Continental Shelf: Area of the ocean found between the bottom of the intertidal zone and the shelf break

CONTINENTAL SHELVES Average width – 150 km (80 miles) Broad along passive margins Narrow along active margins Average depth 150 m (500 ft)

Passive Margin – continental margin on the edges of diverging plates Active Margin – continental margin facing converging or sliding plates

slope continental shelf rise abyssal plain abyssal hills ridge Notice the narrow shelf by Florida Transform faults- fractures along which the lithosphere slides horizontally past one another ridge transform fault

Life on the Continental Shelf Biologically rich major portion of the fisheries global catch is caught over the continental shelf Rich in potential natural resources 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 (230 land miles) off their coast – this includes the continental shelf and its resources

Pelagic – water column away from bottom or shore Epipelagic – sea surface to depth of about 200 meters – divided into: neritic waters over the continental shelf oceanic waters that are past the shelf

Marine areas that are never exposed during low tide SUBTIDAL ECOSYSTEMS Marine areas that are never exposed during low tide always submerged

SUBTIDAL ABIOTIC FACTORS rich in nutrients autotrophs decaying marine organic matter brought in from rivers Lithogenous sediments are formed by the weathering process and are made up of small particles of weathered rocks and oceanic volcanoes. They are often formed together when metal and silicate ions bond. There are two types of lithogenous sediments; terrigenous and “red clay” and they are different because of the process behind their existences. For instance, terrigenous sediments are produced as a result of the weathering process of rocks above the water. These eroded particles are carried by the wind and other natural means to the oceans and are deposited at the bottom. Although it can be easily found in river beds, not much of this finds its way to the deep ocean. Red clay lithogenous sediment, on the other hand, is plentiful in the ocean. It is reddish-brown (hence the name) and is a combination of terrigenous material and volcanic ash. It is transported to the oceans by currents and wind and it settles in deep places along the ocean floor. Biogenous sediments are formed from the insoluble remains of past life forms and parts such as bones and teeth. In many areas where the water is shallow, a majority of these sediments are the remains of shells or fragments from shelled sea creatures as well as corals. In the deep sea where there is no such a high concentration of these life forms, biogenous sediment is made from the microscopic shells that are deposited by tiny plants, animals, and plankton that live on the water’s surface and eventually make their way down to the ocean floor. Nile River Delta Mississippi Delta

SUBTIDAL ABIOTIC FACTORS Influenced by sedimentation processes (especially shallow areas) lithogenic / terrigenous sediments physical and chemical weathering of rocks (turbidities, volcanic ash, red clay) brought in by rivers, reworked by waves biogenic sediments shells and skeletons of marine organisms

Abiotic/Physical Features temperature – varies from place to place; affects distribution of organisms more affected by waves and currents than deep water turbulence – water motion – mixes up nutrients so they don’t accumulate on bottom = highly productive substrate sandy or muddy dominate rocky

Pelagic Benthic SUBTIDAL ORGANISMS Plankton (floaters) Nekton (swimmers) Benthic Demersal (hover) Epifauna (on) Infauna (in)

Distribution of marine life Pelagic Benthic

Sampling the Benthic Habitat Bottom trawls Grabs Dredges

SUBTIDAL COMMUNITIES Soft-bottom Subtidal Seagrass Beds Hard-bottom Subtidal Kelp Forests

SOFT-BOTTOM SUBTIDAL COMMUNITIES Dominant substrate sand & mud

Soft-Bottom Subtidal Communities Dominates the world’s continental shelves Animals are distributed in this community based on: sediment particle size sediment stability salinity light and temperature Organisms usually occur in patches, due to planktonic settlement Metamorphose and “tasting settlement sites”

Soft-Bottom Subtidal Communities Notable absence of large plants and algae Occasional green algae Main producers are diatoms Detritus important food source there is little benthic primary production filter and suspension feeders rely on detritus brought in from estuaries Deposit feeders and bacteria dominate fine sediments due to high organic content

SOFT-BOTTOM SUBTIDAL ORGANISMS Mostly infauna, some epifauna, almost no sessile organisms (nothing to hold onto) Infauna – benthic organisms that bury themselves in the sediment Epifauna – organisms that inhabit the surface of the bottom sediment No. of subtidal species > intertidal (more stable, no desiccation) Distribution of organisms influenced by particle size (mud or sand)

Soft-bottom subtidal communities Infauna:  live within the sediment, mostly soft bottom;  mostly clams and worms (polychaetes)  burrow tubes for food scavenging and oxygen supply primary producers: algae, mostly benthic diatoms and dinoflagellates detritus important food source

Infauna

Epifauna

Soft-bottom subtidal communities

Life on the Continental Shelf Meiofauna – live between sediment grains 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 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

Soft bottom subtidal communities 32,000 polychaetes in sand/m2 vs 50-500 earth worms in soil/m2 Polycheate Ecological Role: clean sediments aerate sediments

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 stingrays as well as sperm whales are well known for their activity as bioturbators

Carnivore feeders

Herbivore feeders

Deposit-feeders

Suspension-feeders

Soft-bottom subtidal communities Generalized food web

Seagrass beds Areas that are carpeted by flowering plants They develop best in sheltered shallow areas along the coast Eel grass is the most common representative

Seagrasses Many sessile animals and epiphytic algae Thick mats of eel grass provide lots of habitat for animals to hide By stabilizing the sediment it decreases the turbidity Dead sea grasses provide lots of detritus that benefit suspension, deposit and filter feeders Many sessile animals and epiphytic algae

Soft-bottom subtidal communities Epibionts Seagrasses support many sessile animals and epiphytic algae

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

Sea Grass Food Web

Hard-Bottom Subtidal Communities Less common than soft-bottomed Often, 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 urchins grazing on seaweed in a hard bottomed subtidal community

PRODUCERS Most important communities - seaweeds Strong competition Amount of light influence distribution of seaweeds Seaweeds found in these areas have higher chlorophyll concentration

GRAZERS invertebrates that move slowly: sea urchins, limpets, chitons, abalone defense against predators - by having food that is not tasty - fast regeneration - calcification (formed of calcium carbonate)

Generalized food web for hard-bottomed subtidal community

Kelp Forests Animals that are present have the same characteristics as those of the rocky intertidal Kelp is a macroalgae which can reach amazing sizes

Kelp and Kelp Forests

Air sack holdfast

Life on the Continental Shelf Kelp community- Kelps are macroalgae that are restricted to cold water distributions 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 - species of kelp common in the North Atlantic, North Pacific and the Asiatic coast of the Pacific one of the smaller species of kelp at maximum heights of only around 10 feet its rapid growth provides an enormous amount of primary production in the areas where it is found 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 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

Kelp Communities Several species of kelp-community fishes sheltering near giant kelp, Macrocystis. © Galina Barskaya/ShutterStock, Inc.

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

Kelp Communities Below the effects of waves and tides, kelp communities dominate in temperate areas General structure of a West Coast kelp forest, with a complex understory of plants beneath the dominant Macrocystis or Nereocystis.

Hard-bottom subtidal communities Examples of N. Atlantic Kelp

Geographic distribution of kelp

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 seastars, and sea otters

Figure 14. 4: Sea urchins, bane of the kelp beds Figure 14.4: Sea urchins, bane of the kelp beds. The urchins can absorb carbohydrates that leak from the algae or gnaw the stipes and holdfasts with their teeth. Too many urchins can destroy a kelp forest by releasing the kelp from its holdfasts.

Sea Urchins What leads to population explosions? Absence of their predators Overfishing, less seals & sea lions, killer whales eat sea otters, more urchins

Pictoral Food Web Example

Figure 16-7 Kelp Forest Community Figure 16-7 Kelp Forest Community. Kelps provide both habitat and food for a large number of marine organisms.