6.1 Pressure

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 consequences of this great pressure can be seen in the video below, showing a hole being cut into a pipe at a depth of 6 miles (about 9.6 km). The pressure outside the pipe is over 960 atm, which is far higher than the pressure inside the pipe, and this pressure differential has serious implications for a passing crab…

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. As seen in section 5.4, this means that deeper, high pressure water may contain more dissolved gases than surface water.

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.