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►The Physics of Water

►How Water Physics Affect Marine Life

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The Physics of Water

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The Physics of Water

Seawater’s chemical properties affect how life functions in the ocean.

Water’s physical properties not only affect life processes of marine organisms, but of human beings in the water.

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Heat and Heat Capacity

Temperature is crucial in determining where organisms can live in the ocean.

The concept of temperature comes from the need to measure the relative heat of two bodies, or the same body after removing or adding heat.

Suppose you’ve filled a bathtub with warm water and scooped out a glassful. If you take the temperatures of the water in the glass and the water in the tub, you’ll find they are the same. But, which has more heat?

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Heat is the kinetic energy in the random movement, or vibration, of individual atoms and molecules in a substance.The faster molecules move, the more heat there

is. Total heat energy is measured based on both the quantity and speed of vibrating molecules.

Temperature measures only how fast the molecules vibrate.The two most common temperature systems are

Fahrenheit and Celsius. Celsius is most used in science because it is based on water’s physical properties.

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Heat and Temperature

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Heat capacity of a substance is the amount of heat energy required to raise a given amount of a substance by a given temperature.

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Scientists express heat capacity in terms of the amount of heat energy it takes to change one gram of a substance by 1°C.

It’s expressed as the number of calories required. It takes more heat energy to raise water’s

temperature than that of most substances.

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Therefore water can absorb or release a lot of heat with little temperature change.

Water’s heat capacity affects the world’s climate and weather.Heat is carried to areas that would otherwise

be cooler, and heat is absorbed in areas that would otherwise be hotter.

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A great example is the island of Bermuda. Bermuda has a moderately tropical climate year round, even though it lies above 30° north latitude. That’s about the same latitude as Birmingham, Alabama, or Fort Worth, Texas, both of which experience some snow and freezing rain in the winter. The difference is that the warm Gulf Stream

current flows around Bermuda.By carrying so much heat north, the Gulf Stream

gives Bermuda a tropical climate.

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Water Temperature and Density

As water cools it becomes denser. At 3.98°C (39.16°F) it reaches maximum density.

Below this point, it crystallizes into ice. As water moves into a solid state* it becomes less dense.

Ice

Liquid Water

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* State is an expression of a substance’s form as it changes from solid, to liquid, to gas with the addition of heat.

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Ice does not form all at once at the freezing point of 0°C (32°F), but crystallizes continuously until all liquid turns solid. Temperature does not drop any further until all the liquid water freezes, even though heat continues to leave. This produces non-sensible heat – a change in heat

energy that cannot be sensed with a thermometer. The non-sensible heat lost when water goes from liquid to

solid state is called the latent heat of fusion. Sensible heat is that which you can sense with a

thermometer.

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Relationship of Density to Temperature in Pure Water

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Relationship of Density to Temperaturein Most Substances

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Latent Heat of Vaporization

Latent heat of vaporization is the heat required to vaporize a substance. It takes more latent heat to vaporize water than to

freeze it because when water freezes only some of the hydrogen bonds break.

When it vaporizes, all the hydrogen bonds must break, which requires more energy.

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Changing from a solid to a liquid.

Changing from a liquid to a vapor.

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Latent Heat of Vaporization and Fusion

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Hydrological CycleShows the Movement ofWater Around the Earth

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Thermal Inertia

The tendency of water to resist temperature change is called thermal inertia.

Thermal equilibrium means water cools at about the same rate as it heats.

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Thermal Inertia

These concepts are important to life and Earth’s climate because:Seawater acts as a global thermostat, preventing

broad temperature swings. Temperature changes would be drastic between night

and day and between summer and winter. Without the thermal inertia, many – perhaps most –

of the organisms on Earth could not survive the drastic temperature changes that would occur each night.

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Ocean Water Density

Seawater density varies with salinity and temperature.This causes seawater to stratify, or form

layers.

Relationship Between Temperature, Salinity and, Density

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Ocean Water Density

Dense water is heavy and sinks below less dense layers. The three commonly found density layers are: Surface zone – varies in places from absent to 500 meters

(1,640 feet). In general it extends from the top to about 100 meters (328 feet). This zone accounts for about only 2% of the ocean’s volume.

Thermocline – separates the surface zone from the deep zone. It only needs a temperature or salinity difference to exist. This zone makes up about 18% of the ocean’s volume.

Deep zone – lies below the thermocline. It is a very stable region of cold water beginning deeper than 1,000 meters (3,280 feet) in the middle latitudes, but is shallower in the polar regions. The deep zone makes up about 80% of the ocean’s volume.

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Ocean Water Density

DensityLayers

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Ocean Water Density

The relatively warm, low-density surface waters are separated from cool, high-density deep waters by the thermocline, the zone in which temperature changes rapidly with depth. The top of the thermocline varies with season, weather,

currents, and other conditions. It depends in part on the amount of heat the surface zone

receives from the sun and is therefore more pronounced in tropical and temperate waters.

Thermoclines are weaker in polar regions because the surface water there is cold.

Thermocline zones account for about 18% of ocean water.

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Ocean Water Density

Below the thermocline is the deep layer. This layer is cold, dense, and fairly uniform because it originates in the polar regions. It begins deeper than about 1,000 meters (3,280

feet) in the middle latitudes but becomes shallower until it reaches the surface in the polar regions.

The deep zone makes up about 80% of the ocean’s volume.

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How Water Physics Affect Marine Life

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Light

Water scatters and absorbs light. When light reaches the water’s surface, some light penetrates, but, depending on the sun’s angle, much may simply reflect back out of the water.Within the water, light reflects off light-colored

suspended particles.Dark colored suspended particles and algae

absorb some of the light.Water molecules absorb the energy, converting

light into heat.Water absorbs colors at the red end of the

spectrum more easily than at the blue end.

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Light

Reflection, Scattering, and Absorption

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Light

Natural Light

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Light

Two zones exist with respect to light penetration: Photic Zone – where light reaches (can be as deep as 590

meters/1,968 feet). The photic zone has two subzones. Euphotic Zone – the upper shallow portion where most

biological production occurs – comprises about 1% of the ocean.

Dysphotic Zone – where light reaches, but not enough for photosynthetic life.

Aphotic Zone – it makes up the vast majority of the ocean. Where light does not reach and only a fraction of marine organisms live.

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Light

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Temperature

Seawater doesn’t fluctuate in temperature nearly as much as air does.Marine organisms rarely encounter temperatures

below 1.9°C or above 30°C. Compared to land-based climates, this narrow range provides an advantage.

Compared to land-based climates, marine organisms live in a much less challenging environment with respect to temperature range.

Generally, temperature dictates the rate of chemical reaction.

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Temperature

Most marine organisms have an internal temperature close to that of surrounding seawater.Their internal temperature changes with seawater

temperature. An organism with this characteristic is called an ectotherm.

Ectotherms are commonly called cold-blooded organisms, and include terrestrial as well as marine organisms.

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Temperature

Other marine organisms, such as certain tuna and sharks, have an internal temperature that varies, but remains 9˚ to 16˚C warmer than the surrounding water. Organisms with this characteristic are called endotherms.

Marine mammals and birds have an internal temperature that is relatively stable. Organisms with this characteristic are called homeotherms.

Some endotherms have a body temperature above their surroundings, but it is not constant and varies with the surrounding temperature. Organisms with this characteristic are called poikilotherms. Endotherms are commonly called warm-blooded

organisms.

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Temperature

Temperature affects metabolism – the higher the temperature within an organism the more energy-releasing chemical processes (metabolism) happen.Endotherms and homeotherms can tolerate a wide

range of external temperatures. Internal heat regulation allows endotherms an

advantage.Their metabolic rate remains the same regardless

of external temperature allowing them to live in a variety of habitats.

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Sound

Sound is energy that travels in pressure waves. It can only travel through matter, which is why

there’s no sound in outer space. Sound travels well in air, but even better in

water. In distilled water at 20˚C/68˚F, sound travels 1,482.4

meters (4,863.4 feet) per second, which is about five times faster than in air.

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Sound

Travels through warm water faster than cool…but it travels faster in deep water due to pressure.

Bounces off suspended particles, water layers,the bottom and other obstacles.

Travels much farther through waterthan light does.

Is eventually absorbedby water as heat.

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Sound

Because sound travels so well in water, marine mammals use echolocation to sense an object’s size, distance, density, and position underwater.

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Pressure

Right now, you’re under pressure. If you’re at sea level, you’re under the pressure of the atmosphere, which is literally the weight of the air.

Water weighs far more than air, so marine organisms exist in an environment with greater surrounding pressure than land-based organisms do.

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Pressure

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Pressure

Pressure exerted by water is called hydrostatic pressure.It’s simply the weight of the water. At 10 meters (33 feet) hydrostatic pressure is equal to

atmospheric pressure – 1 bar/ata. At 10 meters (33 feet) the total pressure is 2 bar – 1 bar from

atmospheric pressure plus 1 bar from hydrostatic pressure. A marine organism living at 10 meters (33 feet) experiences

twice the pressure present at sea level. Pressure increases1 bar for each additional 10 meters (33 feet).

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Pressure

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Pressure

Hydrostatic pressure doesn’t affect marine organisms because it is the same inside the organism as outside.Living tissue is made primarily of water, which

(within limits) transmits pressure evenly. Since it’s in balance, pressure doesn’t crush or harm marine organisms.

Hydrostatic pressure is primarily an issue only for organisms that have gas spaces in their bodies.

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Pressure

Many fish have a gas bladder that they use to control their buoyancy. They must add or release gas from the bladders when they

change depth to keep the pressure in balance. Similarly, scuba divers learn to add air to the space in their

ears (a technique called equalizing because it equalizes the pressure inside the air space with the pressure outside), which allows them to dive without discomfort.

Failure to equalize can cause thepressure to rupture the diver’sear drums.

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Pressure and Gas Volume Relationships

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Size and Volume

Marine organisms thrive by getting all the resources they need from the water around them. Each cell gets the nutrients and gas it needs from the

surrounding environment and excretes waste products into that environment.

Single-cell organisms, such as protozoa or bacteria, make these exchanges directly to and from seawater.

A multicellular organism, such as a sea cucumber or a fish, uses systems to gather nutrients and gas from the environment and excrete waste.

The cells within a multicellular organism make the exchanges via the organism’s systems rather than directly with the surrounding water.

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Size and Volume

High surface-to-volume ratio is important for cell function. The bigger the cell, the lower the surface-to-volume ratio, which means that there’s less relative area through which to exchangegases, nutrients, and waste.This is why large organisms are multicellular

rather than a giant single cell.

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Size and Volume

Using a sphere to substitute for a cell:The volume of a sphere increases with the cube of

its radius and the surface area increases with the square of its radius.

If a cell were to increase diameter 24 times original size, the volume would increase 64 times, but the surface area would increase only 16 times.

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Buoyancy

Archimedes’ Principle states that an object immersed in a gas or liquid is buoyed up by a force equal to the weight of the gas or liquid displaced.

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Buoyancy

Floats

Sinks

Floats

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Buoyancy

Means marine organisms don’t have to expend much energy to offset their own weight compared to a land-based existence.

It allows entire communities to exist simply by drifting. It allows organisms to grow larger than those on land. It allows many swimming creatures to live without

ever actually coming into contact with the bottom.

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Buoyancy

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Movement and Drag

While marine organisms have an advantage over land-based organisms with respect to buoyancy, the situation is reversed when it comes to drag. Because water has a far higher viscosity than air,

it resists movement through it far more than air does.

Consider what happens when you’re in a swimming pool; it takes very little effort to push yourself off the bottom thanks to buoyancy.

However, it takes far more effort to swim a long distance than to run the same distance. This is due to drag.

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Movement and Drag

Viscosity affects smallorganisms, plankton in particular.

Their small size gives them littlestrength to swim through water.

Small marine organisms avoid sinking by:Plumes, hairs, ribbons, spines, and other

protrusions that increase their drag and help themresist sinking.

Others have buoyancy adaptations that help them remain suspended in the water column (oil in tissues).

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Movement and Drag

Some marine organisms need to overcome drag as they swim. Adaptations that help them overcome drag:Moving or swimming very slowly.Excreting mucus or oil that actually lubricates

them to “slip” through the water.The most common is to have a shape that reduces

drag – streamlining.

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Movement and Drag

Drag, Streamlining, and TurbulenceHo

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Currents

It is speculated that drifting provides several advantages.Drifting disperses organisms into new habitats,

ensuring survival should something happen to the original community.

May take organisms into nutrient-rich areas, preventing too many offspring from competing for the same resources in the original community.

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Documents

Choose to view chapter section with a click on the section heading. The Physics of Water