Ocean water is a complex mixture of plants,
animals, dissolved solids and gases. Ocean water
and water in general, have physical properties such
as temperature, density, and salinity that help
describe it. Scientists study these characteristics
and the interactions among the oceans, the
atmosphere, and the land.
Salinity
Every year the Earth’s rivers carry about 2.3 billion
metric tons of dissolved solids into the oceans. The
chemical weathering of rocks on continents is one
source of elements found in seawater. Most of
these dissolved solids are salts.
Gases also enter the water out of the atmosphere
or by undersea volcanoes. The amount of dissolved
materials has stayed balanced for millions of years
because of the organisms that live off these
materials.
About 4 billion years ago the Earth cooled enough for the
moisture in the atmosphere to condense and torrential
rains filled the ocean basins. Ocean water is 96.5% pure
water. The other 3.5% are elements like chlorine, sodium,
sulfur, calcium magnesium, and potassium. Other trace
solids such as gold and zinc exist in small amounts.
The most abundant gases in the atmosphere are
the most abundant gases dissolved in the ocean,
Nitrogen, Oxygen and Carbon Dioxide which
Carbon dioxide is most easily dissolved in water.
Temperature affects the amount of gas
that dissolves in water. Gases dissolve
better in cold water. Dissolved gases can
also be released back into the
atmosphere.
Dissolved solids make up about 3.5% of the mass of ocean
water. Sodium chloride, normal table salt NaCl makes up
78% of the oceans dissolved solids. The amount of
dissolved solids present in ocean water is described by it’s
salinity because most of the solids in ocean water are
salts.
Both evaporation and freezing increase the salinity of ocean water.
This is why tropical waters have a higher salinity than polar waters
especially in summer.
There is also more salinity on the surface than at greater depths.
Ocean water ranges anywhere from 38 or more parts per 1000 to 32
or less parts per 1000. The salinity stays constant because there is so
much fresh water running into the oceans.
Ocean Temperatures
Ocean water also absorbs solar energy depending on which latitude
the water sits. Near the equator ocean temperatures are warmer
than polar oceans because of the amount of direct sunlight received.
The boundary between warm surface water and cold deep water is
called a thermocline. As the depth of ocean water increases the
temperature decreases.
The color of water is blue because that is the only
color of light reflected by the water. The rest of the
colors are absorbed.
Ocean Density
Density is how heavy something is for its size. The density
of sea water determines whether the water floats or sinks.
Seawater is more dense than fresh water. Seawater density
is influenced by two factors salinity and temperature. As
ocean temperature decreases the density increases. The
ocean is divided into 3 layers based on its density, a
shallow surface mixed zone, a transition zone and a deep
zone.
Life in the Oceans
Living organisms in the ocean depend on two major
factors: essential nutrients in the water and sunlight.
Nearly all the life in the ocean is regulated by the life
processes of plants. Animals an plants help maintain a
balance of nutrients and gases that are required for them
to live.
When surface water is blown by the wind, deep nutrientrich water moves up to take its place this is called
upwelling. Plants grow only in the top 100m of the ocean
because the sun can only penetrate to about 200m.
Marine organisms are classified according to where they
live and how they move. Microscopic plants and animals
called plankton serve as the base of food in the oceans.
Plankton includes all organisms like algae, animals and
bacteria that drift with the ocean currents.
Zooplankton
Phytoplankton
Larger animals called nekton are those that move
independently of the currents usually by swimming.
Benthos refers to those animals that live on the
ocean bottom.
The ocean is divided into life zones by three factors, the availability
of sunlight, the distance from shore, and the water depth. The
benthic environment is the bottom or ground zone. This zone is
broken up into 5 zones based on the amount of light and organisms
that live there. The pelagic zone is the open ocean and has a photic
zone near the surface and an aphotic zone at about 200 meters
down.
Some life lives at depth in the ocean and don’t need
light to survive. They use the heat and chemicals
that come out of hydrothermal vents. They are
called chemosynthetic organisms.