Page 142 - (DK) Ocean - The Definitive Visual Guide

140 SHALLOW SEAS

Continental Shelves DISCOVERY
FIORDS

CONTINENTAL SHELVES ARE ESSENTIALLY the flooded edges of continents, Fiords are deep, sheltered sea
inlets originally gouged out by
inundated by sea-level rise after the last ice age. The shelf seabed is now glaciers and then flooded by the
approximately 600ft (200m) below the surface, and its width varies, occasionally sea. They often extend many
kilometres inland and are made
extending to hundreds of miles. The shelf seabed and water quality are up of deep basins, separated from
influenced by land processes. Rivers bring fresh water and nutrients, making the open sea by shallow sills.
This basin-and-sill structure
shelf waters very productive ecologically, while river-borne material settles on has a huge influence on marine
the seabed as sediment. The continental shelf has a huge diversity of marine life life. In this sheltered environment,
still, dark salt water lies beneath
and habitats, but it is also the area of the sea that suffers most from pollutants. peaty fresh water. This mimics
the marine conditions off the
continental shelf, and animals
Fertile Fringes normally confined to much deeper
water, such as cold-water corals,
The coastal fringes have the greatest diversity of life in the inhabit water shallow enough for
oceans. Light penetration is highly variable, from turbid basins divers to explore.
to clear tropical waters. In many places, enough light reaches the
shallow sea bed for good growth of photosynthetic organisms.
Seaweeds, seagrasses, and phytoplankton thrive here, fed by solar
energy, nutrients from land, and sediments stirred up by winds
and currents. The coastal fringes are much more productive than
the open oceans. Combined with diverse habitats, this results in
complex marine communities, making rich feeding and nursery
grounds for animals from deeper water. In higher latitudes,
SHALLOW SEAWEED
Seaweeds grow best on shallow, sunlit rocks, seasonal variations in the Sun’s strength stimulate an annual cycle
thrive in strong water movement, and provide of plankton and seaweed growth. In the tropics, where seasons
food and shelter for many small animals. are less pronounced, seagrasses and seaweeds grow year-round.

Productive Plains

Much of the continental shelf is covered with deep sediments.
Sand, gravel, and pebbles are deposited in shallow water, while
fine mud is carried into deeper water offshore. An important
part of shelf sediments is biogenic (made from the remains of
living organisms). It consists of carbonates (chemical compounds
containing carbon) derived from, for example, coral skeletons,
and microscopic plankton.
At first sight, sediment plains appear barren. However, many
different animals live hidden beneath the surface, either
permanently or emerging from burrows and tubes to feed and
reproduce. Shifting sand and gravel is a difficult place to live, but
more stable sediments occur on deeper sea beds. Varying particle
size makes it suitable for constructing burrows and tubes, and it
can contain huge numbers of animals, providing a rich food
SEDIMENT PREDATORS
source. These animal communities are all sustained by plankton
Fish and starfish are top predators on sediments, eating the many
falling from the continental-shelf surface waters, and by the different animals on the surface or buried beneath. Fish catch a wide
products of decomposition of seagrasses and seaweeds. range of creatures, while starfish capture slower-moving prey.

Shelf Fisheries
The waters and sea bed of the continental shelf QUEEN SCALLOP
OCEAN ENVIRONMENTS 90 percent of the world’s total seawater Scallops feed by filtering seawater,
support most of the world’s major fisheries.
In coastal waters, there is planktonic food for
larvae and cover for juveniles, and this is where

catch reproduces. Demersal fish (living
and can be collected by diving, or
on or just above the seabed) such as cod
farmed, with no damage to the
and haddock feed on seabed life. Pelagic
marine environment.
(open water) shoaling fish such as
sardines and herring feed on zooplankton, and are important food
for larger fish such as mackerel and sharks, as well as for cetaceans
and seabirds. Commercially important invertebrates such as shrimp
are caught in shelf waters. Worldwide,
JUVENILE SHELTER
coastal communities are sustained by
These baby cod are feeding
small-scale, inshore fisheries, which
in horse mussel beds, before
catch a wide range of marine life.
moving offshore as adults.