Lecture 4

Listen to part of a lecture from a marine biology class.

Prof: You’ve been reading this month about food chains and food webs. Today we’ll discuss these in relation to seafood. How many of you like seafood? Mmm, most of you. So do I. In fact, I’m on a seafood diet. I see food, I eat it. (weak laughter; perhaps some groans or hissing). Get it? Ha ha. OK. But seriously folks…Today we’re going to talk about trophic relationships in marine food chains and webs. Who can remind us what a trophic relationship is? Yes, Mr. Li?

S: It’s what an organism eats, and uh, the things that eat that organism.

P: Very good. Trophic relationships describe the relationship between producers and consumers, so they help us diagram food chains and food webs. Now, in marine ecosystems, like other ecosystems, food is needed for matter - growth and reproduction - and for energy - metabolic processes within the body. Also like other ecosystems, marine ecosystems have producers, consumers and decomposers. The primary producers are autotrophic plankton. Auto trophic means these plankton can synthesize their own food. Autotrophs are consumed by heterotrophic organisms. “Hetero” means other; so in this case, heterotrophic means organisms that can’t synthesize their own food. They must rely on autotrophs for food energy. The primary consumers in marine food chains are the plant eaters – herbivores – and the secondary consumers are both the meat eaters - carnivores - and predators that eat both meat and plants: omnivores. The decomposers are heterotrophic bacteria, which get energy from body wastes and dead tissue, thus cycling it back to the producers.

A simple marine food chain, then, might look like this (sound of writing on board): the top predator, trophic level number 4, is a herring. Herring fish eat level 3, carnivorous zooplankton. The carnivorous zooplankton eat trophic level 2, herbivorous zooplankton. And herbivorous zooplankton eat level number 1, phytoplankton, which is a type of autotroph. In marine food chains, energy transfer is not very efficient. Phytoplankton utilize only about one percent of the energy available from the sun. Between 70 and 90 percent of the energy made by producers or eaten by heterotrophs is used in their bodies or expelled as waste. This leaves only 10 to 30 percent that’s retained in the body’s biomass and available for consumers at the next highest trophic level. Thus, the biomass at each trophic level is controlled by the efficiency of the energy transfer. At the lowest trophic level, animals will generally have high biomass, and there will be lots of small producers. At the highest trophic level, animals will generally have low biomass, and there will be only a few large animals.

Now, let’s expand our simple food chain into a food web. In this web, a herring is no longer a trophic-4 predator. There is a bigger fish, um…a tuna, that eats the herring. But there is an even bigger animal that eats the tuna. And that is…?

S: Us!

P: Yes. And there’s something else from the seas that will even eat us.

S: Sharks!

P: Correct. A food web is more complex than a food chain. And then there are gigantic animals, like whale sharks and baleen whales, that are herbivores and only eat plankton. But let’s focus for a few moments on us. What are the implications of trophic levels for the fish we eat? Well, looking at the fish harvest worldwide, 88 percent of the fish we catch are fish with fins. Eight percent are shellfish, and four percent are crustaceans. Fish caught in the open ocean, such as tuna, are high-level predators on an inefficient food chain. Fish caught in coastal areas, such as cod, herring and haddock, are at the top end of shorter, more efficient food chains. This is because there is a high density of phytoplankton, so consumers expend less energy catching food. These fish, then, provide more energy and better nutrition for us. In “upswelling” areas, off the west coasts of America and Africa, the fish are even healthier. Here there are small, very efficient food chains, and the fish are small, fast-growing, eat lots of phytoplankton and travel in dense schools. Two examples of such fish are anchovies and sardines.