Ecosystem Cycling

Clarification Statement: Emphasis is on the idea that matter that is not food (air, water, decomposed materials in soil) is changed by plants into matter that is food. Examples of systems could include organisms, ecosystems, and Earth.


Assessment Boundary: Assessment does not include molecular explanations.

Table of Contents
Representative Image

This activity builds on the content below and guides students in the development of a small group-sized aquaponics system. To learn more about plant growth, check out the other 5th grade Life Science topic on Materials for Plant Growth.
This Activity Extension is an alternate series of the activities Build a Hydroponics System, from the Materials for Plant Growth topic, and Aquaponics in a Bottle, from this topic. This activity extension facilitates guides students in the development of a hydroponics system that can be converted into a medium class-sized aquaponics system.



A Quest to Understand Aquaculture

A.Q.U.A. is an easy-to-follow aquaculture curriculum created to educate residents in Hawaiʻi and U.S. Affiliated Pacific Islands about aquaculture. Go to page 22 for the start of the aquaponics background and activity that guides students to develop a large school-sized aquaponics system. 

The Movement of Matter


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Fig. 1. Matter cycles and recycles through producers, consumers, and decomposers within an ecosystem.

Image copyright and source

Image courtesy of Wikimedia

Ecosystems are complex, interactive systems that include both biological communities (biotic) and physical (abiotic) components of the environment. Ecosystems are sustained by the continuous flow of energy, originating primarily from the sun, and the recycling of matter and nutrients within the system (Fig. 1). 

Trophic Levels

Trophic levels are a way to group species into broad categories based on their energetic, or food resource, contribution to the community.


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Fig. 5. An energy pyramid shows that all energy in an ecosystem began as energy stored in plants from the sun.

Image copyright and source

Image courtesy of Wikimedia Commons

 Only a fraction of energy actually gets transferred from one trophic level to the next. Therefore each successively higher trophic level has less and less energy available (Fig. 2). In a majority of communities, the drop in energy at each trophic levels is reflected as a drop in the relative abundance (number of organisms) and total biomass (amount of living matter per unit area) of organisms representing the different trophic levels. For example, in a terrestrial grassland community, plants are very abundant with high biomass, followed by lower amounts of herbivores like mice, grasshoppers, and deer, and even less of carnivores like owls, foxes, and wolves.


Food Webs in the Ocean


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Fig. 3. Groups of organisms that play similar roles within a food web make up different trophic levels.

Image copyright and source

Image courtesy of the Original SEA activity

In the ocean, food webs can be complex with many overlapping layers (Fig. 3). Plankton are a diverse groups of tiny organisms that live in every part of the ocean. Phytoplankton are primary producers that create and store energy through photosynthesis that is then passed on to other creatures. Phytoplankton form the base of the food web. Zooplankton are animal plankton that do not photosynthesize. A next step in the food web occurs when herbivorous zooplankton, or primary consumers, eat phytoplankton. The herbivores are consumed by carnivores, or secondary consumers, which are then fed upon by successively larger animals. Thus, the energy captured by the primary producers is passed up the food web. When organisms die, decomposers help break down their matter and return it for use by primary producers.


Check out the interactive food web game at the bottom of the page to test your knowledge!

Movement of Matter in a Fish Tank

The rocks in an aquarium play an important role in providing a lot of surface area for beneficial bacteria to live on. These beneficial bacteria are decomposers that help cycle nutrients and matter within a fish tank. The first step in movement of matter within a fish tank is the food fed to the fish—which adds matter to the tank. Fish food contains energy in the form of chemical bonds that make up carbohydrates, proteins, and fats. After the fish eats and digests the food; some of the energy is lost as heat, some of the energy and matter is used to help the fish maintain and grow its body, and the rest of the digested food energy and matter is excreted by the fish.

Fish excrete carbon dioxide (CO2) through their gills. The CO2 released by the fish is used as a carbon source by the algae and plants in the tank. Fish also release energy and nitrogen containing compounds when they poop. The nitrogen released by fish is in the form of ammonia (NH3), which is toxic to fish. However, the bacteria inside the tank help to recycle the nitrogen into usable plant fertilizer. First, ammonia (NH3) is turned into ammonium ion (NH4+) in water and then bacteria convert it into nitrites (NO2-) in a process called nitrification. In the next step, another bacteria converts the nitrites into nitrates (NO3-). The resulting nitrates are then used as fertilizers by aquatic plants and algae.

Using Knowledge of Ecosystem Cycling

Knowledge of food webs and the movement of matter is important to understanding how we can manage and maximize our own food production. With ocean food webs, for example, understanding the connectivity at each level allows fishers to maintain sustainable harvest practices. If they take too much of one type of fish, it can lead to a cascading effect that influences other levels. Aquaponics is another method that sustainably produces food. 

Also check out Food Resources and Fisheries Science for information on sustainable fishing practices corresponding to this activity.

What is Aquaponics?

Aquaponics is a system that combines aquaculture and hydroponics to raise aquatic animals and grow plants. These two systems work symbiotically to provide ideal conditions in which the animals have clean water and the plants get the proper nutrients. There are lots of designs for aquaponic systems, both large and small scales, but the concept is always the same. Water from the aquaculture system, or fish tank, is pumped to the hydroponic system. This water is rich in nutrients from the fish or animal excretions and the plants readily take them in to use for growth. The water is them recirculated back to the tank, providing clean water for the animals. 



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Fig. 4. Aquaponics is a system that combines aquaculture (left) and hydroponics (right) to grow aquatic animals and plants. The aquaponics system shown here on the left is in Honolulu, Hawai'i at the President William McKinley High School. The hydropic farm on the right is on Midway Atoll, Hawai'i. 

Image copyright and source

Left image courtesy of Hawaii Department of Agriculture, Flickr.  Right image courtesy of Wikimedia Commons.


Aquaponic Systems

Aquaponic systems have two main parts; a tank for the aquatic animals and a system for the plants. While aquaculture and hydroponics systems work seperately, combining them allows for the natural flow and cycle of nutrients between plants and animals. Bigger systems are often more complex, requiring pumps and sometimes filters. Smaller systems can work with plant roots simply resting atop the fish tank, absorbing nutrients through the roots that are growing directly into the tank.

Follow the prompts in the interactive game below to learn about food webs in the ocean.
You may need to enable Flash, refresh, or change browsers to view the interactive feature below.

Note: If you cannot view the entire interactive on your screen, press Ctrl-Minus (-) on a PC and Command-Option-Minus (-) on a Mac to zoom out.


Ecosystem Cycling Vocabulary

  • Abiotic: relating to non-living aspects of an environment. 
  • Abundance: the relative representation of a species in a particular ecosystem.
  • Aquaponics: any system that combines aquaculture (raising aquatic animals such as snails, fish, crayfish or prawns in tanks) with hydroponics (cultivating plants in water).
  • Biomass: the total mass of organisms in a given area.
  • Biotic: relating to living features of an environment. 
  • Carnivores: animals that feed primarily or exclusively on animal matter.
  • Cascading effect: an inevitable and sometimes unforeseen chain of events due to an act affecting a system.
  • Consumer: an organism requiring complex organic compounds for food which it obtains by preying on other organisms or by eating particles of organic matter.
  • Decomposer: an organism that breaks down dead or decaying organisms (ex. soil microbes and fungi).
  • Food Web: the combination of many different food chains in a given ecosystem or community that give a more realistic picture of the feeding relationships.
  • Herbivores: animals that eat only plants.
  • Hydroponics: a method of growing plants without soil. In hydroponic systems, nutrient rich waters are delivered directly to plant roots.
  • Photosynthesis: the process by which plants convert water and carbon dioxide into carbohydrates, using sunlight as the source of energy.
  • Phytoplankton: the component of plankton consisting of microscopic plants.
  • Plankton: a diverse group of animals (zooplankton) and plants (phytoplankton) that freely drift in the water.
  • Primary consumers: an animal that feeds on primary producers; herbivore.
  • Primary producers: organisms that produce biomass through photosynthesis and chemosynthesis in a community or group of communities.
  • Secondary consumer: a carnivore that feeds only upon herbivores.
  • Trophic level: any class of organisms that occupy the same position in a food chain, as primary consumers, secondary consumers, and tertiary consumers.
  • Zooplankton: the heterotrophic form of plankton.

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawai?i, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.