Professor – 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? Hmm, most of you; so do I. In fact, I am on a “see-food” diet, I see food and I eat it. Get it? OK but, uh, 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, Mrs. Lee?
Student – It’s an organism eats… No; it’s what an organism eats and the things that eat that organism.
Professor – 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. Autotrophic 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 tissues, thus cycling it back to the producers. A simple marine food chain, then, might look like this: the top predator, trophic level 4, is the herring. Herring, fish, eat level 3, carnivorous zooplankton. The carnivorous zooplankton eat trophic level 2, herbivorous zooplankton. And herbivorous zooplankton eat level 1, phytoplankton, which is a type of autotroph. In marine food chains, energy transfer is not very efficient. Phytoplankton utilize only 1% of the energy available from the sun. Between 70 and 90% 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% that’s retained in the body’s biomass and available for consumers at the next highest trophic level. Therefore, 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, 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…
Student – Us?
Professor – Yes! And there’s something else from the seas that will even eat us!
Student – Sharks?
Professor – 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 a few moments on us. What are the implications of trophic levels for the fish that we eat? Well, looking at the fish harvest worldwide, 88% of the fish that we catch are fish with fins, 8% are shellfish, and 4% are crustaceans. Fish caught in the open ocean such as tuna are high level predators on an insufficient food chain. Fish caught in coastal areas such as cod, herring, and haddock are at the top end of a more efficient food chain. This is because there is a high density of phytoplankton, so consumers expend less energy catching the food. These fish then provide more energy and better nutrition for us. In upwelling 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.
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