Physical &

Chemical

OceanographyUnit 7

Chemical Composition of Sea Water

� Sea water� Has an average salinity of 35‰ (parts per thousand)

� Salinity� The total amount of salt dissolved in seawater

� Usually expressed in number of grams left behind when 1,000 grams of seawater are evaporated (parts per thousand)

� Due to amount of solutes (dissolved materials), usually ions (charged atoms present in seawater

� Salinity has a huge impact on organisms living in the water� Most marine organisms die in fresh water� Even slight changes in salinity can harm animals

� Salinity varies based on:� Location

� Near river mouths or melting glaciers = less salinity

� In dry, arid areas = higher salinity

INB pg.

45

Chemical Composition of Sea Water

� Regardless of where you take samples, percentage of ions within sea water remains fairly constant� Only 6 ions make up 98% of solids

in seawater!

� Sodium (Na+) & Chloride (Cl-) make up 85% - why seawater tastes “salty”

� This phenomenon is the rule of constant proportions� states that the relative amounts

of the various ions in seawater are always the same

� However, ion composition can be locally changed due to:� Volcanic activity� Runoff� Atmospheric dissolution

55%31%

8%

4% 1%1%

0%

Chloride (Cl-)

Sodium (Na+)

Sulfate (SO4-2)

Magnesium

(Mg+2)Calcium

(Ca+2)

Volcanic Changes� Volcanoes emit the following gases which enter sea

water� Carbon dioxide (CO2)

� Sulphur Dioxide (SO2)

� Hydrogen Sulphide (H2S)

� Hydrogen Chloride (HCl)

� These gases dissolve in atmospheric water (evaporation/condensation in clouds) & enter seawater as precipitation (recall: hydrological cycle)

� Underwater (submerged) volcanoes at tectonic plate boundaries emit the above gases

� Gases from volcanoes are major source of chloride ions in seawater

Changes from runoff

� Runoff = flow of water from land� Comes from rain or melting or snow & ice

� As part of hydrological cycle, most will eventually reach oceans

� Water flowing through soil or urban landscapes picks up pollutants � Pesticides

� Fertilizers

� Oils

� Plastics

� Pollutants may have initially low concentrations, but if taken in by organisms, biomagnification occurs� Pollutant eaten/absorbed by organism at low trophic level

� Passed on to each level within food web

� Increases its concentration at each trophic level

� Ex. Minamata Bay in Japan, from 1932 to 1968

Biomagnification through runoff

Changes through Atmosphere� Gases dissolved in seawater are at equilibrium with

atmosphere due to diffusion� Actual concentration of gases is dependent on:

� Relative solubility

� Temperature of water

� Salinity of water

� Dissolved gases include:� Nitrogen

� Can be “fixed” (changed) by nitrogen fixing bacteria into products usable by other organisms

� Great fertilizer

� Oxygen� Necessary for respiration

� Carbon Dioxide� Used in photosynthesis