Falling Into Freshwater

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Estuary: the area at the end of a river where fresh water meets the salty water of the ocean.

Exponential: to increase at a rate that continues to get faster as time goes on; these rates increase so quickly they require the rate to be multiplied by itself.

Moss: a flowerless plant that grows close to the ground and often resembles a fuzzy mat.

Producer: an organism that can make food from simple non-living materials. Producers are also known as autotrophs... more

Temperate: an area that experiences fairly mild seasonal temperature variation (for example, a place that is warm in summer and snows in winter).

Aquatic plant in Brazil

Plants aren't the only organisms that can capture energy from sunlight. Image by Renalle Ruana Pessoa Ramos.

When you think about photosynthesis, or organisms that can take energy from the sun and make it into food, you probably think about plants. Plants are the most commonly recognized producers. However, phytoplankton and bacteria can also be producers.

Producers are organisms that produce their own energy by using sunlight to break down carbon dioxide (one of the molecules in air). This process is called photosynthesis. During photosynthesis, producers use the energy from sunlight to separate carbon dioxide into carbon molecules and oxygen molecules. The producers can then use the carbon to grow, while the oxygen molecules are breathed out by the producers and can then be breathed in by animals, like humans.

The Tiny Ones

The tiniest freshwater producers are phytoplankton and algae. Phytoplankton and algae can be attached to an object or rock in the water, or they may simply float around.


Image of a diatom created with an electron microscope. Click for more detail.

Individual phytoplankton and algae are too small to be seen by the naked eye of humans. However, phytoplankton and algae are often grouped together in large communities and we can see these large communities as discolored or mossy looking regions of water. And even though each of these individual producers is very small, together, they are responsible for the majority of photosynthesis in freshwater systems. Phytoplankton and algae also serve as the lowest level on the food chain, providing food for other freshwater animals.

But what exactly is phytoplankton or algae? The term algae stands for many different types of producers that include bacteria, green algae (an evolutionary ancestor of plants), and diatoms (single-celled organisms that can either live individually or as part of a colony). Some diatoms are free floating (like phytoplankton), where they move through the water with very little control over their movements, but some male diatoms also have a flagellum, a whip-like tail, which they can use to move around. Additionally, many unattached planktonic algae also have a flagellum to help move around water. 

Lake Ontario plankton bloom

A massive plankton bloom in Lake Ontario, one of the five Great Lakes. Click for more detail.

Human activities, such as pollution, can result in a plankton bloom. These plankton blooms occur when cyanobacteria or blue-green algae grow at exponential rates, causing all the oxygen to be sucked from the water. This results in a "dead zone" where animals cannot live.

The loss of oxygen in these dead zones can occur even though blue-green algae produce oxygen, because of the huge amounts of blue-green algae that exist within these blooms. When these algae die, oxygen is used to break down and decompose their bodies. This breakdown of the dead algae is what sucks all of the oxygen out of the system.


Many lakes and rivers have plants that are attached to the ground beneath the water and grow to either near or past the surface of the water.

Cattails in Seney NWP marsh

Many rivers and lakes have marshes along the edges. Image by Seney National History Association.

In temperate marshes, lakes, and stream edges you can see attached plants such as reeds, watercress, and bulrushes.

In tropical marshes, rivers, and lake edges you can see different attached plants such as reeds and water lilies.

Estuaries are where fresh and salt water meet. These habitats can contain various grasses such as eel grass and widgeon grass.

In deep regions of lakes, you can find willow moss and various kinds of worts, such as quillwort and stonewort. Ponds and slower streams can support water lilies, pond weeds, coontails, and milfoil. 



Floating fern

Some plants float on top of the water. Click for more detail.

Additionally, some plants are not actually attached to the ground at all. Instead they just float on the surface of water and have roots that dangle freely in the open water.

Lakes and slow streams that are warm year round can have water lettuce, water hyacinth, and water ferns. Lakes and backwaters that experience more seasonal variation can have water ferns and duckweeds.

Decomposers – Taking Out the Waste

Microbes and fungi all help break down the dead plant and animal life that falls to the floor of rivers and lakes. By eating dead plants and animals, decomposers are breaking this dead matter back down into its most basic nutrients. Phytoplankton can then take in these nutrients and use them to grow and restart the circle of life.

Water mold

Water mold is a microorganism that breaks down dead materials. Image by TheAlphaWolf.

Bacteria are tiny single-celled organisms that can exist in very large numbers in the soil, and to a lesser amount in the water, of freshwater systems. Bacteria are one of the main types of organisms responsible for breaking down dead matter in freshwater systems. However, if bacteria reproduce to higher than normal levels, it can result in a health hazard for both humans and animals that are exposed to the water. 

Fungi also take part in breaking down dead matter. You can find various types of fungi such as water molds, mildews, and yeast in freshwater systems. And despite fungus’s appearance and people’s initial beliefs about fungi, fungi are now actually thought to be more closely related to animals than plants.

Additional images via Wikimedia Commons. Spirogyra by Bob Blaylock.

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Bibliographic details:

  • Article: Producers and Decomposers of Freshwater
  • Author(s): Jason Borchert
  • Publisher: Arizona State University School of Life Sciences Ask A Biologist
  • Site name: ASU - Ask A Biologist
  • Date published: April 5, 2015
  • Date accessed: May 16, 2022
  • Link: https://askabiologist.asu.edu/producers-and-decomposers-freshwater

APA Style

Jason Borchert. (2015, April 05). Producers and Decomposers of Freshwater. ASU - Ask A Biologist. Retrieved May 16, 2022 from https://askabiologist.asu.edu/producers-and-decomposers-freshwater

American Psychological Association. For more info, see http://owl.english.purdue.edu/owl/resource/560/10/

Chicago Manual of Style

Jason Borchert. "Producers and Decomposers of Freshwater". ASU - Ask A Biologist. 05 April, 2015. https://askabiologist.asu.edu/producers-and-decomposers-freshwater

MLA 2017 Style

Jason Borchert. "Producers and Decomposers of Freshwater". ASU - Ask A Biologist. 05 Apr 2015. ASU - Ask A Biologist, Web. 16 May 2022. https://askabiologist.asu.edu/producers-and-decomposers-freshwater

Modern Language Association, 7th Ed. For more info, see http://owl.english.purdue.edu/owl/resource/747/08/
Spirogyra algae

Green algae is also found in freshwater, where it performs photosynthesis.

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