Unit 4: The Atmosphere and the Oceans Chapter 16: The Marine Environment     The Marine Environment What is the largest structure ever built by living things? It is not the Great Wall of China or the pyramids of Egypt. The largest structure built by living things is the Great Barrier Reef of Australia. This reef is 2000 km long, and it was built mainly by corals, tiny marine animals that are only 1 to 30 mm in diameter. Each individual coral secretes a calcium carbonate cup to provide protection for its soft body. Many corals live in colonies, and together their cups form a structure called a coral reef. Corals are just one example of a population of marine organisms. A population is a group of organisms of one species that live in the same place at the same time. Corals are not the only species that live in a coral reef, however. Populations of sea anemones, sponges, and small fishes may live there, too, and they, along with the corals, form a coral reef community. The community of organisms, and its interactions with the physical conditions of the coral reef, together form an ecosystem. An ecosystem is made up of all the populations in a community and their interactions with their environment. Marine Ecosystems In each type of marine ecosystem, every species has a particular role to play. In coral reefs, corals live in the calcium carbonate cups that they secrete. These cups, and the reef built from them, are the habitat of the corals. The coral reef, in turn, provides food and shelter for many other organisms, including sponges, sea anemones, and fishes. All of these functions make up the niche of corals. A niche is the role and position a species has in its ecosystem, including how a species meets its needs for food and shelter, how it reproduces, and all of its interactions in its habitat. Relationships Among Organisms Within any ecosystem, there are relationships among organisms. Many of these relationships involve feeding. Corals, for example, are consumers-organisms that obtain energy by eating other organisms. Plants, cyanobacteria, and algae are producers-organisms that use the Sun's energy to produce food through photosynthesis. Some producers, such as those in hydrothermal vent communities, use chemicals to produce food in a process called chemosynthesis. Other organisms, called scavengers, feed on dead things, while still others, such as bacteria and fungi, are decomposers, organisms that break down and absorb nutrients from dead organisms. Other kinds of relationships also exist among organisms. For example, corals are predators, but they also have a relationship with dinoflagellate protists called zooxanthellae (zo zan THE lee) that live in their bodies. The zooxanthellae are producers that make food both for themselves and for the corals. The corals provide shelter and protection for the zooxanthellae. A close and permanent association between two species that live together, such as the one that exists between corals and zooxanthellae, is called symbiosis. LINK UP: Find out more information about coral reefs. Energy Flow through Ecosystems As was stated earlier, many of the relationships among organisms in an ecosystem involve feeding. Producers capture the Sun's energy and store it as food in their cells. This stored energy is released to consumers when they eat the producers. Recall that energy cannot be created or destroyed, but it can change form and be transferred. Ecologists identify the transfer of energy in an ecosystem through simple models called food chains, which usually are drawn with arrows to show the direction in which energy is transferred from one organism to another, as shown below. Green algae -> Snail -> Fish -> Human In this food chain, green algae, which are producers, are eaten by a snail, a consumer. The snail is eaten by a fish, another consumer, and finally, the fish is eaten by a human, the final consumer in the food chain. Each step in a food chain is known as a trophic level and represents the passing on of energy and matter. As energy moves from the first trophic level, a producer, to the second trophic level, a consumer, some of the stored energy is used for the consumer's maintenance, some is used for its growth or reproduction, and some is lost as heat. Only about 10 percent of the energy available at each trophic level can be passed on to the next level. This is why most food chains have only three or four trophic levels. Energy Pyramids Ecologists illustrate the energy flow through ecosystems with pyramids that represent the trophic levels involved in a food chain. The base of such a pyramid always represents the producers in the food chain, or the first trophic level. The next few steps represent the second, third, and occasionally the fourth trophic levels. Pyramids may be based on the numbers of organisms at each level; the amount of energy transferred to each level; or the biomass, the total mass of the organisms at each level of the food chain. The figure below illustrates a pyramid of biomass for a coral reef. A food chain is a simple way to show how energy and matter flow through an ecosystem, but most organisms eat more than one type of food. Thus, ecologists also use food webs as models of energy flow that express all possible feeding relationships in ecosystems. Food webs consist of many interconnecting food chains. The simplified food web illustrated below shows only producers and consumers. In a real food web, there also are scavengers and decomposers. The loss of any species from an ecosystem can weaken a food web. Each organism, big or small, plays an important role in the overall health of the marine environment. Activity: A food chain in the Antarctic Ocean can be represented by the following: Diatoms -> Krill -> Blue whale Describe a pyramid of biomass for this food chain. Would this pyramid be similar in shape to the pyramid for organisms around Eniwetok Atoll? Explain.