The surface layers of the pelagic biome receive light and so support phytoplankton,
small single-cell algae, and diatoms. These support zooplankton, a mixture of small
crustaceans, molluscs, worms, and many other forms which are fed upon by fish.
Small fish are transparent as a way of avoiding predation. Larger species such as tuna
(Thunnus) are fast swimming and move in large shoals. The essential chemicals
for growth (nutrients) in these waters are not high and so the amount of plant and
animal material is also low.
In the deep pelagic zone there is no light and the animals have to survive on the
dead material that sinks from the surface layers. These are called heterotrophic sys-
tems because they depend on food from outside sources rather than on plants, which
trap their own light and make carbohydrates (these are called autotrophic systems).
One still finds crustacea, colonial protozoans (foraminifera, radiolaria), and fish, many
of which cannot see or have extraordinary adaptations to lure other fish within catch-
ing distance. This biome also contains the giant squids.The deep ocean benthos is one of the most extreme of all environments: cold, dark,
and pressured. Nevertheless, a diversity of animals live in the bottom mud. Some are
attached to the mud (sea anemones, sponges, and brachiopods), others are burrowers,
and yet others crawl over the surface.The continental shelf and the surface waters above it are the richest in nutrients,
plankton, and animal life. Dense algal forests can grow because light reaches the sea
bottom and these in turn support communities of inshore fish. The higher density
of marine invertebrates and fish in these environments supports larger mammal pre-
dators such as seals, sealions, and some whales, but mostly in temperate regions. Tropical
continental shelves are less productive and support fewer mammals: the dugong (Dugong
dugon) and manatees (Trichechus) which graze on submarine vegetation.
Cold-water currents high in nutrients well up at the edge of the continental shelf.
Upwellings occur particularly in arctic and antarctic waters, but there are some in
the tropics such as the Humboldt current off Peru. The upwellings are rich in
plankton, and a wealth of fish, seabirds, and whales feed on them.
Coral reefs are a special biome forming a rim around oceanic islands. Although not
usually associated with a continental shelf, they have similar ecological characteristics.The world can be divided into broad ecological divisions, each of which has a char-
acteristic vegetation and wildlife. The forest biomes are diverse, being subdivided into
boreal, temperate, tropical, woodland, and shrubland. Grassland biomes include
tropical savanna, temperate grassland, alpine grassland, and tundra. The deserts
constitute a further biome. Each of these can be divided further into ecosystems and
communities based on groupings of plants and animals. Within these larger group-
ings each animal species selects its habitat.18 Chapter 2
2.8.1Pelagic
2.8.2Benthos
2.8.3Continental
shelf