Life on the Continental Shelf

Plankton, Nekton, Benthos

Oceanic Life:

Oceanic life exists in a three layer cake.

Plankton - Organisms that drift with currents close to the surface.

Nekton - Organisms capable of swimming against currents which can be found a wide variety of depths.

Benthos - Organisms that live on the bottom, generally avoiding the upper water column.

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. Rocky substrate places for such organisms to attach themselves. Soft bottoms are sandy, silty, or muddy. They are often unvegetated by sea grasses or algae's.  However, they are often rich with organisms burrowing in the sediments. 

Life in the Mud

Unvegetated soft bottoms are often unstable, shifting constantly in response to waves, tides, and currents.  They can include sandy bottoms and beachs as well as mud flats. These are difficult places for plants or kelp to grow. Most of the producers are plytoplankton within the water column. The organisms that live in these softbottom communitiesorganisms are classified as either epifauna (organisms living on the surface) or infauna (organisms 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.  The remove food material from the mud.  Suspension feeders feed from the water column above them.

Rocky Bottoms

Rocky bottoms are rich and productive, often colonized by kelps, seaweed's, suspension feedering organisms (sponges, hydroids, sea anemones, soft corals, bryozoans, and barnacles), grazers (urchins, chitons, limpets, abalones, gastropods, fishes), and carnivores (crabs, octopus, and fishes.) In Alaska, rocky fjord walls are dominated by a variety of attached filter-feeding organisms that include corals, hydroids, sea pens, sponges, sea anemones, barnacles, scallops and mussels. All suspension feeders, these animals take water into their bodies and filter out detritus and dissolved organic matter. They provide the bottom layers to a food web that includes numerous species of shrimp, fish, and marine mammals. Anyplace with a hard surface to cling to is considered prime habitat for kelps and seaweeds that provide a seasonally abundant food source. Click to learn more about rocky bottoms.

Sediment Types:

Lithogenous or Terrigenous Sediments - several different types but all inorganic, formed by the breakdown of rocks on land. These sediments are

Biogenous Sediments - organic, composed of the skeletons and shells of marine organisms.

Calcium Carbonate and the CCD-  Calcium Carbonate (or Calcite) is one of the primary materials used by marine organisms to make hard parts like shells.  This is true of plankton as well as larger organisms like mussels.  Carbonate oozes (the remains of those shells) cover about half of the world's sea floor. They are present chiefly above a depth of 4,500 meters; below that they dissolve quickly. This depth is named the Calcite Compensation Depth (or CCD). It represents the level at which the rate of carbonate accumulation equals the rate of carbonate dissolution. If you recall from earlier in the year, water is a universal solvent, meaning it will dissolve almost anything (including Calcite). eelgrass

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 go to:

 

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; have a canopy where the fronds lie on the water surface or in mid-water; next layer is the understory where fronds are erect and stand above the bottom or lie directly upon it; final layer is the substrate upon which many algae lie (such as coralline algae).

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. To learn more about kelp forests and the species that live there, take an interactive tour of Moneterey Bay's kelp forests . Kelp

Kelp anatomy

Kelps attach to the substrate 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. Complete the table (found below) about different types of sediments.
  2. Identify three types of organisms that live in soft bottom habitats.
  3. What does the word infauna mean?  Why are most soft bottom organisms considered "infauna"?
  4. Describe the difference between a suspension feeder and a deposit feeder, and provide an example of each.
  5. Explain the key ecological role that sea grass plays in an undersea environment.  In your explanation be sure to talk about photosynthesis, nutrients, and habitat.
  6. Explain where sea grass communities can be found and three environmental conditions needed for their growth.
  7. Seagrass communities are highly endangered.  What are three things threatening sea grass communities.
  8. 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?
  9. Describe the interaction between Sea Otters and Sea Urchins in a kelp forest.
  10. Identify four organisms that live in  the kelp forest (include one invertebrate, one fish, one mammal, and one type of kelp).
Sediment Type Source of Sediment Examples of Sediment Distribution of Sediments

Biogenous

 

      

Lithogenous

 

      

Hydrogenous