The Chemistry of Water
Aquatic Science Unit 4 Notes
Introduction:
Bodies of water, both fresh and salt, are actually giant chemical solutions, containing dissolved salts,
metals, nutrients, gases, and organic compounds. Chemical oceanographers examine these chemical
properties in seawater, and gain valuable information used by oceanographers in all of the other fields.
The presence or absence of certain elements can be very important for aquatic organisms. They either
help the organisms to live or they can harm of kill organisms. We are going to focus on the various parts
of seawater, but remember that freshwater has many of these same chemical properties, just not on such a
grand scale as oceans.
Salinity
Salinity is an expression of the amount of salt in a fixed amount of seawater. It is usually expressed as
parts per thousand (ppt) or 0/00. Parts per thousand literally means “x” amount of solutes per thousand
parts of water. (Remember that solutes are the substances being dissolved by water…the universal
solvent).
Salinity varies over the surface of oceans. The salinity at any particular spot depends on what processes
or factors are operating at that location that either add or remove water.
Factors that add water cause lower salinity.
Ex. high rainfall, river input, melting of sea ice
Factors that remove water cause higher salinity.
Ex. high evaporation, sea-ice formation
At depth, salinity remains about the same in all the oceans. Between the surface and deep water, however,
is a region called a halocline. This is the area of the water where salinity gradually increases or decreases
with depth.
Surface salinity is highest in the subtropics, where a lot of water is lost or removed as a result of
evaporation. Salinity is also high in enclosed or partially-enclosed basins (such as the Mediterranean.)
Salinity is lowest in colder regions or where there are large inflows of river water.
Where does the salt come from?
The salt in the ocean exists in the form of dissolved ions, some positively charged, and some negatively
charged. (Remember that water is the universal solvent, and salts are hydrophilic…meaning they love
water)
There are 6 types of salt ions found in the ocean, and regardless of how salty the water is, they are found
in definite proportions. The most common salt ions are sodium (Na+) and chloride (Cl-)…both
components of table salt (NaCl). The next four most common are sulfate (SO4), magnesium (Mg+),
calcium (Ca+), and potassium (K+). Each of these is evenly distributed throughout salt water. No matter
how the salinity of the water varies, the ratio between these ions remains the same. This is called the
Principle of Constant Proportions
85% of sea
Na+
salt
Contributions to Aquatic Science
ClNAME: Alexander Marcet (1770 – 1822)
SO4
DESCRIPTION: Swiss chemist and doctor who performed some of the
Mg+
earliest research in marine chemistry.
Ca+
CONTRIBUTION: Best known for his discovery, in 1819, leading to the
K+
Principle of Constant Proportions - that all the main chemical ions in
seawater are present in exactly the same proportions throughout the
world’s oceans.
Sources of Salt
The ions that make up the salt in the oceans have arrived there through various processes.
 Some were dissolved out of rocks on land by the action of rainwater and carried to the sea in rivers
 Others entered the sea from hydrothermal vents
 Some came from dust blown off the land
 Others come from volcanic ash
There are also “sinks” for every type of ion. Sinks are processes that remove dissolved ions from
seawater, such as salt spray onto land, or the falling of ions onto the seafloor as mineral deposits. Each
type of ion has a certain length of time in which it will stay dissolved in water before it is removed, or
sinks. The most common ions in seawater will remain dissolved for a few hundred years to hundreds of
millions of years.
Gases and Gas Exchange in Water
The main gases found in water are in the form of dissolved gases, and include nitrogen (N), oxygen (O2),
and carbon dioxide (CO2). The levels of O2 and CO2 vary according to the numbers of phytoplankton
(microscopic organisms that perform photosynthesis) and the activity of other aquatic animals in the area.
O2 levels are usually highest near the water’s surface. This occurs for two reasons:
1. O2 in the atmosphere above the surface is diffusing into the water
2. Phytoplankton are found in the top layers and are producing oxygen as a product of
photosynthesis.
O2 levels begin to drop as you go deeper into the water, and become the lowest at about 3,300 ft. High
levels of bacteria and other animals break down or eat the organic matter at this level, and use up a lot of
the available oxygen in the process. As you continue to move deeper, O2 levels begin to rise again.
** Oxygen levels in the upper ocean depend on the balance between its being produced by
photosynthesizing organisms, such as seaweed, and being used by animals, such as fish.
CO2 levels are the highest at depth and the lowest at the surface. This is because the phytoplankton that
live near the surface use much more available CO2 for photosynthesis, at a faster rate than it is produced
by organisms that exhale it. Carbon dioxide also diffuses out of bodies of water and into the atmosphere,
continuing the carbon cycle.
** Carbon Sink - Many aquatic organisms make shells out of carbonate, a compound of carbon and
oxygen. When they die, their shells may fall to the ocean floor, and become sediments and rocks over
time.
Nutrients and Nutrient Cycles
There are many nutrients and substances found in small amounts in oceans that are necessary for aquatic
organisms to grow. Phytoplankton, for example, are microscopic floating life-forms that obtain energy
by photosynthesis. They form the base of every aquatic food chain. Phytoplankton need nitrates, iron, and
phosphates to grow and multiply. If these nutrient supplies dry up, their growth stops. If the supply
rapidly increases, than blooms (rapid growth phases) occur. Although some nutrients return to bodies of
water from sources outside the aquatic world, most of the nutrients come from a continuous cycle within
the bodies of water itself. As organisms die, they sink to the floor, where their tissues decompose and
release or return nutrients. Upwelling of seawater from the ocean floor recharges the surface waters with
new nutrients, where they are taken up by the phytoplankton, refueling the chain. We will learn more
about upwelling in the near future.
Temperature
Surface temperatures vary widely across the globe. Surface waters in the tropics and subtropics remain
warm throughout the year because of constant heating from the sun. In the middle latitudes, there is more
seasonal changes in surface temperature. In higher latitudes and polar oceans, the water is constantly cold,
often below 32oF (0oC).
Below the surface, the temperature of ocean waters drops quickly to about 46-50oF (8-10oC) at 3,300 ft.
This region of steep decline is called a thermocline. As you move even deeper, temperature continues to
drop, but it drops more gradually to a uniform 36oF (2oC) on the sea floor. This temperature remains
constant throughout the deep oceans around the globe.
Density
The density of any small portion of seawater depends mostly on two things: its temperature and its
salinity. Any decrease in temperature or increase in salinity makes seawater more dense. There is an
exception to this rule, however. When the temperature drops below 4oC (39oF), water becomes a little less
dense (remember that it freezes at 0oC and floats on water.)
In any part of the ocean or other body of water, the density of the water increases with depth, because
dense water always sinks if there is less dense water below it.
This changing density of ocean water is extremely important because seawater either rises or sinks as a
result. This drives the large-scale circulation of water in the oceans between the surface and deep water. It
also causes circulation of oceans from middle latitudes toward the poles, and vice versa.
Example: The oceans each contain distinct,
named water masses that increase in density
from the surface downward. The denser,
cooler masses sink and move slowly toward
the equator. The cold, high-density deep and
bottom waters comprise 80% of the total
volume of the ocean!
Density Layers in the Atlantic