9. The Marine Food Web and Trophic Pyramid– From Primary Producers to Top Level Predators
and Back to Detritus for the Decomposers


Marine Ecosystem

1. Introduction
2. Nutrients
3. Carbon in the Sea
4. Carbon Cycle
5. Conditions for Life
6. Limiting Nutrients
7. Nutrient Distribution
8. Life in the Sea
9. Food Webs

cocco4.gif (3931 bytes)

Coccolithophorid, a marine algae (phytoplankton)

Ecosystems are areas where the community, or communities, are rather self-sustaining. This involves food webs - that is describing who eats who and following the energy flow (in the form of food) .

Marine life interacts in a variety of complex ways.

Primary producers, such as single-celled phytoplankton as seen to the left, make their own energy, either through photosynthesis in the sunlit upper layers of the ocean or through chemosynthesis as we learned about in our studies of hydrothermal vents.

Phytoplankton include diatoms, dinoflagellates, coccolithophores, and photosynthetic bacteria, for example cells without a nucleus. As we learned earlier in this expedition, all use solar energy to convert CO2 and nutrients into carbohydrates and other molecules used by life.

Phytoplankton form the base of the marine food web as shown in the diagram below, as they produce their own food. This is also referred to as the first trophic level.


Diatom, a marine algae (phytoplankton)


Marine Food Web Diagram

Marine Food Web

After the primary producers, come the herbivores, such as small and large zooplankton, including copepods and marine invertebrates, that eat the phytoplankton. These organisms compose the second trophic level. You may have seen some of these, if you attended the bay voyage this semester.

The herbivores are then consumed by the carnivores, such as small fishes and squid, which form the third trophic level.


Marine Food Web Diagram

From "Fishing down marine food webs' as an integrative concept" by Daniel Pauly (University of British Columbia, Canada)

Also, notice that feeding relationships, even as schematically shown in this simplified diagram are far more complex than in the oversimplified concept of a food chain.

UniTunaImage.jpg (12986 bytes)

Do not click here 
Image from Saving the Bluefin Tuna
©Copyright Hewlett Packard

The fourth trophic level is composed of the carnivores that eat other carnivores.

Last, there is the detritus, which is produced from the decaying remains of dead organisms, by the decomposers (bacteria) , which returns the nutrients to the phytoplankton at the base of the food web, once again.



Like all ecosystems, marine ecosystems are mostly self-sustaining systems of life forms and the physical environment. In these ecosystems materials are cycled and recycled.


-- from Marine Bionet

Let's go on with a trip onto Monterey Bay..........


NPR logoMorning Edition logo

Morning Edition, July 7, 1997 · -- N-P-R's Alex Chadwick goes on a National Geographic Radio Expedition to a marine sanctuary off the coast of California to see and hear whales. The marine sanctuaries are like the National Parks-- protected areas in the ocean that provide safe areas for threatened species

Let's now recap our work on food webs and flow of energy through a marine trophic pyramid.

From https://www.sciencelearn.org.nz

©Copyright 1999
July 28, 2015
Send to Don Reed

Department of Geology
San Jose State University

Remember to post in your Expedition learning group according to instructions on worksheet.