Sea

1.

2.

3.

The sea, connected as the world ocean or simply the ocean, is the body of salty water
that covers approximately 71 percent of the Earth's surface. The word sea is also used
to denote second-order sections of the sea, such as the Mediterranean Sea, as well as
certain large, entirely landlocked, saltwater lakes, such as the Caspian Sea.
The sea moderates Earth's climate and has important roles in the water cycle, carbon
cycle, and nitrogen cycle. Humans harnessing and studying the sea have been
recorded since ancient times, and evidenced well into prehistory, while its modern
scientific study is called oceanography. The most abundant solid dissolved in
seawater is sodium chloride. The water also contains salts of magnesium, calcium,
potassium, and mercury, amongst many other elements, some in minute
concentrations. Salinity varies widely, being lower near the surface and the mouths of
large rivers and higher in the depths of the ocean; however, the relative proportions
of dissolved salts vary little across the oceans. Winds blowing over the surface of the
sea produce waves, which break when they enter the shallow water. Winds also create
surface currents through friction, setting up slow but stable circulations of water
throughout the oceans. The directions of the circulation are governed by factors,
including the shapes of the continents and Earth's rotation (the Coriolis effect). Deepsea currents, known as the global conveyor belt, carry cold water from near the poles
to every ocean. Tides, the generally twice-daily rise and fall of sea levels, are caused
by Earth's rotation and the gravitational effects of the orbiting Moon and, to a lesser
extent, of the Sun.

4.

Tides may have a very high range in bays or estuaries. Submarine earthquakes arising
from tectonic plate movements under the oceans can lead to destructive tsunamis, as
can volcanoes, huge landslides, or the impact of large meteorites.
A wide variety of organisms, including bacteria, protists, algae, plants, fungi, and
animals, live in the sea, which offers a wide range of marine habitats and ecosystems,
ranging vertically from the sunlit surface and shoreline to the great depths and
pressures of the cold, dark abyssal zone, and in latitude from the cold waters under
polar ice caps to the colourful diversity of coral reefs in tropical regions. Many of the
major groups of organisms evolved in the sea and life may have started there.
The sea provides substantial supplies of food for humans, mainly fish, but also
shellfish, mammals and seaweed, whether caught by fishermen or farmed
underwater. Other human uses of the sea include trade, travel, mineral extraction,
power generation, warfare, and leisure activities such as swimming, sailing, and
scuba diving.

5.

The sea is the interconnected system of all the Earth's oceanic waters, including the
Atlantic, Pacific, Indian, Southern and Arctic Oceans. However, the word "sea" can
also be used for many specific, much smaller bodies of seawater, such as the North
Sea or the Red Sea. There is no sharp distinction between seas and oceans, though
generally seas are smaller, and are often partly (as marginal seas or particularly as
mediterranean seas) or wholly (as inland seas) bordered by land. However, the
Sargasso Sea has no coastline and lies within a circular current, the North Atlantic
Gyre. Seas are generally larger than lakes and contain salt water, but the Sea of
Galilee is a freshwater lake. The United Nations Convention on the Law of the Sea
states that all of the ocean is "sea".
Wind blowing over the surface of a body of water forms waves that are perpendicular
to the direction of the wind. The friction between air and water caused by a gentle
breeze on a pond causes ripples to form. A strong blow over the ocean causes larger
waves as the moving air pushes against the raised ridges of water. The waves reach
their maximum height when the rate at which they are travelling nearly matches the
speed of the wind. In open water, when the wind blows continuously as happens in
the Southern Hemisphere in the Roaring Forties, long, organised masses of water
called swell roll across the ocean. If the wind dies down, the wave formation is
reduced, but already-formed waves continue to travel in their original direction until
they meet land. The size of the waves depends on the fetch, the distance that the wind
has blown over the water and the strength and duration of that wind. When waves
meet others coming from different directions, interference between the two can
produce broken, irregular seas.

6.

Constructive interference can cause individual rogue waves much higher than
normal. Most waves are less than 3 m high and it is not unusual for strong storms to
double or triple that height; offshore construction such as wind farms and oil
platforms use metocean statistics from measurements in computing the wave forces
(due to for instance the hundred-year wave) they are designed against. Rogue waves,
however, have been documented at heights above 25 meters.
The top of a wave is known as the crest, the lowest point between waves is the trough
and the distance between the crests is the wavelength. The wave is pushed across the
surface of the sea by the wind, but this represents a transfer of energy and not a
horizontal movement of water. As waves approach land and move into shallow water,
they change their behavior. If approaching at an angle, waves may bend (refraction)
or wrap rocks and headlands (diffraction). When the wave reaches a point where its
deepest oscillations of the water contact the seabed, they begin to slow down. This
pulls the crests closer together and increases the waves' height, which is called wave
shoaling. When the ratio of the wave's height to the water depth increases above a
certain limit, it "breaks", toppling over in a mass of foaming water. This rushes in a
sheet up the beach before retreating into the sea under the influence of gravity.

7.

There is a broader spectrum of higher animal taxa in the sea than on land, many
marine species have yet to be discovered and the number known to science is
expanding annually. Some vertebrates such as seabirds, seals and sea turtles return to
the land to breed but fish, cetaceans and sea snakes have a completely aquatic
lifestyle and many invertebrate phyla are entirely marine. In fact, the oceans teem
with life and provide many varying microhabitats. One of these is the surface film
which, even though tossed about by the movement of waves, provides a rich
environment and is home to bacteria, fungi, microalgae, protozoa, fish eggs and
various larvae.
The pelagic zone contains macro- and microfauna and myriad zooplankton which
drift with the currents. Most of the smallest organisms are the larvae of fish and
marine invertebrates which liberate eggs in vast numbers because the chance of any
one embryo surviving to maturity is so minute. The zooplankton feed on
phytoplankton and on each other and form a basic part of the complex food chain that
extends through variously sized fish and other nektonic organisms to large squid,
sharks, porpoises, dolphins and whales. Some marine creatures make large
migrations, either to other regions of the ocean on a seasonal basis or vertical
migrations daily, often ascending to feed at night and descending to safety by day.
Ships can introduce or spread invasive species through the discharge of ballast water
or the transport of organisms that have accumulated as part of the fouling community
on the hulls of vessels.

8.

The demersal zone supports many animals that feed on benthic organisms or seek
protection from predators and the seabed provides a range of habitats on or under the
surface of the substrate which are used by creatures adapted to these conditions. The
tidal zone with its periodic exposure to the dehydrating air is home to barnacles,
molluscs and crustaceans. The neritic zone has many organisms that need light to
flourish. Here, among algal encrusted rocks live sponges, echinoderms, polychaete
worms, sea anemones and other invertebrates. Corals often contain photosynthetic
symbionts and live in shallow waters where light penetrates. The extensive
calcareous skeletons they extrude build up into coral reefs which are an important
feature of the seabed. These provide a biodiverse habitat for reef dwelling organisms.
There is less sea life on the floor of deeper seas but marine life also flourishes around
seamounts that rise from the depths, where fish and other animals congregate to
spawn and feed. Close to the seabed live demersal fish that feed largely on pelagic
organisms or benthic invertebrates. Exploration of the deep sea by submersibles
revealed a new world of creatures living on the seabed that scientists had not
previously known to exist. Some like the detrivores rely on organic material falling to
the ocean floor. Others cluster round deep sea hydrothermal vents where mineral-rich
flows of water emerge from the seabed, supporting communities whose primary
producers are sulphide-oxidising chemoautotrophic bacteria, and whose consumers
include specialised bivalves, sea anemones, barnacles, crabs, worms and fish, often
found nowhere else. A dead whale sinking to the bottom of the ocean provides food
for an assembly of organisms which similarly rely largely on the actions of sulphurreducing bacteria. Such places support unique biomes where many new microbes and
other lifeforms have been discovered.