Profundal Zone
Below the depth of effective light, beneath the thermocline, lie the profundal waters. Here, diversity and abundance of life are influenced by oxygen and temperature. The organisms depend upon the rain of organic material from the layers above as their energy source. In highly productive waters, decomposing organisms deplete the profundal waters of oxygen that little aerobic life can exist there.
The profundal zone of a deep lake is a relatively larger portion of the lake volume, so the highly productive water of the epilimnion is small in comparison to the volume of profundal water and decomposition does not deplete the oxygen. Here the profundal zone supports some life, particularly fish, some plankton and such organisms as certain cladocerans which live in the bottom ooze.
Other zooplankton may occupy this zone during some part of the day, but migrate upward to the surface to feed. Only during the spring and autumn overturns when organisms from the upper layers enter this zone is life abundant in the profundal waters.
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