Lecture 08Water

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

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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.

Bonds formed between water molecules – break and reform – like velcro

Unequal sharing of electron in water molecule results in positively and negatively charged regions

• 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/images/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

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• 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