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 Oceanography Lecture
  Ocean Structure
Chapter 6 and 7

Properties of water:

Water is a polar molecule

Molecular vibration of hydrogen bonds absorb small amounts of red (long wave length) light.   What is the obvious observable consequence of this?




Water has a high heat capacity

Where does climate tend to be most moderate and why?  Is global climate likely to be affected by changes in ocean current patterns?



Properties of seawater:

Seawater is about 96.5% pure water an d 3.5% dissolved solids and gases (note that important nutrients for productivity are trace elements in seawater). 
                    

What is the source of some ions (e.g. chloride) that are not common in water entering the ocean from rivers?

Silica and calcium ions are not as common relative to river water; where do these go?



Ocean water is ~35 parts per thousand (ppt or 0/00) but varies between 33 to 37 ppt
Water between 0.5 ppt and 17 ppt is called brackish; >17 ppt is considered seawater

Salinity can be measured by:


The maximum density of ocean water is at the freezing point which is lower than pure water (0°C). 

Ice is much less dense (0.917 g/cm3)

(Salt in water also decreases heat capacity, reduces evaporation rate, and alters osmotic pressure)


Density

So density of water is affected by temperature and salinity (and to a lesser extent, pressure)


Will a water mass that is a 5°C and 33 ppt mix easily with a water mass that is 15°C and 36 ppt?

Will it be harder to mix
a water mass that is a 5°C and 33 ppt with a water mass that is 5°C and 35 ppt?








Why is the ocean divided vertically
into 3 zones
(surface mixed layer, pycnocline, and deep zone)?













Why does this zonation vary with latitude?


What will happen to a water mass that increases in density (for example in polar regions where water cools  and salt is excluded as ice is formed, or where evaporation is particular high)?

The tropics tend to have low density surface water due to greater heating.
The arctic tends to have low density surface water due to large freshwater inputs from Candian and Russian rivers.
So where on the globe is exchange between surface and deepwater likely to be the greatest?
  
What is  the consequence of such vertical exchange of water on the horizontal structure of water?




Light in water

Fate of light in aquatic systems:

            
 
Measuring light properties in water:
Vertical illumination (light penetration) - illumination at some depth as measured by underwater photometer (often as PAR: photosynthethically-active radiation). Light is absorbed exponentially with depth (a constant percentage of light available is extinguished at each meter)
When plotted as depth vs log % incidence of light, line is straight in a homogeneous solution.
    What would a deflection in the line indicate?
   
Compensation depth
- Respiration exceeds photosynthesis within a cell at about 1% of incident surface light (the euphotic zone is the region from the surface to where 99% of light has disappeared).

 
   
Turbidity - an expression of the optical properties that cause light to be scattered and absorbed rather than transmitted in a straight line. Turbidity is primarily caused by total suspended solids but a direct relationship is varies from system to system. Why?  Nephlometer (turbidometers) measures intensity of light scattered at 90°.
 
Visibility - measure of the depth at which one can see into the water. Measured using a secchi disc.  
What factors affect this measurement?

Visibility can be used to estimate photic depth.
 

http://dipin.kent.edu/secchi.htm           http://www.hao.ucar.edu/public/education/sp/images/secchi.html
  What factors affects the depth of light pentration?


Dissolved gases
Unlike major salts, gases tend to by "nonconservative"
Saturation point of a gas increases with decreasing water temperature.

           
Why do oxygen and carbon dioxide concentrations vary with depth in the graph above?

Why is pH highest nearer the surface of the ocean?







 

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