A body of water goes through a marked level of temperature change as one moves farther from the surface. The waters settle into stratified temperature layers, though at some point the temperature starts to drop far more quickly than the waters above or below. This mixed layer where the temperature drops more rapidly than the bordering waters is called the thermocline.
The thickness of this mixed layer changes with the seasons and can affect local fish populations - a more resilient thermocline prevents the upwelling of nutrients from deeper waters, causing fish to move to more hospitable climates or, in smaller ecosystems, to die off.
It's also worth mentioning that the thermocline is mostly impermeable to sound waves - sound reflects back off the water of a different density, rendering SONAR ineffective. Therefore a submarine is able to travel at high speeds below the thermocline and not be heard by vessels on the other side of the gradient. Note that the impermeable layer works both ways - a ship under the thermocline is equally ignorant of anything happening on the upper side of the layer.
Sources:
Thermocline: a transition layer between deep and surface water. University of Illinois WW2010 Project.
http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/thermocline.rxml?hret=/guides/mtr/eln/econ.rxml
January 18, 2005
Earth and Space Research Organization. Thermocline Definition.
http://www.esr.org/outreach/glossary/thermocline.html
January 18, 2005