OCEAN AND PRESSURE
July 13, 2022
UPDATED 12:00 PM EST
When we talk about pressure in the ocean, we are referring to hydrostatic pressure, which is a result of the weight of the water column pressing down on an object due to gravity. The deeper you go, the more water that is above you, and the greater the weight (and thus pressure) of that water. At the surface we experience one atmosphere of pressure (1 atm = 101.3 kPa) due to the weight of the atmosphere above us. As you descend into the ocean, pressure increases linearly with depth; there is an increase in pressure of 1 atm for every 10 m increase in depth. So at 1000 m depth the pressure would be 101 atm (100 atm of pressure due to the 1000 m depth, plus the 1 atm that is present at the surface). This leads to very high pressure in the deeper parts of the ocean; if you consider that the average depth of the ocean is about 3800 m, the pressure at that depth would be 381 times greater than the pressure at the surface.
The density of seawater is increased by increasing pressure. Density changes about 2% because of the pressure difference between the surface and the deep seafloor. The effect of pressure on density usually can be ignored because most applications require density comparisons between water masses at the same depth.
CONSEQUENCES OF PRESSURE AT DEPTH:
There are several important consequences of high pressure at
depth. First, due to Boyle's Law, which states that the volume of a gas is inversely related to pressure, high pressure will act to compress air spaces, such as the lungs of a diving animal (or person), or the space inside a submarine. Submarines and submersibles must therefore have very strong hulls to resist this compression at extreme depths. Second, Henry's Law provides that at higher pressures a fluid will contain more dissolved gas.
THE HUMAN DIVERS AND PRESSURE:
This also has implications for human divers. According to Henry’s Law, if you increase pressure, you increase the amount of gas that can dissolve in a fluid (such as blood). Conversely, when you reduce pressure, the fluid holds less dissolved gas, and the excess gas will leave the solution, often in the form of bubbles. This is what happens when you open a bottle of a carbonated beverage. The contents in the bottle are sealed under pressure, and as you open the bottle, you release the pressure, and the fluid can no longer hold all of the CO2 that was dissolved in it, so the CO2 escapes, forming bubbles. Decompression sickness, or “the bends” occurs in SCUBA divers if they ascend too quickly after breathing compressed air. A slow ascent allows this excess gas to be removed from the blood and exhaled, but if the diver ascends too quickly, these gases will come out of solution and form bubbles in the blood that congregate near the joints, causing intense pain and perhaps death.
THE SEA ANIMALS:
Diving response is a reflex that all vertebrates have, including humans, but it is most strongly developed in diving animals. They automatically slow their heart rate and send blood flow away from the muscles and to the most important organs such as the brain to conserve oxygen. They also have high levels of hemoglobin and myoglobin, which store oxygen, in their blood.
Their main defense against the bends is that their lung structure can collapse under high pressure. This forces the air away from the alveoli and into the upper airways where the gas can’t enter the bloodstream, and that’s what keeps the blood from absorbing too much nitrogen at depth. This also preserves a reservoir of oxygen that becomes available again during the trip back to the surface.
There is another important factor: the symptoms of the bends were often found in the carcasses of animals that were exposed to extremely loud noise, such as naval sonar testing. Sonar has been linked to marine mammal deaths for a long time, though scientists don’t know yet exactly how or why. It may disorient them or force them to surface too quickly.
While there is a lot that we still don’t know about how marine mammals dive, we do know that even these animals who are so well adapted to a deep-diving lifestyle, aren’t immune to decompression sickness – and human activities may even be causing it.
[Photo Credit: UCLA Newsroom]
Norton College, “CC.6.Salinity, Temperature,Pressure and Water Density”
Roger Williams University , “Introduction to Oceanography” , Chapter 6, 6-1:Pressure
One world one ocean, ocean stemuation:” How mammals avoid bends?” , Sarah Bedolfe