Lecture 08
Water
Hydrologic Cycle, Properties of
Water, Factors affecting Life in
Water
• Over 71% of the earth’s surface is covered by water:
– Oceans contain 97%.
– Polar ice caps and glaciers contain 2%.
– Freshwater in lakes, streams, and ground water make up
less than 1%.
• Precipitation  infiltration (or surface
runoff)  groundwater
– Special issue as we create hard surfaces
– Special value of wetlands
• Water returns to atmosphere via:
– Transpiration: evaporation from internal
surfaces of leaves, etc.
– Evapotranspiration: movement from plant and
ground surfaces to atmosphere
The Hydrologic Cycle
• Turnover time is the time required for the
entire volume of a reservoir to be
renewed.
– Atmosphere
9 days
– Rivers
12-20 days
– Oceans
3,100 years
http://digital.films.com/PortalViewVideo.aspx?xtid=47289&psid=0&sid=0&State=&titl
e=Liquid Assets: The Big Business of Water&IsSearch=N&parentSeriesID=
Evaporation – Loss of Water from
Organism to Atmosphere
• Important for terrestrial organisms
–Provides cooling
–Represents major loss of water.
• Greatest in dry climates – water vapor
in air less – where ‘humidity’ is lower
–Concentration gradient greater
• Cooling from evaporation greatest in
dry climates.
Unequal sharing of electron
in water molecule results in
positively and negatively
charged regions
Bonds formed between
water molecules – break
and reform – like velcro
•
•
•
•
•
Cohesive and adhesive
Viscous
High specific heat
High heat of vaporization
Greatest density is as a
cold liquid, less dense as
solid
• Solvent
• Properties altered by
dissolved substances
• Changes in density with temperature
• Greatest density at 4C
• Ice floats – expands due to intermolecular
interactions
• Develops layers of stratification
– Surface waters warmed (in summer)
– Deeper waters cool
– Thermocline – region of rapid change in temp.
with depth
Penetration of Water by Light
• % of surface light at various depths:
•
Depth
% of surface light
1 cm
73
1 meter
44.5
10 meters
22.2
100 meters
0.53
•varies with turbidity – assume clear water
•Different wavelengths penetrate water to
different degrees – blue penetrates the furthest
http://staffwww.fullcoll.edu/tmorris/elements_of_ecology/image
s/light_spectral_absorption_water.jpg
• Estimation of
turbidity of water
using senchi disc
• Turbidity is a
function of
suspended
plankton growth
and amount
particulate matter in
water
• Oxygen and Depth
• Dissolves at surface (diffusion): function of temperature
• Reaches minimal concentration between surface and ~ 1000
meters depth
• Anoxic or Anaerobic = ?
– Certain deep waters
– Consequence of metabolic activity
Gasses dissolved in water: Enter and move
by diffusion
Oxygen - solubility function of
– Temperature – greater at lower temperatures
– Salinity – more soluble in fresh water
– Atmospheric pressure
• Carbon dioxide – creates carbonic acid
pH and water – acids and bases
• Due to dissociation of water molecules into
Hydrogen and hydroxyl ions
• pH is a measure of hydrogen ion
concentration
• Impacted by dissolved substances –
organic materials, gasses, salts
Acidity – concentration of hydrogen ions
• pH is a measure of hydrogen ion
concentration
• Acid = substance which increases [H+]
• Base = substance which decrease [H+]
Acids and Bases
• Acid: excess of H+ ions
• Base: excess of OH- ions
pH is a measure of H+ ion
concentration on a log scale:
pH = -log [H+]
• lower number indicates a higher
hydrogen ion concentration or a
more acidic condition
Buffers
• A buffer _______________________
– sort of like a chemical shock absorber
• Important in living systems – pH is
critical to maintenance of life
processes
• CO2 is absorbed from atmosphere
• Enters rain water and diffuses directly into
surface waters
– Creates moderately acidic condition but also
some buffering capacity
• Other atmospheric gasses may increase
acidity of rain water: = acid rain
– Sulfur oxides  sulfuric acid
– Nitrogen oxides  nitric acid
• Strong acids, overcome buffering capacity, create
acidic bodies of water
• Particular problem for areas with granite substrate
•
•
•
http://digital.films.com/PortalViewVideo.aspx?xtid=44017&psid=0&sid=0&State=&title=The%20Weather%20Video%20Clip
%20Collection&IsSearch=Y&parentSeriesID=&loid=111194
http://digital.films.com/PortalViewVideo.aspx?xtid=10780&loid=32209&psid=0&sid=0&State=&title=The%20Environment&
IsSearch=Y&parentSeriesID=
http://digital.films.com/PortalViewVideo.aspx?xtid=44363&psid=0&sid=0&State=&title=The%20Adirondacks&IsSearch=Y&
parentSeriesID=&loid=124929
• Summary
• Life on earth depends on water and its
properties
• Water is a polar compound
– Ends of each molecule have different charges
• Water is a solvent for ionic solids – salts which
dissociate into positively and negatively charged
ions
• pH is a measure of H+ ion concentration
– Lower pH means higher H+ ion concentration
• Light is quickly absorbed by water meaning is in
only available at the surface of bodies of water
• Water is much more viscous than air