Marine Fisheries Food Webs
All our ideas of life in the sea are rapidly changing. The plants and animals described on this page are mostly large. Yet, the marine food web is dominated by astronomical numbers of micro, nano, and pico plankton described in the chapter on the Microbial Food Webs. We know little about these organisms. They are too small to be easily seen and studied. They cannot be cultured. They tend to all look alike.
We can separate marine microbes using DNA analysis, and the analysis is leading to remarkable discoveries. Life in the sea is much more diverse than we expected. For example, we now know, since the 1970s that all life is separated into three domains. All three are very common in the ocean. Click on any of the three domains and explore the microscopic world.
Since the discovery of the astronomical number of microbes in the ocean, we now recognize two important, overlapping food webs in the ocean.
The two webs are coupled in many ways that are not yet understood. Microbes cycle nutrients, they produce other nutrients such as vitamins needed by primary producers discussed here, and they infect, sicken, and kill many organisms in the marine fisheries food web. We are not the only large animals that get viral and bacterial diseases.
Phytoplankton: Primary Producers of the Marine Fisheries Web
The sunlit upper layers of the ocean, called the euphotic zone, are home to vast numbers of single-cell marine primary producers called phytoplankton. They include diatoms, coccolithophores, cyanobacteria especially synechococcus and prochlorococcus, and dinoflagellates. Of these major classes of organisms, all but the cyanobacteria are eukaryotes, organisms with cells with a nucleus. Microscopic, eukaryota phytoplankton were formerly lumped together under the term protists, but this is a catch-all term.
The term algae is another catch-all term for primary producers with chlorophyll that formerly included many unrelated organisms, excluding land plants.
Phytoplankton are primary producers because they use solar energy to convert CO2 and nutrients into carbohydrates and other molecules used by life. Together, they account for about 95% of the primary productivity in the ocean and about half of all primary productivity on earth.
Phytoplankton are most common in cooler, mid-latitude zones with sufficient nutrients, especially nitrogen. Thus they are common in the north Atlantic and Pacific, and along coasts. They are much less common in the central regions of the ocean and in the southern hemisphere. For more information, see the chapter on phytoplankton distribution.
The major primary producers include:
Example: Diatoms are single-celled organisms with a nucleus (unicellular, eukaryotic organisms) capable of converting sunlight into carbohydrates. The diatoms are located near the bottom of the Tree of Life. They belong to the Stramenopiles which are included in the chromalveolates supergroup of the eukaryotes. The relationships among the phytoplankton and organisms near the base of the tree of life is not yet well understood (see the Discussion of Phylogenetic Relationships) and the primary interrelationships are just now becoming clear. There are perhaps 200,000 different species of diatoms ranging is size from micrometers to millimeters. Some are no more closely related than fish and mammals (Armbrust 2009). They account for 20% of the photosynthesis on earth.
The phytoplankton are eaten by the smallest floating animals, the zooplankton. They range in size from single-celled organisms to larger multi-celled organisms. Small zooplankton are eaten by larger zooplankton. Zooplankton include
Zooplankton are eaten by small predators:
Example: Clupeus harengus (Atlantic herring) is a small bait fish. It schools in coastal waters. It feeds on small planktonic copepods in the first year, thereafter mainly on copepods. Adults are about 30-35 cm in length, and they live about 20 years. They are eaten by many species of birds, fish, and marine mammals.
At the top of the marine food web are the large predators:
Example: Thunnus alalunga (Albacore) is large, fast-swimming fish. Their average weight is about 9-20 kg. They are thought to become sexually mature when they are 5-6 years old and about a meter long. They have a maximum lifespan of 8 years. They are well adapted to swim fast, and they prey on many species of fish.
Food Chains and Food Webs
Phytoplankton, small zooplankton, larger zooplankton such as jellyfish, larger animals including bait fish and squid, (see also here), and top predators such as tuna, all interact in a marine food web. Each species eats and is eaten by several other species at different trophic levels.
The interactions in a food web are far more complex than the interactions in a food chain. Furthermore, the branching structure of food webs leads to fewer top predators compared with the numbers of top predators in a food chain.
Using the illustration above, a food chain would go from phytoplankton to large zooplankton such as krill to marine mammals such as baleen whales with no branches. Food chains are much rarer than food webs in marine ecosystems, although the example I just gave which leads to baleen whales is a common food chain in the Antarctic Circumpolar Current.
Over Fishing Changes Food Webs
We saw earlier that Cod stocks on Canada's East Coast have failed to rebound more than a decade after the fishery was closed. Now, Kenneth Frank and colleagues have reported the results of their study of changes in the food web in the large eastern Scotian Shelf offshore of Nova Scotia, Canada. They found that the removal of cod and other large fish changed the entire structure of the food web from top to bottom:
The changes in marine ecosystems due to over fishing is often called fishing down the marine food web. As top predators are removed by fishing, fishers target smaller fish lower in the food web, reducing their numbers. This reduces the average trophic level of the food web. Trophic levels are based on the food eaten at that level. Level 1 includes phytoplankton, level 2 includes zooplankton, level 3 includes bait fish, etc.
Scripps Institution of Oceanography graduate student Loren McClenachan studied historical photographs spanning more than five decades that she collected from Florida. The study showed a drastic decline of so-called "trophy fish" from Key West.
Armbrust, E. V. (2009). The life of diatoms in the world's oceans. Nature 459 (7244): 185–192.
Berger, W. (1976). Biogenous deep-sea sediments: production, preservation and interpretation. In J.P. Riley, R. Chester (eds.), Treatise on Chemical Oceanography. Academic Press, London, pp. 265-388.
Pauly, Daniel (2003). Ecosystem impacts of the world's marine fisheries. Global Change Newsletter, 55, page 21.
Scheffer, M., S. Carpenter, et al. (2005). Cascading effects of
overfishing marine systems. Trends in Ecology & Evolution 20(11):
Revised on: 3 August, 2009