Ocean Water
Density, Salinity, and Temperature
Why is the Ocean Salty?
 Minerals & chemicals erode and dissolve into fresh
water & Waves erode coastal rock
 Hydrothermal vents add minerals
 The ocean is in chemical equilibrium
– Conservative & nonconservative constituents
Ocean Water
 Salinity = the total
amount of salt
dissolved in seawater
– Measured in parts per
thousand (ppt or ‰)
 Rule of Constant
Proportions
– the proportion of ions
stays the same
whether salinity
increases or
decreases
Concentration
ppm, mg/kg
part of
salinity
%
molecular
weight
mmol/
kg
Chloride Cl
19345
55.03
35.453
546
Sodium Na
10752
30.59
22.990
468
Sulfate SO4
2701
7.68
96.062
28.1
Magnesium
Mg
1295
3.68
24.305
53.3
Calcium Ca
416
1.18
40.078
10.4
Potassium K
390
1.11
39.098
9.97
Bicarbonate
HCO3
145
0.41
61.016
2.34
Bromide Br
66
0.19
79.904
0.83
Borate BO3
27
0.08
58.808
0.46
Strontium Sr
13
0.04
87.620
0.091
1
0.003
18.998
0.068
Chemical Ion
Fluoride F
Salinity
 The ocean’s average salinity is about 35 ‰
– Salinity is NOT uniform
 Decreases in salinity
– Precipitation (rain)
– Rivers
 Increases in salinity
– Evaporation removes
fresh water
– Volcanic activity
Effect of Salinity on
Living Things
 Marine organisms are
adapted to survive within a
specific salinity range
– Water moves into or out of
their bodies through
osmosis
Two ways of coping
with salinity:
 Osmoregulators
– organisms whose
cells adjust the
concentration of
water in their cells
 Osmoconformers
– organisms whose
internal salinity rises
or falls along with the
water’s salinity
Density of Ocean Water
 Density = the mass of a given volume of a
substance
– Density = Mass/Volume
 Ocean water density depends on 2 factors:
– Temperature
 Lower temperature (colder) = higher density
– Salinity
 Higher salinity = higher density
 Ocean water weighs about 2% - 3% more
than pure water
As the water gets colder and more
salty, density increases
Ocean Temperature
 Varies between 28o F and 86o F
 Water temperature depends on latitude and
depth
– Colder at the poles, warmer in the tropics
– Colder/lower temperature as you go deeper
 Density is controlled more by temperature
than salinity
Seawater Stratification
 Ocean water stratified
– It forms layers
– Denser water sinks below less
dense waters
– Defined based on temperature
& salinity variations
 Layers vary by region and
seasonal changes
 Surface zone
Ocean Layers
– Waves & currents
continually mix water
– Exposed to sunlight
 Thermocline
– Rapid temperature change
with depth
– Varies by season
– Do not exist in polar regions
 Deep zone
– Cold, dense, and uniform
– Originates in polar regions
Light in the Ocean
 Water scatters & absorbs light
 Zones of light penetration:
– Photic = where light penetrates
 Euphotic = upper, shallow portion where
photosynthesis occurs (about 1% of oceans)
 Dysphotic = light reaches, but not enough for
photosynthesis
– Aphotic = no light
 Majority of oceans
 Not much life
Light in the Ocean, cont.
 Clear ocean water is most
transparent to blue light
– Red light is more easily absorbed
– Blue light travels the greatest
depths
 Total darkness at 1000 meters
 Light penetration depends on
materials suspended and
dissolved in the water
– Algae, pollution, sediments
Pressure
 Changes with depth
– Deeper water = more pressure
 Measured in atmospheres
– Pressure on land = 1 atm
– Water pressure increases 1 atm
every 10 meters of depth
 Organisms use gas-filled
structures to counter the
pressure
– May cause injury or death if
brought to the surface