This is a backgrounder about physical oceanography. Deep is dangerous. It may be more dangerous for man to dive to the depths, than to go out in space. Learn more about these deep-sea challenges!
The weight of the air presses down on everything on earth. High up on mountains or in flying planes the weight or pressure is less because there is less air above. This weight or pressure is greatest at sea level where it is about 1kg/cm3. This pressure is defined as being equal to 1 atmosphere.
The weight of the water in the sea also creates pressure. Because water is so much heavier than air, pressure in the sea increases much more rapidly with changes in depth. It increases by 1 atmosphere for every 10m of depth, or so. Thus at 1000m the pressure is about 100 times what it is at the surface. In the deep trenches at 10 000m of depth the pressure is 1000 times that at the surface. In the Challenger Deep, at 11 000m, the pressure is 1100atm!
The bodies of most marine creatures are largely made up of water. As liquids are not very compressible, marine creatures are not greatly affected even by large pressure changes. Many fish use a gas-filled bladder to help them to maintain neutral buoyancy in the water column. The gas-filled volumes are very sensitive to pressure changes, and fish have to be able to add or remove gas from the bladders to their blood streams in order to move up and down in the water.
When fish are collected in deep water, they may not have time to manage the pressure changes involved bringing them to the surface.
Left: Expanded swimbladder may push on eyes Right: Swimbladder decompression in a rattail from a trawl to nearly 3000m Pictures of pressure effects from Dr. Paul H. Yancey's Deep-Sea web pages, Whitman College, Walla Walla WA USA (see web site for larger pictures)
Recent evidence is showing that marine organisms may use special proteins to help them adapt to conditions of high pressure. more on pressure
Although the ocean’s surface temperatures vary greatly (from 40°C to -2°C ), the average ocean water temperature (including all depths) is 3-4°C . No matter how warm the surface layers, the temperature decreases steadily with depth until it reaches around 5°C at 1000m, and 4°C at 2000-3000m.
The notable exception to this, of course, are the super heated waters gushing out of deep-sea hydrothermal vents. Although this water may attain temperatures of 300-400°C when it first emerges, it quickly cools and the temperature drops to “normal” with a metre or so of the vent. More on temperature
Light does not pass easily through water. At 100m it is around 1/100 of the intensity it is at the surface. As the light passes through the water it becomes less red and bluer.
Light penetration is greatly affected by turbidity, or the clarity of the water. In the open ocean in winter it may penetrate to 1000m, but usually its effects are most noticeable in the top 100 or more metres.
High concentrations of suspended particles, phytoplankton or pollution, can also greatly reduce the amount of light penetration. More on light. More on physical oceanography - deep-sea zones