Chemical and Physical Features of Seawater and the World Ocean

The “Weather” of the Marine Environment

• Wind• Waves• Tides• Currents• Temperature• Salt

• Where organisms are found in the marine environment is determined by the chemical and physical factors

• To understand the biology of marine organisms, we must know something about the environment in which they live

The Waters of the Ocean

• Marine organisms are mostly made of water• 80% or more by weight in most cases• Jellyfish – 95%• Water makes life possible

The Unique Nature of Pure Water

• All matter is made of atoms• Elements – made of a single kind of element• Molecules – two or more atoms joined together – ex.

Water• Water molecules stick together because of their polarity• These weak bonds are known as hydrogen bonds

• Hydrogen bonds make water different from any other substance on earth

Three States of Matter

• Solid, liquid, gas• Water is the only substance that naturally occurs

in all three states on earth

Special Characteristics of Water

• In liquid water hydrogen bonds hold most of the molecules together in small groups

• Temperature is a reflection of the average speed of the molecules – faster they move the higher the temperature

• When the molecules move fast enough they escape the hydrogen bonds and enter the gaseous phase (evaporation)

• In water vapor the molecules are not held together by hydrogen bonds

• As water cools the molecules pack closer together and take up less space

• Therefore the density of water increases as water cools until it reaches 4oC

• Below 4oC water becomes less dense• Cool seawater will sink since it is denser

• Once water cools hydrogen bonds reform• Ice crystals (solid made of regular patterns of

molecules)• Water molecules are spaced farther apart than in

liquid water making ice less dense than water• Ice will float on top of water – special property

that makes life in the water possible – insulates water below

Heat and Water

• It takes a large amount of heat to melt ice• As heat energy is added and the temperature of

ice rises, the molecules vibrate faster, breaking some of the hydrogen bonds that hold the crystal together

Latent Heat of Melting

• Amount of heat required to melt a substance

Melting Ice

• Once ice begins to melt added heat breaks more hydrogen bonds rather than increasing the speed of molecular motion

• Any heat put in goes into melting the ice, not into raising the temperature

Heat Capacity

• Amount of heat needed to raise a substance’s temperature by a given amount

• How much heat a substance can absorb• Water has one of the highest heat capacities of

any substance

Importance of Water’s High Heat Capacity

• Most marine organisms are not subjected to the rapid and sometimes drastic temperature changes that occur on land

Latent Heat of Evaporation

• The amount of heat energy that is needed to evaporate a substance

• Change from a liquid to a gas• Water absorbs a great deal of heat when it

evaporates

Water as a Solvent

• Universal solvent• Especially good at dissolving salts• Salts are made of combinations of particles that

have opposite electrical charges• The polarity of water allows it to break down the

salts

• Ion – electrically charged particles• Ions have stronger charges than the ends of water

molecules• When a salt enters water the ions break apart and

become surround by water molecules which break there hydrogen bonds to surround the ion

• Ions pull apart or dissociate and the salt dissolves

Seawater

• Characteristics of seawater are due both to the nature of the pure water and to the material dissolved in it

Solids Dissolved in Seawater

• Come from the chemical weathering of rocks on land and are carried to the sea by rivers

• Earth’s interior – Hydrothermal vents– Volcanoes

Salt Composition

• Solutes – dissolved materials• 6 ions compose over 99% of the solids dissolved in

seawater• Na and Cl account for 85% of the dissolved solids

in seawater

Salinity

• Total amount of salt dissolved in seawater

• Usually expressed as the number of grams of salt left behind when 1,000 grams of seawater are evaporated

1 = dissolved trace elements

• Ions are good conductors of electricity• Electrical conductivity of seawater therefore

reflects the concentration of dissolved ions• Practical Salinity Units – psu – measurement of

salinity determined from conductivity measurements

Importance of Salinity

• Salinity of water greatly affects the organisms that in it

Rule of Constant Proportions

• Percentage of various ions in seawater remains constant even though the total amount of salt in the water can vary slightly

• Oceans are chemically well mixed and ocean salinity varies almost entirely as a result of the addition or removal of pure water rather than the addition or removal of salt

Addition and Removal of Water

• Water is removed from the ocean primarily by evaporation and to a lesser extent by freezing

• Water is added to the ocean by precipitation

Average Salinity of the Ocean

• 35 ppt (parts per thousand)• Red Sea 40 ppt• Baltic Sea 7 ppt (from river runoff)

Salinity, Temperature and Density

• The saltier the water the denser it is

• The density of seawater therefore depends on its temperature and its salinity

Measuring Temperature and Salinity

• Can be measured by lowering specially designed bottles and thermometers on a wire to the desired depth

• A weight called a messenger is released to slide down the wire, triggering the bottles to snap shut and trap a water sample

Temperature Profile

• A graph that shows the temperature at different depths in the ocean

• Water column – vertical shaft of water

Modern Technology

• Oceanographers usually use electronic sensors to quickly and accurately record salinity, temperature and depth throughout the water column, rather than at certain depths

• CTDs – Conductivity Temperature Depth meters• XBTs – Expendable bathythermographs – measure

temperature

Problem

• Measurements can only be made at one place at one time – difficult to get information over a large area

• Ship had to move to a new place to make more measurements

• Conditions change because of currents or weather• Many ships would help but it is expensive

Part Solution

• Make measurements with automated instruments that are left in the ocean

• Satellites can measure surface conditions

Dissolved Gases

• Gases are dissolved in seawater as well as solid materials

• The 3 most important gases are: oxygen, carbon dioxide and nitrogen

• Found in the atmosphere and dissolve at the sea surface

Gas Exchange

• movement of gases between the atmosphere and the ocean surface

• Gases dissolve better in cold than warm water• Dissolved gas concentrations are higher in polar

waters than in the tropics

Oxygen

• Not very soluble• 0 to 8 milliliters per liter of seawater• On average 4 to 6 ml/L• Air has 210 ml/L

Carbon Dioxide

• More soluble than oxygen because it reacts chemically when it dissolves

• 80% of the dissolved gas in the ocean• .04% in air• Stores more than 50 times as much total CO2 as

the atmosphere

Transparency

• Biologically important property• Sunlight can penetrate into the ocean• Allows for photosynthesis• Not all colors penetrate seawater equally well• Water is most transparent to blue light

• As depth increases more colors are filtered out• Red is the first to be filtered out• Something that is red at the surface looks black or gray at

depth because there is no red light to reflect off them and be seen

• At depths of 1000 m or 3300 ft there is total darkness

Turbidity

• Transparency of water is strongly affected by material that is suspended and dissolved in the water

• Ex. Muddy water, lots of plankton

Pressure

• Factor that changes dramatically with depth• On land – 1 atm of pressure• With each 10m (33 ft) of increased depth another

atmosphere of pressure is added• As the pressure increases the gases are compressed –

limits range of orgs – ex. Swim bladder

Water Density and the Three Layered Ocean

• Much of the three dimensional structure of the sea, especially in relation to depth is controlled by the density of the water

Stability and Overturn

• Densest water sinks so the ocean is usually layered or stratified

• Deep water – cold and dense• Surface water – warm and light

Water Column Stability

• Stable Water Column - Less dense on top, dense on bottom

• Low stability – surface water is only slightly less dense

• Highly stable – large density difference• Unstable – surface water more dense than

bottom water

• Downwelling – when surface water sinks • Overturn – when dense surface water displaces

deeper water• Temperature and density profiles are vertical

straight lines for water columns experiencing overturn

• Overturn usually occurs in temperate and polar regions during the winter when the surface water cools

• The water descends to a depth determined by its density

• The processes that change salinity in the open ocean (precipitation, evaporation and freezing) occur only at the surface

• Temperature changes occur only at the surface

Water Mass

• Once surface water has sunk its properties do not change

• The volume of water has a “fingerprint”, a characteristic combination of temperature and salinity

• Oceanographers can tract the movement or circulation of water masses

Thermohaline Circulation

• Circulation driven by changes in density which in turn is determined by temperature and salinity

• Extend throughout the ocean depths• Important in regulating earth’s climate and chemically

mixing the oceans• Brings dissolved oxygen to the deep sea• Helps determine the abundance of life in the deep sea

The Three-Layered Ocean

Surface Layer

• 100 to 200 m thick (330 to 660 ft)• Mixed by wind, waves and currents• Also known as the mixed layer

Thermocline

• Sudden changes in temperature over small depth intervals

• seasonal

Intermediate Layer

• Below the surface layer of around 1500 m (5000 ft)

• Contains the main thermocline

Main Thermocline

• zone of transition between warm surface water and cold water below

• lies in the intermediate layer• rarely breaks down• feature of the open ocean

Deep and bottom layers

• Below 1,500 m or (5,000 ft)• Uniformly cold

• Typically less than 4oC (39 oF)

Motion in the Ocean

Surface Circulation

• Most intense motion of the ocean occurs at the surface in the form of surface currents and waves

• Driven by wind which is driven by heat from the sun

• Coriolis effect also strongly influences

Coriolis Effect

• Earth is round and rotating so anything that moves over its surface tends to turn a little rather than moving in a single straight line

• Mostly effects winds and ocean currents that move over large distances

• Northern Hemisphere – deflects things to the right

• Southern Hemisphere – deflects things to the left

Winds Patterns

• Winds in our atmosphere are driven by heat energy from the sun

• Most of the solar energy is absorbed near the equator

• Warm air rises at the equator• Air from adjacent areas gets sucked in to replace

the rising equatorial air creating wind

• The wind does not move straight to the equator but are bent by the Coriolis effect – approach at a 45 angle

Trade Winds

• winds near the equator (northeast and the southeast)

• steadiest winds on earth • between 0 and 30 degrees

Westerlies

• driven by solar energy• more variable • between 30 and 60 degrees • move in the opposite direction to the trade winds

Polar Easterlies

• Most variable• Between 60 and 90 degrees

Surface Currents

• The major wind fields of the atmosphere push the sea surface creating currents

• All major surface currents of the open ocean are driven by the wind

• When pushed by the wind the uppermost layer of water begins to move

• The water does not move in the same direction as the wind but at a 45o angle because of the Coriolis effect

• The top layer pushes the water below but at a 45o angle and so on

Ekman Spiral

• Spiral change in the movement in the water column when the water is pushed by the wind

• At a depth of a few hundred meters the wind in not felt at all

• Ekman Layer – upper part of the water column that is affected by the wind

• Ekman transport – taken as a whole the Ekman layer moves at 90o from the wind direction

Consequence of the Coriolis Effect

• Trade winds move towards the equator the equatorial currents that these winds produce move parallel to the equator

Gyres

• Wind driven surface currents combined into huge more less circular systems

• Under the influence of the Coriolis Effect

Transportation of Solar Heat

• Warm currents on the western sides of the gyres carry vast amounts of solar heat from the equator to higher latitudes

• Cold currents flow in opposite direction on the eastern sides

• Ocean currents act as a giant thermostat warming the poles and cooling the tropics

• Large scale fluctuations in current patterns can dramatically effect weather around the world - El Nino

Role of Surface Currents

• Surface water temperatures are higher on the western sides of the oceans where currents carry warm water away from the equator

Waves

Waves

• Wind causes• Most familiar of all ocean phenomena• Affect the organisms that live on the shore

Wave Parts

• Crest – highest part of a wave• Trough – lowest part of a wave• Wave Height – vertical distance between trough

and crest• Wavelength – distance between two successive

crests or troughs• Period – time a waves takes to go by any given

point

Water Movement

• In a wave crest, water moves up and forward• In a wave trough, water moves down and back• On the whole water particles do not go anywhere

at all – just move in circles• Waves carry energy across the surface, not water

Formation of waves

• Begins when the wind starts to blow• The faster and longer the wind blows the larger

the waves get• Fetch – span of open water over which the wind

blows – determines size of waves

Seas

• waves that have sharp peaks and relatively flat wave troughs

Swells

• Waves with smooth rounded crests and troughs• Similar to ideal waves

Surf

• Waves that becomes so high and steep as it approaches the shoreline that it breaks

• Waves become closer together

• Energy is released on the shoreline when the wave breaks

Tsunamis

• Deadly waves• Japanese word for “harbor wave”• Produced by earthquakes, landslides, volcanoes,

and other disturbances of the sea floor• Tidal waves – properly called – seismic sea

waves

• Long fast moving waves• Wavelengths of 240 km (150 mi)• Travel 700 km/hr (435 mi/hr) – as fast as a jet

plane• Open ocean – not very high – 1 m

Warning

• Worldwide network of seismic monitoring stations that provide instant notice of an earthquake or other seismic disturbance

• System has saved lives but is far from perfect• Can’t predict which earthquakes produce killer tsunamis • Also many people in developing countries do not get the

warnings

Tides

Tides

• Dominant influence on near shore sea life• Expose and submerge organisms on the shore• Drive the circulation of bays and estuaries• Triggers spawning

Causes of the Tides

• Caused by the gravitational pull of the moon and sun by rotations of the earth moon and sun

• Earth and the moon rotate around a common point (their combined center of mass)

• This rotation produces a centrifugal force

• The centrifugal force just balances the gravitational attraction between earth and the moon

• The centrifugal force and the moon’s gravity are not in perfect balance everywhere on earth’s surface

• On the side nearest the moon, the moon’s gravity is stronger and pulls the water toward the moon

• On the side away from the moon the centrifugal force dominates and pushes the water away from the moon

• If earth were completely covered with water, the water would form two bulges on opposite sides of the planet

• Water would be deep under the bulges and shallower away from the bulges

• Earth is spinning like a top on its own axis• As it does this any given point would be under the

bulge and then away from the bulge• High tide occurs when a point is under a bulge

and low tide occurs when it is away from a bulge

• The earth rotates on its axis every 24 hours so a point will have two high tides and two low tides

• The moon advances on it orbit each day so a full tidal cycle takes 24 hours and 50 minutes

The Sun’s Bulge

• Sun produces a bulge like the moon but is it smaller

• When the sun and the moon are in line there bulges add up and when they are at right angle to one another they cancel each other out

Tidal Range

• Difference in water level between successive high and low tides

Spring Tide

• When the sun and moon bulge add together• High high tides and low low tides• Named because they seem to surge up like

spring water• Occur when there is a full or new moon

Neap Tide

• Occur when sun and moon are at right angles to one another

• Moon is in the 1st and 3rd quarters• Average tides• Low high tide and a high low tide

Variations in Tides

• Tides vary from place to place depending on the location and on the shape and depth of the basin

Bay of Fundy, Canada

Tide Terms

• Semidiurnal tides – two high and two low tides• Mixed semidiurnal tides- successive high tides of

different height• Diurnal Tides – one high and one low -

uncommon

Tide Tables

• Give the predicted time and height of high and low tides

• Very accurate

The End