Chapter 9: The Continental Shelf

Life on the Shelf:

Most of the world’s living things live in the relatively shallow waters of the ocean’s continental shelf. It is easy to think of these organisms as being divided into three basic categories.  Plankton are organisms that drift with currents close to the surface. Many are very small, but there are large examples as well. The Portuguese Man-o-war can grow to over 100 feet long. What makes a plankton is not the size, but whether or not you are at mercy to the flow of air and wind currents along the ocean’s surface. Nekton are organisms that are capable of swimming against currents. They can be found at a variety of depths, from shallow to deep. They move through the water column with ease and can be highly migratory. Benthos (or benthic) are organisms that live on the bottom. Some are capable of swimming, but most walk or slither instead of swim.

Benthos Composition

Benthic communities vary greatly based on what is on the bottom. Roughly speaking, the bottom is either hard or soft. Hard, or rocky, bottoms are often rich with plant or coral life. These hard surfaces provide places for organisms to attach themselves. Soft bottoms are sandy, silty, or muddy. They are normally un-vegetated by plants or algae’s, but they are often rich with organisms burrowing or burying themselves in the sediment.

Rocky Bottoms

Image from:

Image from:

Rocky bottoms are rich and productive places, often colonized by kelps and seaweed’s. They are home to many types of animals including; suspension feeding organisms (such as sponges and barnacles), grazers (such as urchins, chitons, limpets, and abalone), and carnivores (such as crabs, octopus, and fishes). In Alaska, rocky fjord walls are dominated by a variety of attached filter-feeding organisms that include corals, sea pens, sponges, sea anemones, barnacles, and mussels. Like all suspension feeders, these animals take water into their bodies and filter out detritus and dissolved organic matter. They make up the bottom layer of a food web that includes numerous species of shrimp, fish, and marine mammals. Any place with a hard surface to cling to, and provides a seasonally abundant food source, is considered prime habitat for kelps and seaweeds.  Click to learn more about rocky bottoms.

Soft Bottoms

Unvegetated soft bottoms are often unstable, shifting constantly in response to waves, tides, and currents.  They can include sandy bottoms and beaches as well as mud flats. These are difficult places for plants or kelp to grow. However, animals are abundant. The animals that live in these soft bottom communities, are classified as either epifauna (animals living on the surface) or infauna (animals living below the surface). Infauna can be further divided into suspension feeders or deposit feeders.  Deposit feeders eat amongst the muck and mud on the bottom, and remove food material from the mud.  Suspension feeders, such as this peacock worm, feed from the water column above them.

Benthic Sediment Types

There are three main types of sediments found on the ocean bottom. They are classified by their source of origin. Lithogenous or Terrigenous Sediments are inorganic, formed by the breakdown of rocks on land. Biogenous Sediments are organic, composed of the skeletons, shells, and decomposed remains of marine organisms. Biogenous sediments tend to be found close to the continental shelf. Click to read more about these different sediment types.

Seagrass Communities

Soft bottoms may be carpeted by seagrasses (turtle grass, eelgrass, manatee grass are just a few examples).  Roots keep plants anchored in the face of turbulence and help stabilize the soft bottom; leaves cut down wave action and currents; finer sediments get deposited; and water is generally clearer since less sediment is suspended.  Seagrass beds are highly productive.  They feed herbivores and detritovores directly, and other organisms indirectly, by providing sheltered habitat.  To learn more about sea grass communities, check out:

By Colin.faulkingham at en.wikipedia (Create by Colin Faulkingham in Summer 2007.) [Public domain], from Wikimedia Commons

By Colin.faulkingham at en.wikipedia (Create by Colin Faulkingham in Summer 2007.) [Public domain], from Wikimedia Commons

Kelp Forests and Kelp Beds

Kelp is a generic term for accumulation of brown algae’s.  If they do not form a surface canopy, they are called kelp beds, but if they do form a canopy, they are called kelp forests.  Kelp forests have distinctive layers, each made up of species that grow at a characteristic height above the bottom. First is the canopy, where the fronds lie on the water surface or in mid-water. Next is the understory, where fronds are erect and stand above the bottom or lie directly upon it. The final layer is the substrate upon which many algae lie down. Kelp forests communities are rich in species diversity, including a wide variety of invertebrates, cephelopods, fish, and sea birds. Sea urchins are one of the most important grazers of kelp forests and can be responsible for kelp forest destruction when their populations increase too much.  Sea otters play key roles in regulating the abundance of sea urchins and, as such, act as a keystone species.

Kelp anatomy

Kelp attaches to the substrate (or hard bottom) via a holdfast, have a stemlike stipe, broad flat blades, and a float or pneumatocyst. Fronds are defined as a stipe with many leaf-like blades.

Questions to Research:

As always, your textbook may be used as a resource.  For questions about life on the continental shelf, see chapter 13.

  1. What is seastar wasting disease?  Listen to an NPR story to help you.  Is it affecting Alaska?  What are some possible environmental stressors that may be contributing to seastar wasting disease.
  2. Create a table that describes three (of the four) different types of sediments.  In the table, identify where each type of sediment comes from and where each type of sediment can be found.
  3. Check out this map of ocean sediment thickness.  Based on your observations, is ocean sediment thicker along the continental shelf or in the deep ocean?  Explain why you answered the way you did.
  4. What is a dead zone, how do human activities contribute to them, and what impact can they have on the species that inhabit the continental shelf?
  5. What does the word infauna mean?  Describe an example of an infauna species.
  6. Explain the difference between a suspension feeder and a deposit feeder, and provide an example of each.
  7. Explain one key ecological role that sea grass plays in the continental shelf.  In your explanation, be sure to talk about photosynthesis, nutrients, or habitat.
  8. Seagrass communities are highly endangered.  Use the link from question 7 to explain three things threatening sea grass communities.
  9. Identify four organisms that live in  the kelp forest (include one invertebrate, one fish, one mammal, and one type of kelp).  You may also check out – rocky bottoms.
  10. Describe the interaction between Sea Otters and Sea Urchins in a kelp forest.  In your description use the words keystone species and grazer.