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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.

The Movement of Matter

<p>Fig. 1. Matter cycles and recycles through producers, consumers, and decomposers within an ecosystem.</p>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

<p>Fig. 2. An energy pyramid shows that all energy in an ecosystem began as energy stored in plants from the sun.</p>Trophic levels are a way to group species into broad categories based on their energetic, or food resource, contribution to the community. 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

<p>Fig. 3. Groups of organisms that play similar roles within a food web make up different trophic levels.</p>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 eat phytoplankton. The herbivores are consumed by carnivores, which are then fed upon by successively larger animals. Thus, the energy captured by the primary producers is passed up the food web.


Copepods, which are small crustaceans, are the most abundant zooplankton. Copepods typically account for at least 70% of the zooplankton. Most copepods are omnivorous, consuming both phytoplankton and zooplankton. Other crustaceans, such as krill, are also important members of the plankton. Krill are commonly found in colder water, especially in the polar seas. Because of their large size, up to 6 cm (2.36 inches), krill are an important prey resource for many fishes, seabirds, and large, filter-feeding whales.


Many zooplankton spend their entire lives in the plankton and are called holoplankton. In contrast, meroplankton spend only the early stages of their life history in the plankton, and comprise the larval stages of many fish and invertebrate species.

What is Aquaponics?

Aquaponics is a system that combines aquaculture and hydroponics to raise aquatic animals and grow plants. Thses 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. 

<p>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.&nbsp;</p><br />

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. 

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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).
  • Autotrophic: any organism capable of self-nourishment by using inorganic materials as a source of nutrients and using photosynthesis or chemosynthesis as a source of energy, as most plants and certain bacteria.
  • 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.
  • Consumer: an organism requiring complex organic compounds for food which it obtains by preying on other organisms or by eating particles of organic matter
  • Copepods: a group of small crustaceans found in nearly every freshwater and saltwater habitat.
  • Food Chain: simplistic linear models that describe the feeding relationships among various species of organisms in an ecological community.
  • 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
  • Heterotrophic: organisms that obtain nourishment from the ingestion and breakdown of organic matter, such as plants and animals
  • Holoplankton: Organisms that live in the water column and cannot swim against a current for their entire life cycle
  • Hydroponics: a method of growing plants without soil. In hydroponic systems, nutrient rich waters are delivered directly to plant roots
  • Keystone species: a species whose impacts on its community or ecosystem are much larger and more influential than would be expected from mere abundance
  • Meroplankton: a wide variety of planktonic organisms, which spend a portion of their lives in the benthic region of the ocean
  • Omnivorous: heterotrophs that feed on both plants and animals
  • 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
  • Predators: an organism that primarily obtains food by the killing and consuming of other organisms
  • Primary Consumers: an animal that feeds on primary producers; herbivore.
  • Primary production: the biomass produced through photosynthesis and chemosynthesis in a community or group of communities
  • Secondary Consumer: a carnivore that feeds only upon herbivores.
  • Symbioses: interaction between two different organisms living in close physical association, typically to the advantage of both.
  • 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



NERRS: Ecosystem Cycling


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