These exercises were created by Dr. Karen Grove (© 1998 Grove) for use in the Introduction to Oceanography course offered by the Department of Geosciences at San Francisco State University. Other educators are welcome to use these exercises in their own classes. Please send comments to Course home page: Commercial use is prohibited.

VOYAGE 9: The Ocean: Habitat for Life

Instructions: First print copies of Voyage 9, Lag-time questions, and Real-time questions. Read the text and examine the images. Write answers to the questions on your copies. When you are satisfied with your answers, type them into the computer and submit. Keep the copies to study from.

Don't forget to check out the course web site where you can obtain preparatory information before each class and summaries that are posted after the class meets. Click here to bring up a new browser window with the course site. You can also use this site to display the text and images part of the voyage on a second window so that you don't have to print anything.

Life environments

The physical and chemical structure of the ocean that we have discussed in class provides habitats that marine organisms depend on. Variations in such factors as temperature, light, and nutrient concentration create distinct conditions to which organisms have adapted. For example, the planktonic seaweed in the Sargasso Sea grows slowly and lives a long time, an adaptation to low nutrient concentrations. Fish that live at deeper levels of the ocean have developed bioluminescence and photic organs that help them to cope with low-light conditions. For more information about the characteristics of different life environments, see pages 231-238 in the textbook.

Figure 1. Divisions of the marine environment. The two large divisions are the benthic realm (organisms that live on the seafloor) and the pelagic realm (organisms that live in the water). The benthic and pelagic environments are subdivided into zones based on depth. From deepest to shallowest, the zones are hadal, abyssal and bathyal zones. In the pelagic environment, the shallowest zone is called the photic or surface zone. In the benthic environment, the shallowest zones are called the intertidal and subtidal zones.

NOTE: this diagram has a large amount of vertical exaggeration (slopes look MUCH steeper than they really are).

Figure 2. General characteristics of the marine environment.

Figure 3. This diagram shows some of the physical and chemical conditions that change with depth in the ocean. The first panel shows variation in density with depth. This density change is a function of temperature, which decreases downward in the ocean. Panels 2, 3, and 4 all show variation in nitrates, phosphates, and silicates, which are nutrients essential for plant growth. The fifth panel shows variation in dissolved oxygen, which is supplied to the ocean from the atmosphere and by plant photosynthesis.

Note that the vertical axis in all graphs is depth (deeper toward bottom of graph) and that the horizontal axis in all graphs is concentration, accept panel one, which is density. In all graphs, concentration or density increases toward the right side of the graph. These characteristics are not uniform throughout the ocean, but these graphs represent common conditions.

Figure 4. Figures 1, 2, and 3 show typical variations with depth in the ocean. Specifics, however, vary from place to place. For example, in some locations, the water does not contain many sediments or plankton (has low turbidity), and solar light energy is available at greater depths than at locations where the water contains lots of sediment and/or plankton (has high turbidity).

Figure 5. Animals need to eat, and so their choice of living environment must take into account their favorite food's location. This diagram shows that zooplankton biomass (amount of living matter) varies directly with phytoplankton biomass. This diagram shows an example from a polar latitude (Siberian Sea). Note that biomass increases upward and that is changes drastically throughout the year.

Gray whales and their environment

California gray whales live in the pelagic environment and are an example of animals that have well adapted to conditions in their physical environment.

Figure 6. California gray whales migrate thousands of miles each year from lagoons in Baja California, where calm, warm water provides excellent habitat for mating and parenting, to their summer homes in the Chukchi and Bering Seas near Alaska.

Figure 7. Although the populations of many marine animal species are declining, gray whales are an exceptional success story. Demand for whale oil and bone led to their drastic decline between the mid-1800s and the mid-1900s. In 1946, the International Whaling Commission was formed and passed laws that prohibited the killing of whales. Since then, their populations have rebounded.

Intertidal life environment

The part of the benthic realm that is typically most accessible to human viewing is the rocky intertidal zone. In this difficult environment, organisms must cope with breaking waves, exposure above water, and daily variations in water temperature and salinity. Organisms have developed adaptations, such as firm attachment to rocks and shells to hold in moisture, to deal with these conditions.

Figure 8. Marine organisms are adapted to life in the water. Thus, exposure above water is a difficult condition to cope with. This diagram shows the length of time that organisms living at a particular level (between the highest tide and the lowest tide) are exposed to air during a tidal cycle (here called a tidal day).


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