Chemical and Physical Features of Seawater and the World Ocean

Introduction Marine organisms have evolved and adapted to the environment in which they live. In order to understand marine organisms we must first take a look at the environment in which they live.

The Waters of the World Part 1 of 3

Introduction Water is common on this planet but in the solar system, it is rare. The Earth is the only known planet with liquid water on it. Water not only fills the oceans, it also makes life possible. Most marine organisms are composed of between 80% and 95% water.

The Unique Nature of Pure Water All matter is made from atoms. There are over 115 elements. Each element is made up of a different type of atom. Atoms come together to form molecules. Water is made up of a large oxygen atom and two small hydrogen atoms. Water has hydrogen bonds that cause the molecules to stick to each other. This is what makes water unlike any other substance.

The Unique Nature of Pure Water The Three States of Water Any substance can exist in one of three different states solid, liquid, or gas. Water is the only substance to be found naturally on earth in all three states. As temperature increases, so does the speed of the molecules. Until EVAPORTION. As seawater cools, it becomes denser and sinks (that’s why deeper water feels colder). When the molecules slow down enough, they form crystals which increase the volume. This causes the ice to float.

The Unique Nature of Pure Water Heat and Water Water melts at a high temperature because of the hydrogen bonds. It is also able to absorb a lot of heat. The amount of heat needed to melt a substance is called the latent heat of melting. It takes a large amount of heat to raise the temperature of water. This is measured by the heat capacity (amount of heat needed to raise the temperature by a certain amount). Water also absorbs a lot of heat when it evaporates (it has a high latent heat of evaporation). Fast moving molecules leave the surface the molecules left behind have a slower average velocity so the surface is cooler (this is known as evaporative cooling)

The Unique Nature of Pure Water Water as a Solvent Water can dissolve more things than any other natural substance (it is often called the universal solvent). Water is especially good at dissolving the salts because they are composed of ions. See Figure 3.5

Seawater The characteristics of seawater come from the nature of pure water and to the material dissolved in it. Some of the materials dissolved are from weathering of rocks on land. Other materials come from the earth’s interior through volcanoes, or hydrothermal vents.

Seawater Salt composition Contains a little of almost everything, but there are six main solutes (dissolved materials) that make up 99% of the dissolved solids. Salinity is the total salt dissolved in seawater (it is measured per 1,000 grams of seawater) and use to be found by evaporating 1,000 grams of seawater and massing the resulting salts. Now instruments are used to calculate salinity based on how well it conducts electricity.

Seawater Salt Composition (Cont.) Scientists use to use parts per thousand, now they use psu practical salinity units (the two units are numerically the same). The rule of constant proportions – the salinity may change but the percentage of each ion remains the same (this is not exactly true) The salinity in the open ocean is around 35 ppt or psu.

Seawater Salinity, Temperature, and Density The saltier water is, the denser it is. Also, the cooler water is (before it freezes), the denser it is. Temperature can be measured at various depths to form a temperature profile of water column like the one in figure 3.8 Nowadays, CTDs (conductivity, temperature, depth meters) are used

Seawater Dissolved Gases Gasses are exchanged between the water and the atmosphere. The three most important dissolved gasses are nitrogen, oxygen and carbon dioxide. Gasses dissolve better in cold than warm water. So there is a much higher concentration of oxygen in polar waters than in tropical waters. These gasses (oxygen and carbon dioxide) are used and released by photosynthesis and respiration. Carbon dioxide makes up over 80% of the dissolved gasses in the ocean.

Seawater Transparency It is important that sunlight be able to travel through water because it provides the light energy for photosynthesis. Water is most transparent to blue light. Other colors are absorbed more than blue, as depth increases more of these colors are filtered out. At depths greater than 1,000M even blue light gets absorbed so clear water will appear as total darkness.

Seawater Pressure Organisms on land are under 1 atmosphere (ATM) of pressure (from the weight of all the air above them). Marine organisms also have the pressure from water on them. Every 10M one more ATM is added. At 90M, you would be at 10ATMs. See figure 3.13

Water Density and the Three-Layered Ocean Part 2 of 3

Introduction Oceans are 3D. They have two horizontal dimensions (E/W, and N/S) but they also have depth. Depth is very important because living conditions change drastically with depth.

Salinity and Overturn The ocean is layered (stratified) this is due to cooler water sinking. The warmer and “lighter” water floats on the cooler “denser” water. It is pretty stable unless wind or storm waves mix the water. The greater the difference is in density between surface and deep water, the more stable the water column is and the harder it is to mix the column vertically.

Salinity and Overturn Sometimes the surface water becomes more dense than the water below, in this situation it is unstable and downwelling occurs. It sinks and replaces deeper water. This is called overturn. When the water mass sinks, the salinity does not change. This can be used by scientists to fingerprint the column of water. This can be used to track the circulation of water over great distances. This type of circulation is known as thermohaline circulation (because it is driven by density differences or differences in temperature/salinity)

The Three Layered Ocean There are three principle layers of the ocean. The surface layer Intermediate layer Deep layer

Motion in the Ocean Part 3 of 3

Introduction The ocean is in a state of constant motion. This constant movement has a large effect on sea life.

Surface Circulation The surface is where the most intense motion occurs (in the form of ocean currents and waves). They are both driven primarily by the wind. They are also driven by the Coriolis effect

Surface Circulation The Coriolis Effect The earth is round and rotating, so things on its surface tend to turn instead of moving in a straight line. This bending is called the Coriolis effect. The wider it is, the faster it spins, therefore things traveling on its surface seem to bend. This causes things in the northern hemisphere to bend to the right, while in the southern hemisphere things are deflected to the left.

Surface Circulation Wind Patterns Winds in the atmosphere are driven by solar heat. The warm air at the equator rises, air from adjacent areas is sucked in to replace this rising air (this creates wind). These winds are bent by the Coriolis effect. They are called the trade winds. At middle latitudes (blowing the opposite direction) are the westerlies. The polar easterlies are the most variable. See Figure 3.20

Surface Circulation Surface Currents All major surface currents of the ocean are driven by the wind. The surface of the water moves at 45 degrees to the wind (due to the Coriolis effect). The top layer pushes the second layer (again the Coriolis effect moves the water a little to the right). Each layer moves a little more to the right (Ekman spiral). Wind only effects water to a certain depth, this depth is called the Ekman layer. If you view the Ekman layer as a whole, it moves about 90 degrees from the direction of the wind this is called Ekman transport.

Surface Circulation Surface Currents A comparison of wind and surface currents shows that the ocean current moves at 90 degrees to the wind. This is why equatorial currents move parallel to the equator. The Coriolis effect forms large circles of movement called gyres. See figure 3.22. These gyres regulate climate by transporting heat from tropical area to polar regions. (The gulf stream)

Waves Wind drives surface currents and causes waves. The highest part of a wave is called the crest. The lowest part the trough. The size is the wave height. Wavelength is the distance between crests. The time it takes for a wave to pass a point is called the period.

Waves As a wave passes, water particles do not move much (only in small circles). The size of waves depends on the strength of the wind and the fetch (or distance of open water that the wind blows). While the wind is blowing, it pushes the wave crests up into sharp peaks and stretches out the troughs (these waves are called seas). Once away from the wind, the seas settle into swells.

Waves As waves approach shore, the water gets shallower and shallower. They begin to move in long ellipses instead of circles. This slows the wave. The next wave gets closer (decreasing wavelength). The wave gets steeper and steeper. Eventually the wave falls forward and breaks creating surf. See figures 3.25 – 3.27

Tides Tides are the rhythmic rising and falling of the sea surface. The tides have a very large effect over life in the ocean (especially life near the shore).

Tides Why Are There Tides? The tides are caused by the gravitational pull of the moon and sun and by the rotations of the earth, moon, and sun. The moon and earth rotate around the sun (at their combined center of mass). This produces a centrifugal force that opposes gravity. On the side of the earth nearest the moon, the moons gravity pulls the water more than on the side farther away from the moon. Centrifugal force has the opposite effect pulling water away from the moon this essentially (on a water filled earth) makes two bulges, one near the moon and one far away.) See Figure 3.30

Tides Why Are There Tides? It takes 24 hours to complete a rotation, a point on the earth will therefore experience two high tides and two low tides per 24 hours. However, the moon itself advances a little bit in its orbit (it takes the earth an extra 50 minutes to catch up). Therefore a full tidal cycle takes 24h 50 min.

Tides Why Are There Tides? When the sun and moon are aligned, there pulls are combined and we experience great tidal range (difference between high and low tides). These tides are called spring tides (b/c they surge up like a spring of water) When the moon and sun are at right angles, their effects cancel each other and we experience neap tides (tidal range is small). Occurs when moon is in 1st and 3rd quarters.

Tides Tides in the Real World Tides vary from place to place depending on the shape and depth of the ocean. Most places have semidiurnal tides (two high and two low tides) Some places have mixed semidiurnal tides (with successive high tides of different heights). Diurnal tides (one high, one low) are rare. Tide tables can be used to accurately predict the times of high and low tides.