Energy Flow and Nutrient Cycles

Study Questions

Find the answers as you read.

  1. How does the flow of energy through the food web affect an ecosystem?
  2. Which nutrient cycle is the basis for most of the biomass in all ecosystems?
  3. Which nutrient cycle is thought to be limited in marine ecosystems compared to terrestrial ecosystems?
  4. Why are the most productive marine ecosystems found in cold, temperate regions?


Trophic relationships and nutrient cycles are both concepts fundamental to ecology. They describe how energy and matter form the basis for interaction among organisms and between organisms and the environment. Photosynthesizers bring energy from the sun into the foodweb. This energy transfers up through the food web, but most of the energy gets lost as heat in the process. Only about 10% of the available energy passes from one trophic level to the next.

The energy flow through the food web affects an ecosystem by determining how much energy is available for organisms at higher trophic levels. In most ecosystems, there are fewer high-level predators than low-level prey. The amount of primary production shapes the ecosystem. High and diverse primary production creates the potential for more organisms at high trophic levels and the potential for more trophic levels. Similarly, anything that affects energy flow will also affect the ecosystem. For example, if pollution causes a substantial decline in an ecosystem’s primary consumers, it disrupts energy flow to higher trophic levels. Therefore, even with ample primary production, the ecosystem would lose many of the high-level organisms in its community.

Interrupted Energy Flow
A substantial decline in an ecosystem’s primary consumers disrupts energy flow to higher trophic levels. Compare the first and the second diagrams above. In the diagram on the right, we see a reduction of the amount and types of prey available to killer whales. The whale population will suffer in this ecosystem unless they move on to an area with more productivity, or more primary consumers to transfer energy to higher trophic levels.

Energy flows through an ecosystem, eventually being lost as heat into the water, atmosphere, and space. Nutrients, on the other hand, are not lost. As discussed in Unit 3, carbon, nitrogen, phosphorus, and other crucial elements cycle through the Earth’s ecosystems.

The carbon nutrient cycle is the basis for most of the biomass in all ecosystems. As discussed in Unit 3, carbon is fundamental for all life. The ocean appears to play an important role in the global ecosystem by helping absorb the excess carbon released by the burning of fossil fuel.

Nitrogen Cycling
Atmospheric nitrogen must be fixed into other compounds, such as nitrate or ammonia, before it can be used by organisms. The nitrogen-fixing bacteria that do this were once thought to live primarily in terrestrial ecosystems. However, recent evidence indicates that nitrogen-fixing bacteria are much more common in marine ecosystems than scientists first thought. Beyond natural sources, two of the biggest sources of organic nitrogen in coastal waters are fertilizer and industrial wastes from the land.

The nitrogen nutrient cycle is thought to be more limited in marine ecosystems than in terrestrial ecosystems. This is because inorganic nitrogen stored in the atmosphere must be fixed into organic compounds before organisms can use it. Nitrogen-fixing bacteria live primarily in terrestrial ecosystems. However, nitrogen- fixing bacteria are more common in marine ecosystems than previously thought. Terrestrial sources of organic nitrogen are especially important in coastal water. Seabird droppings, erosion, and runoff carry organic nitrogen compounds (and phosphorus) from terrestrial environments into the marine environment. The source of a great amount of organic nitrogen in coastal waters is fertilizer and industrial waste. Runoff carries the nitrogen into rivers and on into coastal waters. Once there, the nitrogen fertilizes the water, creating algal blooms. This is an example of how ecosystems do not exist in isolation.

Nutrient cycles and energy flow are equally important to an ecosystem. However, nutrients are usually a limiting factor, whereas energy is usually not. You can see the importance of limiting factors by comparing warm, tropical marine ecosystems with cold, temperate marine ecosystems. Tropical ecosystems generally have more energy (sunlight) available, but fewer nutrients than temperate ecosystems. The reason for fewer nutrients is that in the tropics, the strong thermocline prevents vertical mixing that brings nutrients up from deeper water. One of the few highly productive marine ecosystems found in tropical waters is the coral reef. Temperate coastal waters, by comparison, have less overall sunlight, but receive far more nutrients. For this reason, the most highly productive marine ecosystems are found in colder water.

 Continue to Unit 7 Chapter 2. 

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