1. Salinity and Buoyancy: Fish Swim Bladder

2. Buoyancy
The relationship between density of a fluid, weight of an object, and buoyancy is an absolutely basic concept for all students of the sea or fresh waters, because density directly affects everything in the water.
Scientific understanding of this basic science concept was first elucidated more than 2,000 years ago by Archimedes, a Greek scientist and middle school teacher
Archimedes was interested, among other things, in why things float.
He answered this question by a series of careful experiments in which he weighed the water displaced by each of the objects he tried to float and then weighed the object.
Archimedes found that the weight of the water displaced by the floating object is greater than or equal to the weight of the object.

3. Buoyancy
Buoyancy counteracts the earth’s gravitational force, which pulls the objects downwards, towards the center of the earth.
Because the density of saltwater is greater than the density of fresh water, objects in seawater float higher than they do in freshwater.
Buoyancy is why ships float.
Ships float because the force of gravity pulling the ship down is less than the force of the water pushing the ship up.
Empty ships ride high in the water.
When loaded with cargo a ship rides lower in the water.
It is important not to load too much cargo aboard or the ship will take on water and sink.
A “Plimsoll mark” is found on all large ships.
This mark shows proper loading in fresh or seawater.
A ship can hold more cargo in seawater because it floats higher.

5. Bell Work: Fish Buoyancy
Why do you think fish don’t float to the top of the water or sink to the bottom in ocean water?
Why do different fish swim at different depths in the open ocean?
Do you think freshwater fish would have trouble swimming in ocean water and saltwater fish have trouble swimming in freshwater? Explain.

6. CWM Objectives: The student should be able to:
Analyze the relationship between buoyancy and water displacement
Explain how salinity relates to buoyancy of an object in water
Identify how and why the fish regulates its buoyancy in water
Describe the way fish fill their swim bladder
Propose some advantages of a counter-current exchange system

7. Density of ocean water varies with location and time do to:
Buoyancy Neutral: having same density as the water (you don’t sink or float)
Density of ocean water varies with location and time do to:
Freshwater inflow from rivers (estuaries)
Evaporation rates (slow or fast)
Amount of rain
Depth of water (layering)

8. The swim bladder of fish...
Swim Bladder (red oval) – maintains neutral buoyancy
Contains oxygen gas which can be filled or emptied to achieve the desired density for the whole fish.
Gills (black crescents) - the functional equivalent of our lungs; blood adsorbs oxygen from the water and releases carbon dioxide
FYI - Swim bladder in the adult is not connected to the mouth or digestive system.
However, during embryonic development it was formed from a pouch of the digestive system.

9. Moving gas into and out of the swim bladder
Oxygen gas is carried by hemoglobin in the blood of the fish
Hemoglobin enables the fish to fill or empty its swim bladder
If blood becomes more acidic, the hemoglobin releases oxygen into the bladder.
If the blood becomes less acidic the oxygen is reabsorbed by hemoglobin.
Root effect -The hemoglobin of fish with bladders is extremely sensitive to acidic conditions, releasing about half of bound oxygen even at high oxygen concentrations.

10. Oxygen Binding Curve: Response of hemoglobin to acidity
A binding curve shows how the amount of oxygen bound to hemoglobin increases with an increase in the free oxygen concentration.
Black curve - shows the binding of oxygen to hemoglobin under normal conditions.
Blue curve - Root effect- approximately half of the oxygen is released, even if the free oxygen concentration is very high.
Red curve - Bohr effect - The hemoglobin of most animals is sensitive to acidic conditions, but to a much reduced degree.

11. The fish uses lactic acid to force oxygen into the bladder
Cells on the wall of the swim bladder convert glucose to lactic acid.
The lactic acid diffuses into blood circulating over the outer surface of the bladder.
Due to the Root effect, oxygen is released from hemoglobin and then diffuses into the bladder
expands and gives the fish greater buoyancy.
The acidic blood and high oxygen concentration is shown as red in the Figure.
mainly restricted to vessels on the surface of the bladder.

12. How does the fish do it?
Blood entering and leaving the bladder surface pass through the rete mirable (Latin for wondrous network).
In this network of capillaries the two blood flows are separated by only thin walls of capillaries.
This is a counter-current exchange system, in which acid and free oxygen in the blood leaving the bladder diffuses back into the less acidic blood entering the bladder.

13. The fish uses lactic acid to force oxygen into the bladder
The advantage of this localization is three fold:
Acidic blood flowing into the main circulation of the fish would depress the performance of the swimming muscles
(remember how your muscles ache after a hard race).
Keeping the acid in the region of the bladder means the acid cells of the bladder don't have to produce as much lactic acid.
Recycling the free oxygen back to the bladder increases its concentration and forces it into the bladder.

14. Bohr effect (decrease in oxygen affinity in acidic conditions) appears to have been present some 500 million years ago (when fish first appeared in the oceans).
Root effect (release of oxygen in acidic conditions) evolved 350 million years ago.
However, evolution of a swim bladder rete occurred independently at least four times (but shown only twice in the simplified time line on the left).
In recent times (if you call the last 100 millions years recent) the swim bladder rete, the choroid rete (for the eye), and even the Root effect has been lost several times by various groups of fish as they evolved.

15. Lab Quiz
Turbidity is a measure of _____________, specifically, of how much the solid matter suspended in water decreases the passage of light through the water.
Turbidity impacts aquatic life in the following ways:
Turbidity interferes with the penetration of __________ needed for the growth of large plants.
Suspended particles can transport heavy _______ and other toxic substances into the aquatic habit.
Fish cannot see very well in turbid water and may have difficulty finding __________.
What is the order of the solutions that results with three distinct layers?(ocean, brackish, freshwater)
Why did this result happen in that order?
In an estuary where the river meets the sea, where would the salinity of the water be the greatest?
What was your hypothesis for the “Pirate Booty” lab?
Does changing the salinity of the water affect the buoyant properties of a “ship”?
Explain why saltwater can hold up ships better than freshwater? (include the relationship between water displacement and mass of saltwater vs freshwater)
water clarity
Sunlight; metals; Food
Ocean bottom, brackish middle, freshwater top
Densities of water: saltwater is most dense
Salinity greatest near the bottom
If you increase salinity, then the buoyancy of an object in water will increase.
Yes, increase salinity = increase buoyancy
Adding salt increase density, therefore, the volume of saltwater displaced by a floating object will have a greater mass than the same volume of freshwater would.