Coastal wetlands are particularly important because they protect coastal land from flooding and from damage and erosion caused by storms. In the United States there are many coastal wetlands. Examples are bays and lagoons. Examples of plant and animal species found commonly on coastal wetlands are grasses and shrimps.
Along tropical coasts with too much silt for coastal wetlands we may find swamps. These help protect the coastline from erosion and are the environment for over 2,000 species of fish, birds and plants.
In clear and warm coastal waters of tropics and subtropics, coral reefs may form. Coral reefs are the most biologically divers aquatic life zones. In coral reefs many species live and interact with one another in complex ecological relationships. An example of a large coral reef is the Great Barrier Reef in Australia.

Open ocean life zones

The open sea contains only about 10% of all marine species. The open ocean is divided up into three life zones, the euphotic zone, the bathyal zone and the abyssal zone. The subdivision is based on the penetration of sunlight.
The euphotic zone is the upper oceanic zone, where producer species produce oxygen. Nutrient levels are low and dissolved oxygen levels are high. The euphotic zone makes up about 90% of the oceanic surface, whereas only about 10% of the world's fish species are found here. Sunlight penetration rates are high in this oceanic zone.
The bathyal zone is hardly lit and the abyssal zone is very dark. These zones are only found in the open sea and do not contain any producers, because of a lack of penetrating sunlight. In the abyssal zone the water is very cold and dissolved oxygen levels are very low. There are high nutrient levels that support many of the species found in the open water. Below the abyssal zone, on the bottom of the ocean, there are many species of decomposers, which break down the organic material of dead oceanic organisms into nutrients.
The open ocean has a very high productivity. This makes the above-mentioned life zones of great importance.

Overview of oceanic life zones:

What kind of freshwater life zones are there?

Freshwater life zones are found in waters with a dissolved salt concentration of less than 1%. Freshwaters are divided up into standing bodies of freshwater, such as lakes, ponds and inland wetlands and flowing bodies of freshwater, such as rivers and streams. Only 1% of the earth's surface is covered with freshwater. However, about 41% of all known fish species live in freshwater. Run-off from land provides the water with nutrients, such as nitrogen and phosphorus, because the freshwater zones are close to terrestrial (land) ecosystems.
Here we will give an explanation of the various types of life zones found in standing and flowing bodies of freshwater, along with a schematic overview of all these life zones.

Life zones in standing freshwater bodies

Lakes and ponds are large, natural bodies of standing water. They are fed mainly by rainfall and melting snow, and they consist of various different life zones.
The first life zone in freshwater lakes is called the littoral zone. The littoral zone is found near the shore where rooted plants grow. It is the most productive zone of a lake, because it gets abundant sunlight and it receives nutrients from land run-off. The littoral zone sustains floating plants, surfaced plants, submerged plants and phytoplankton. There are also large quantities of decomposers and some animal species, such as frogs, fish and insects.
Under the littoral life zone there is the limnetic life zone. This is the zone on the surface of the lake, extending to the depth where sunlight penetrates. Depending on the available nutrients it contains phyto- and zooplankton, and various fish species.
Under the limnetic life zone is the profundal life zone. The profundal life zone is the deep, dark water that cannot be reached by penetrating sunlight. In this zone we can only find fish that can survive in cooler, darker circumstances.
Finally, at the bottom of freshwater bodies there is another life zone called the benthic life zone, mainly inhabited by decomposers and insect larvae.

Life zones in flowing freshwater bodies

Flowing bodies of freshwater, such as rivers and streams, are watersheds for precipitation water. This water becomes land run-off and flows with the rivers and streams to the sea. The flow of surface water to the sea takes place in three different life zones. These life zones all have their own environmental conditions, which causes species diversity to vary between the three zones.
In the first life zone streams of clear, cold water rush over waterfalls and rapids. While the water falls down, it dissolves large amounts of oxygen from the air. In this life zone plant species like algae and mosses and fish species like trout are most common. Few plankton species are found in these streams and fishes are usually flat-bodied.
In the second life zone there are wider and deeper streams and fewer obstacles. The water is warmer and the oxygen content lower, so that this life zone supports more producer species, such as phytoplankton.
In the third zone streams flow into wider and deeper rivers, which stream through flat valleys. The water in this life zone has a lower oxygen content and higher temperatures than the water in the first two zones. The rivers and streams are slowly moving and contain producers, such as algae and cyanobacteria, as well as rooted aquatic plants. Because of land erosion the water is often muddy and filled with suspended solids.
Flowing freshwater bodies usually receive their nutrients from land run-off.

Overview of freshwater life zones:

Which species live in the aquatic life zones and how do they interact?

Both saltwater and freshwater life zones contain a wide variety of organisms, which all interact with one another in various food webs. A food web is a system in which certain organisms consume other organisms, plant or animal, to form a sustainable system in which species will be in balance and will not experience overpopulation.
The main kinds of organisms in aquatic ecosystems are free-floating, very small organisms called plankton, strongly swimming organisms called nekton, bottom-dwelling organisms called benthos and decomposers, such as bacteria.
Plankton is a kind of organism that sustains many species, because it is consumed. Phytoplankton is the most important kind of plankton, because phytoplankton consists of producers. This basically means it produces matter that will sustain the lives of other aquatic organisms, such as oxygen. Phytoplankton is also the most widely eaten kind of plankton. The other species of plankton are either half-producers (nanoplankton) or consumers (zooplankton).
Nekton is a group of larger, swimming consumers, which eat plankton or other consumers. Examples of nekton are such as fish and turtles.
Examples of benthos, the bottom-dwelling organisms, are oysters and lobsters.
Decomposers have one single task in an aquatic ecosystem; to break down (decompose) organic material. They decompose dead plant and animal material into nutrient compounds, which can be used by producers to built life-sustaining matter.

To simplify this explanation, we have added a schematic representation here:

(The arrows represent matter flows and the dashed arrows represent consumption.)

Remember that decomposers will not start to decompose consumer material until after the consumers have died.

Which are the limiting factors for aquatic life?

Aquatic environments have many advantages. Water has many properties and because of that it is a unique kind of environment to live in.
Water pressure provides physical support. Temperature fluctuations are very limited, which reduces the risk for aquatic animals to become overheated or to dry out. The required nutrients are readily available, because they are dissolved in water. When toxins enter the water they are quickly converted or dispersed.

These are all very positive effects of living under water. However, there are also factors that limit aquatic life. Factors that determine which life forms can be sustained in an aquatic life zone are:
- Temperature
- Access to sunlight
- Dissolved oxygen concentrations
- Availability of nutrients, such as nitrogen and phosphorus

The water temperature usually falls with water depth, because less sunlight will penetrate the water at a greater depth. Most aquatic organisms have a limited range of tolerance to temperature changes. This is all they need, because temperatures are not likely to undergo great changes beneath the water surface. However, when sudden temperature changes do occur it will have a significant effect on the performance and survival of aquatic organisms.
Sunlight can only penetrate the water up to a depth of about 30 metres below the surface. Producers need sunlight to produce oxygen and other required substances that will sustain consumers. Production can only occur in the zone where sunlight can penetrate. Suspended matter may interfere with the penetration of sunlight into water. This may decrease the size of the zone in which production takes place.
Oxygen enters the aquatic ecosystem from the atmosphere and on account of production by (phyto)plankton. Dissolved oxygen concentrations deriving from atmospheric oxygen are influenced by water temperatures. When dissolved oxygen levels fall below 3 ppm many consumers, such as fish and zooplankton, will die.
This makes dissolved oxygen and water temperature very important limiting factors in aquatic life zones. Oxygen levels, like temperatures, also decrease with depth.
Nutrient supplies are usually satisfactory in freshwater ecosystems. However, in open oceans nutrients are often in short supply. They are an important limiting factor for productivity in aquatic life zones. Phosphorus is the main limiting nutrient in freshwater life zones, whereas nitrogen is the main limiting nutrient in saltwater life zones.

What impact do humans have on aquatic life zones?

Human's desire to live near the coast has aided to the degradation of aquatic life zones. Today, around two-thirds of the worlds' population lives near coasts. People have been drying wetlands and other coastal areas, in order to gain land for urban development.
Most water bodies are now severely polluted, because we have been discharging wastewater in it for a very long time, before we realised this was very bad for the water quality. Coastal life zones are particularly sensitive to toxic contamination, because they are a dump for pesticides, heavy metals and other pollutants, which will soon accumulate to high levels. Run-off from sewage treatment plants and other water treatment systems is threatening all aquatic life zones nowadays and we are trying very hard to solve this problem.
A human activity that seriously threats life in aquatic life zones is fishing. People do not know where to stop, so over fishing has become a common problem in oceans, as well as in inland waters.
We are trying very hard to protect coral reefs right now, because we are responsible for the destruction of about 10% of the world's coral reefs. Coral reefs are the most widely developed ecosystems on earth with an enormous variety of plant and animal species. But protecting coral reefs is complex and expensive. Only half of the countries with coral reefs have the resources to protect them.
Wetlands and swamps are disappearing because people convert them to farmland, or use them for mining, forestry, oil and gas extraction and highways. Many wetlands have already been converted. Today nature organisations are trying to restore wetlands. However, replacing wetlands is expensive and it does not guarantee the restored wetlands to resemble the natural ones that were once there.

For more effects on freshwater ecosystems take a look at nutrients in freshwater or take a look at the definitions that has something to do with freshwater pollution

For water terminology check out our water glossary, or return to the water FAQ index

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