© 2006 thomson-brooks cole chapter 16 continental shelves and neritic zone

© 2006 Thomson-Brooks Cole

Chapter 16

Continental Shelves and Neritic Zone


Key Concepts

• The number and kinds of benthic organisms on continental shelves are influenced by sediment characteristics.

• Hard-bottom communities are dominated by epibenthic organisms.

• In areas north and south of the tropics, kelps (a type of brown algae) dominate the subtidal zone where the water is cold and the sediments are hard.


Key Concepts

• Kelps are important primary producers and provide habitats for many animals.

• Soft-bottom communities are dominated by suspension feeders and deposit feeders.

• The distribution of organisms in benthic communities of the continental shelf is patchy.


Key Concepts

• The neritic zone is the water column that lies above the continental shelves.

• The neritic zone receives high levels of nutrient input from rivers, coastal runoff, and upwellings.

• The neritic zone supports enormous amounts of phytoplankton.

• The high productivity of coastal seas supports large numbers of fishes, birds, and marine mammals.


Continental Shelves

• Average 67 km (40 miles) wide• Descend gradually from shore to

depths of 130 m (430 feet)– at this point, bottom may become steep

slope or shear drop-off• Rivers carry large amounts of sediment

to coastal seas, providing nutrients that settle on the shelves or are dissolved in the seawater

• Plenty of sunlight


Benthic Communities

• Role of sediments– epifauna are adapted to bottoms

composed of coarse sediments (where currents on the bottom are strong)• epifauna—animals that live on surface

sediments

– infauna and interstitial animals are adapted to bottoms of fine sediments (where currents are weak)• infauna—animals that burrow in the sediments• interstitial animals—animals that live in the

spaces between sediment particles


Benthic Communities

• Hard-bottom communities– hard-bottom habitat = large sediments

that cannot be pushed apart (e.g. rocks)– many sessile organisms– epibenthic organisms—organisms that live

on the surface of the bottom sediments– characterized by patchiness—uneven

distribution of benthic organisms• sunlight exposure• landslides and shifting sediments


Benthic Communities

• Kelp communities– kelp beds

• may be underwater forest with canopy and understory; kelp may be distanced or dense


Benthic Communities

• Kelp communities– kelp life cycles

• spores germinate with sufficient light• microscopic form establishes itself only if it is

not over-consumed by herbivores• stipes grow upward and spread out into a

canopy• mature kelps constantly grow and erode


Benthic Communities

• Kelp communities (continued)– kelp community

• kelps provide food, shelter or both• kelps may increase usable habitat• many filter feeders and some herbivores rely

on kelp forests


Benthic Communities

• Kelp communities (continued)– impact of sea urchins on kelp communities

• kelps are a favorite food of sea urchins• sea urchins are usually held in check by wave

action and predators• decline in predators (e.g. otters) can lead to

urchin population explosion and mass destruction of kelp forests


Benthic Communities

• Soft-bottom communities– patchiness in soft-bottom communities

• changes in sediment distribution• bottom currents• larval settlement

– soft-bottom food chains• detritus is primary source of food• suspension feeders eat detritus and plankton• deposit feeders eat detritus and its bacteria• these inactive animals are eaten by more

active predatory species


Benthic Communities

• Soft-bottom communities (continued)– succession in soft-bottom communities

• disturbance of sediments by erosion or landslides removes or kills many animals

• disturbance exposes deeper, anoxic sediments• larvae arrive and recolonize• surface dwellers like polychaete worms feed on

organic material and aerate the sediments• increased oxygen allows other animals to

move in


Neritic Zone

• Food chains in the neritic zone– phytoplankton growth is supported by

nutrients from freshwater runoff from land– zooplankton feed on phytoplankton

• most abundant are copepods (crustaceans)

– benthic filter feeders eat phytoplankton– small fish eat zooplankton– large fish eat filter feeders– fewer trophic levels than in the open sea


Neritic Zone

• Productivity in the neritic zone– areas of upwelling, where nutrients are

brought from the ocean floor to the surface where plankton live, are the most productive

• Other roles of plankton in coastal seas– many animals spend some part of their

lives as members of plankton– having planktonic larvae allows sessile

organisms to disperse to new areas

© 2006 thomson-brooks cole chapter 16 continental shelves and neritic zone

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