Chapter 13 Life on the Continental Shelf

The continental shelf is the submerged edge of a continental plate. The continental shelf is the submerged edge of a continental plate. The shelf extends from the low tide line to an area known as the shelf break. The shelf extends from the low tide line to an area known as the shelf break. The slope of the continental shelf is normally more gradual, whereas, the slope past the shelf break is normally very steep. The slope of the continental shelf is normally more gradual, whereas, the slope past the shelf break is normally very steep.

Life on the Continental Shelf Water turbulence created by waves and currents keep the water column well mixed. Water turbulence created by waves and currents keep the water column well mixed. Nutrients never have a chance to “rain down” on the bottom and be lost the way they do in the deep sea. Nutrients never have a chance to “rain down” on the bottom and be lost the way they do in the deep sea. This constant water motion means the nutrients are available to the benthic organisms as well as those in the neritic zone (the pelagic environment above the continental shelf). This constant water motion means the nutrients are available to the benthic organisms as well as those in the neritic zone (the pelagic environment above the continental shelf).

Pelagic – water column away from bottom or shore Pelagic – water column away from bottom or shore Epipelagic – sea surface to depth of about 200 meters Epipelagic – sea surface to depth of about 200 meters –The epipelagic can be divided into (1) neritic waters over the continental shelf and (2) oceanic waters that are past the shelf

Types of Communities on the Continental Shelf 1. Soft-bottomed benthic 2. Seagrass beds 3. Kelp forests 4. Hard-bottomed benthic

Life on the Continental Shelf Soft-bottomed benthic- Soft-bottomed benthic- –Dominant bottom type on the continental shelf –Benthos of infauna, epifauna and meiofauna are present as well as _____ species (pelagic species above continental shelf) above the benthos –Sessile types are rare due to lack of “hard substrate” for attachment –There are more species present in this area than in the adjoining intertidal, but the distribution is often patchy

Life on the Continental Shelf Soft-bottomed benthic- Soft-bottomed benthic- –Infauna like snails, clams, urchins, and annelids as well as epifauna like crustaceans and brittle stars. –Many are tube- builders to assist with the stability of the habitat.

Life on the Continental Shelf Meiofauna Meiofauna Many of these species are found no where else and include protozoa, cnidarians, crustaceans, nematodes, & annelids Many of these species are found no where else and include protozoa, cnidarians, crustaceans, nematodes, & annelids These species have appendages and attachment structures to lock themselves to sediment grains These species have appendages and attachment structures to lock themselves to sediment grains

Life on the Continental Shelf Soft-bottomed benthic- Soft-bottomed benthic- –Bioturbators (organisms that disturb bottom sediments) constantly “churn” the bottom –This churning helps to oxygenate benthic soils –Bioturbators can include worms, mollusks, fish and even whales –Skates and stringrays as well as sperm whales are well known for their activity as bioturbators

Life on the Continental Shelf Seagrass Beds Seagrass Beds –They are best developed in sheltered, shallow waters with good water clarity –There are over 50 known species and most common genus is Zostera, commonly called eelgrass –Species have particular tolerance levels with regards to temperature, water clarity and nutrient availability that determines their distribution –Many sessile animals and epiphytic algae

Life on the Continental Shelf Seagrass Beds Seagrass Beds –Because of their different tolerances, seagrass species normally prefer either cool waters OR tropical/subtropical waters, but not both normally –They are often found in combination with algal species –The density of individual seagrass plants provides large amounts of detritus

Life on the Continental Shelf Seagrass Beds Seagrass Beds –Thalassia (turtle grass) is a common seagrass in tropical and subtropical areas –It is named turtle grass because it is often directly consumed by green sea turtles –In fact, patches of turtle grass have been shown to be “farmed” by individual turtles –In this farming, green turtles bite the tops off the turtle grass –When the tops grow back, they are much more tender and easier to digest by green sea turtles

Life on the Continental Shelf Hard-bottomed subtidal- Hard-bottomed subtidal- –Less common –Often, it is the result of a submerged rocky shoreline creating lots of hiding places –Rock formations attract a wide variety of organisms including many sessile types that cannot survive in soft bottomed communities –Oyster reefs, worm tubes and calcareous algae can all serve as hard-bottomed communities as well

Sea _____ grazing on seaweed in a hard bottomed subtidal community

Generalized food web for hard-bottomed subtidal community

Life on the Continental Shelf Kelp community- Kelp community- –Kelps are ____ algae that are restricted to cold waters in distribution –In addition to requiring cold water, kelps are very nutrient-needy –This is the result of the fast growth seen in these algae (up to 20 inches a day)

Life on the Continental Shelf Macrocystis is a large species of kelp that can grow heights of over 100 feet Macrocystis is a large species of kelp that can grow heights of over 100 feet It is found mainly in the Pacific on the coasts of North and South America It is found mainly in the Pacific on the coasts of North and South America This species forms thick forests that serve as a MAJOR ocean ecosystem in the areas where it is found This species forms thick forests that serve as a MAJOR ocean ecosystem in the areas where it is found

Geographic distribution of kelps

Life on the Continental Shelf Kelp community- Kelp community- –Sea urchins are by far the largest direct consumer of kelp –In turn, many species feed on the sea urchins including their “cousins” the starfish and sea otters

Life on the Continental Shelf The diagram at the right shows the anatomy of a kelp forest The diagram at the right shows the anatomy of a kelp forest Thousands of organisms are dependent on the kelp forests for shelter (habitat) or for trophic needs Thousands of organisms are dependent on the kelp forests for shelter (habitat) or for trophic needs