Life on the Continental Shelf Chapter 13 Life on the Continental Shelf

Life on the Continental Shelf 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 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 The slope of the shelf, the depth of the water at the shelf break and the distance of the shelf break from shore varies greatly by location. Estuaries are much more well developed in areas of the world where the continental shelf has a gradual slope and is very wide. This is the case on the East Coast and Gulf of the US, which is the reason for the well developed estuaries there.

Life on the Continental Shelf The depth of water at the shelf break varies greatly with an average depth of around 490 feet. The width of the continental shelf also varies greatly from 0.6 miles to 470 miles with an average of 48 miles.

Life on the Continental Shelf The continental shelf is biologically rich. This is also an area of potential natural resources such as oil and natural gas. For this reason, countries wanted a way to protect these resources from exploitation by other countries. In the 1950’s, the United Nations agreed that each country shall have control over the marine resources out to 200 nautical miles (=1.15 land miles) off their coast – this includes the continental shelf and its resources.

Life on the Continental Shelf A major portion of the fisheries global catch is caught over the continental shelf. The shelf and associated estuaries are an important area for larvae and juveniles of many species as well as adults. A wide variety of organisms can be found on/in the continental shelf.

Life on the Continental Shelf Temperature: Temperature of the water over the continental shelf varies much more than the water in the deep sea (but less than in an estuary). The temperature fluctuates more because the water is shallower than in the deep sea and because waves and currents keep the water circulating. This means that warmer surface waters are mixing readily with the rest of the water column.

Life on the Continental Shelf Water turbulence created by waves and currents keep the water column well mixed. This affects not only the temperature of the water, but also affects the nutrients available in the water. 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).

Pelagic – water column away from bottom or shore 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

Life on the Continental Shelf Water may have less clarity due to the many nutrients in the water and due to suspended sediments. Nutrients come from many sources, including: Autotrophs Decaying marine organic matter or _____ Nutrients delivered from estuaries and terrestrial ecosystems via rivers

Life on the Continental Shelf Thousands of animals consume nutrients on the seabed. Several species of worms may share an area, burrowing into the sediment and cementing the grains together into tubes of all shapes and sizes. Other burrowing animals include clams, lobsters, mud shrimp, and fish. The abundant life in the sediment attracts predators like cephalopods, fish, and marine mammals.

Types of Communities on the Continental Shelf Soft-bottomed benthic Seagrass beds Kelp forests Hard-bottomed benthic

Life on the Continental Shelf 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- 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 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

Life on the Continental Shelf Soft-bottomed benthic- Many species in this area are deposit feeders (directly getting nutrients from the soft bottomed material) in less turbulent waters Many other species are filter feeders and water turbulence brings in detritus and some animals are bioturbators

Life on the Continental Shelf 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 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 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 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 Seagrass beds Zostera or eelgrass prefers cooler, temperate waters Like turtle grass, it can be directly consumed by herbivores Both species are often important to the food chain after decaying to form detritus

Life on the Continental Shelf 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- 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 Laminaria is a species of kelp common in the North Atlantic, North Pacific and the Asiatic coast of the Pacific This is one of the smaller species of kelp at maximum heights of only around 10 feet Like other species, its rapid growth provides an enormous amount of primary production in the areas where it is found It can be found growing with larger kelp species in the North Pacific primarily

Life on the Continental Shelf 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 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- 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 Thousands of organisms are dependent on the kelp forests for shelter (habitat) or for trophic needs