SALINITY, TEMPERATURE & DENSITY
The ocean is not a uniform body of water. The ocean is made up of many water masses that flow
past each other. These distinct masses of water each have a characteristic density. Density is the
relative heaviness of a substance; it is mass per unit volume. Dense water masses will sink while
less dense ones will float above them. It is similar to density differences that lead to oil floating
on top of water in a bottle of salad dressing. Water masses of different densities will similarly
layer out. This layering is known as stratification in the ocean.
Density of seawater is primarily determined by two factors: temperature and salinity. Warmer
water is less dense than colder water. Therefore, warm water floats near the surface while cold
water will sink toward the bottom. Salinity also affects density. Higher salinity (more salts in
the water) leads to higher density. So salty water sinks while fresh water floats at the surface.
Anywhere in the ocean where water masses of different salinity and/or different temperature
meet, the ocean will be stratified. There will be distinct layers of water found at different depths.
In this laboratory you will examine the effects of varying salinity and varying temperature on the
density of water and therefore on stratification in the ocean.
Temperature in the Ocean
The major source of heat for the ocean is the sun. Therefore, it is only surface waters that get
heated. Deep-ocean water is cold with temperatures hovering around 4oC. The sun does not heat
the surface of the ocean evenly. Polar regions receive very little, diffuse sunlight and even
surface waters are cold there. Therefore the entire column of water from the surface to the
bottom is cold; there is no thermal stratification. Tropical regions receive the most solar energy
and tropical surface waters are warm. The warmer surface waters, with their low density, float
on top of the colder deep water and the ocean is thermally stratified in the tropics. Temperate
surface waters are cold in the winter but warm up in the spring and summer. Therefore, in these
regions, there is no thermal stratification in the winter. It builds up as the seasons change and
there is strong stratification in the summer months.
Plotting the change in temperature with depth, in the tropics for example, clearly shows that
temperature does not just decrease uniformly with depth. Instead, there are three distinct layers
or zones. The warm upper zone, known as the mixed or surface zone, is kept uniformly warm as
waves and currents distribute the solar energy from the sun. The middle zone is a region where
temperature decreases with depth; this is known as a thermocline. Within the thermocline,
warm surface water mixes with cold deeper water. Below the thermocline is the deep layer,
which is uniformly cold.
Exercise 1: Effect of temperature on density and stratification
Procedure:
1. Install a partition to divide a 10 gallon tank into two halves.
2. Tape four thermometers to the inside of the tank at four different depths. Place the
thermometers near each other but tape each one a a different depth: one near the bottom of the
tank, one near where the surface of the water will be (about ¾ up the tank) and the other two
spaced in between.
3. Fill a bucket with warm water from the tap. Fill another bucket with cold water from the tap.
Add several drops of food coloring to each bucket to color them different colors. Take the
temperature of each solution in oC. Record the color and temperature of each solution:
Hot water:
color ____________ Cold water:
color______________
temperature_______
temperature_________
4. With a partner holding the partition in place, simultaneously pour one bucket of water into
each side of the tank. Both sides of the tank should be filled to approximately the same height.
5. Briefly allow the water to settle and then quickly but carefully pull up and remove the
partition. Observe what occurs as the partition is removed by watching through the side of the
tank.
6. Once the water has settled, record the depth (distance from the surface of the water) of the
bottom of each thermometer in cm and enter it in the following table. You may need to adjust
the thermometers (especially the surface one) once the water is in the tank. Take temperature
readings from the four thermometers at the appropriate times after the partition was removed and
record them in the following table:
Thermometer
#1 Surface
_____ cm
#2
_____ cm
#3
_____ cm
#4 Deep
_____ cm
1 minute
5 minutes
10 minutes
15 minutes
7. At the same times that you take temperature readings, observe the water through the side of
the tank and note the colors. Draw what you see in the following boxes:
1 minute
5 minutes
10 minutes
15 minutes
8. After the 15 minutes are over, blow hard at a low angle across the surface of the tank for
about 30 seconds. Observe the water through the side of the tank and take temperature readings.
Record your data in the following table.
Observations through
Thermometer
Temperature
side of the tank
#1 Surface
#2
#3
#4
9. When all of your data has been recorded. Place your hand in the tank and slowly lower it.
You should be able to feel the thermocline.
10. Carefully bail out the water from the tank into the sink. Rinse out your tank.
11. Plot your group’s temperature data for each time interval on the following pages. Each
time will have its own graph of temperature plotted on the x-axis and depth plotted on the yaxis. Be sure that zero depth appears at the top of the graph and increases as you move down the
axis. Label each graph with the appropriate time. Label the surface layer, thermocline and
deep layer in each graph.
Questions:
1. Briefly describe what happened when the partition was removed.
2. Why did the water moved as described above?
3. In which regions of the globe would the mixing of warm and cold water occur in the ocean?
4. Describe what happened when you blew across the surface of the water.
5. a)Does stratification from temperature appear to be permanent in your lab simulation?
b) Would thermal stratification be permanent in the ocean? Explain.
Salinity in the Ocean
Salinity is a measure of the total amount of dissolved solids (salts) in the ocean. The average
salinity in the ocean varies from about 33 – 37 parts per thousand (ppt or o/oo). Salinity can
approach 0 (fresh water) where rivers enter the ocean and may be very high in areas where there
is little rain and an excess of evaporation. The amount of rainfall, input from rivers and streams
and the level of evaporation will all affect the salinity of the ocean in any area. Therefore, most
salinity variation take place near the surface where these environmental influences occur.
Diffusion is the slow mixing that occurs due to random motion of molecules. The salts and
water molecules in seawater are vibrating and this vibration causes them to bounce off each other
and mix. Salts will slowly spread away from areas of high salinity and toward areas of low
salinity due to diffusion and the salinity of those areas will change.
The salts dissolved in seawater are heavier than the water molecules themselves. Therefore
increasing the salinity of water increases its density. Water with low salinity will float on top of
water with a high salinity, as happens when river water flows into the ocean. Salinity, as
temperature, does not increase uniformly with depth. A plot of salinity versus depth shows three
distinct zones. The upper mixed zone is characterized by lower salinity. The middle zone is a
zone where salinity increases with depth; this is known as a halocline. Below the halocline, the
deep zone contains water of fairly uniform higher salinity.
Exercise 2: Effect of salinity differences on density and stratification
Procedure:
1. Install a partition to divide a 10 gallon tank into two halves.
2. Fill a bucket with cold water from the tap and add 400ml of salt. Stir until the salt is
dissolved. Fill another bucket with cold water from the tap. Add several drops of food coloring
to each bucket to color them different colors. Use the refractometer to measure the salinity of
each solution in ppt. Record the color and salinity of each solution:
Salt water:
color ____________ Fresh water:
color______________
salinity_______
salinity_________
3. With a partner holding the partition in place, simultaneously pour one bucket of water into
each side of the tank. Both sides of the tank should be filled to approximately the same height.
4. Briefly allow the water to settle and then quickly but carefully pull up and remove the
partition. Observe what occurs as the partition is removed by watching through the side of the
tank.
5. Once the water has settled, take small water samples from four different depths by sucking
out water with a pipette at the appropriate times. Using the china pencil make a mark on the
outside of the tank at the four depths that you will use: place one mark near the surface, one near
the bottom of the tank and the other two evenly spaced between. Record the depth from the water
surface to each mark in the table below. To take each sample, first lower the pipette to the
desired depth, then use the pippetteman to suck up a small (<1ml) sample of water, then slowly
remove the pipette from the tank and put the water sample in a labeled test tube. Be careful not
to stir or mix the water in the tank as you take your samples. Use the refractometer to determine
the salinity of water sampled from each depth and record them in the following table:
Pipette
#1 Surface
1 minute
5 minutes
10 minutes
15 minutes
0 cm
#2
_____ cm
#3
_____ cm
#4 Deep
_____ cm
6. At the same times that you take water samples, observe the water through the side of the tank
and note the colors. Draw what you see in the following boxes:
1 minute
5 minutes
10 minutes
15 minutes
7. After the 15 minutes are over, blow hard (or use a fan, if available) at a low angle across the
surface of the tank for about 30 seconds. Observe the water through the side of the tank and take
salinity readings. Record your data in the following table.
Observations through
Pipette
Salinity
side of the tank
#1 Surface
#2
#3
#4
8. Carefully bail out the water from the tank into the sink. Wash and dry your tank.
9. Plot your group’s salinity data for each time interval on the following pages. Each time
will have its own graph of salinity plotted on the x-axis and depth plotted on the y-axis. Be sure
that zero depth appears at the top of the graph and increases as you move down the axis. Label
each graph with the appropriate time. Label the surface layer, halocline and deep layer in
each graph.
Questions:
1. Briefly describe what happened when the partition was removed.
2. Why did the water moved as described above?
3. Where would the mixing of salt and fresh water occur in the ocean?
4. Describe what happened when you blew across the surface of the water.
5. a) Does stratification from salinity appear to be permanent in your lab simulation?
b) Would this stratification be permanent in the ocean? Explain.
6. Is a halocline a zone of rapid salinity increase or decrease with depth?