Note-before we started today’s lecture, I reviewed some key concepts about density driven circulation (thermohaline circulation).  We looked at the following figures from Garrison, which are key figures in the idea of density layering and density – driven circulation.
                     Figures 6.17 (6.13) (really key figure-the T-S diagram) and figure 9.25 (9.21)
                     Figure 9.26 (9.22) - where deep bottom water forms today
                     Figure 6.18 (6.14) -what profiles of temperature, salinity, and density generally look like

1. Today we will take a look at 3 extremely important aspects of the ocean: sound, light, and what we call the Biological Pump, which is the remarkable balancing of  ocean water chemistry by plants and animals.

2. Sound in the ocean:  Sound travels as a wave.  Sound travels faster in water than air, and faster in salt water than in fresh water.  Sound velocity in the ocean is determined by the interplay of salinity and temperature (just as these 2 characteristics determine density, they also determine how fast sound can go in the ocean!).  The pressure from overlying seawater also is important.

            1. The SOFAR Channel or Layer is a zone at approximately 1000 meters where sound is focused and can travel extremely long distances without being absorbed (lost).  Sound waves created within this layer cannot escape unless directed outward at a sharp angle, and thus will travel for great distances until eventually absorbed.  Pressure, temperature and Salinity are just right to create this phenomenon at approximately 10000 m depth.  See Garrison, fig. 6.23, p. 174 (6.21, p. 155), and fig. 6.24, pg. 175 (6.22, p. 155).

            2. Whales have made use of this channel for communication between family “pods”.  During migrations, whales can call to each other across an entire hemisphere of ocean.

3. Light in the Ocean:  As light enters the ocean water, it is gradually absorbed until completely gone at about 600 meters.
            1. The surface water (upper 70 meters) receives the most light, and that is the euphotic zone, where   photosynthesis can occur.
            2. The depths from 70 to 600 meters have reduced amounts of light, just enough for vision, and this is the disphotic zone.
            3. Below 600 meters, its dark!  This dark region is referred to as the aphotic zone, where it is too dark for vision or photosynthesis.
            4. Visible light is one part of the electromagnetic (EM) spectrum, and it is the only part of the EM spectrum that penetrates ocean water. Visible light is made up of the visible colors (ROYGBIV), and these colors penetrate water differently.  The Red end of the visible light spectrum shows least penetration into the water, whereas the Blue-Violet end (especially Blue) of the visible light spectrum penetrates furthest (about 600 meters).  There are many consequences to this differential penetration of light.
                        a. Photosynthesis is confined to the upper 70 meters of water where enough light is available.

                        b. Some types of marine algae called phytoplankton can harvest different colors of light, and thus live at different depths within the top 70 meters of the water.

                        c. Animals that live beyond the range of light penetration have various adaptations to the dark-e.g.  enlarged eyes, bioluminescence.

                        d. Since photosynthesis is confined to the surface water of the ocean, then the only way oxygen can enter the ocean is at the surface, from plants or from the atmosphere.

                        e. Since animals live throughout the ocean water, carbon dioxide can enter at any level (carbon dioxide is given off when animals respire).

                        f. There are more colors available for animal/plant coloration in surface waters, and this situation is expressed in the brightly colored animals and plants of the surface waters (e.g., reef communities) compared with those animals that live in the deeper parts of the ocean.
 

4. Photosynthesis-a chemical process that uses carbon dioxide and water to produce carbohydrates and oxygen, with the energy source for this process coming from sunlight.  Photosynthetic organisms in the ocean include the tiny floating plants called the phytoplankton of the surface water, large algae (seaweeds), and photosynthetic bacteria.  The phytoplankton form the base of the food chain for most ocean life, with the exception of the hydrothermal vent communities.
 
5. Respiration-a chemical process that uses oxygen and carbohydrates to produce carbon dioxide and water vapor-basically, breathing!  Animals in the ocean all respire, and therefore need oxygen to do this, and give off carbon dioxide when they exhale.

6. Carbon dioxide can be exhaled by animals at any depth in the ocean, and so we see CO2 being added anywhere in the ocean depths.  The oceans absorb CO2 very easily from the atmosphere, and it is being produced from marine animals all the time.  Consequently, the oceans are rich in CO2, and in fact, control the CO2 on the surface of the earth.  More than 60% of the CO2 on the earth’s surface is in the oceans.

7. Oxygen can only enter the ocean from the surface-via photosynthesis from the phytoplankton, or directly from the atmosphere-there is no other way in!   See Garrison, Figure 7.8 pg. 195 (172) and Figure 7.10, pg. 197 (173)
 

8. The Biological Pump:  The chemical balance or buffering of the ocean is actually managed by the plants and animals that live in it.  They keep the pH of ocean water at about 8, just a bit higher than neutral (neutral pH is 7). See Garrison figure 7.11 and 7.12, p.198 (pgs. 173-174).
        1.  pH is a measure of the acidity or alkalinity of a substance, and it is always shown on a scale from 1 to 14.  A pH of 7 is neutral ( pure water has a pH of 7). Increasingly lower numbers below 7 indicate increasingly acidic conditions, whereas increasingly higher numbers above 7 indicate increasingly basic or alkaline conditions.

        2. Plants and animals maintain the chemical balance, or do what we call buffering, by the balance of respiration and photosynthesis.  As the plants photosynthesize, they give off oxygen, and drive the pH to higher (more alkaline or basic) values.  As animals respire, they give off CO2, and this drives the pH to lower (more acidic) values.  The dynamic balance struck by these two major processes keeps the ocean in its pH balance, and makes life possible and tolerable in ocean waters
 

Next Lecture:  Wind driven Circulation:
Focus Question:  Why is the Ocean Like a River?
Readings:  Ch. 8, pgs. 202-232, Ch. 9, pgs. 233-237 (ch 8, pgs 177-199, ch 9, 200-204)