Thermocline

The thermocline (also called metalimnion in lakes) acts as a density barrier — warm, less-dense surface water (epilimnion) floats above cold, denser deep water (hypolimnion), and mixing between layers is minimal. Oxygen is typically depleted below the thermocline during summer stratification as decomposition occurs without replenishment from the surface. Baitfish congregate along the thermocline depth because it provides both food (zooplankton tracking optimal temperatures) and a visual barrier from below. Large pelagic predators like tuna hunt at the thermocline depth, exploiting the baitfish concentration. Seasonal thermocline disruption (autumn overturn in temperate lakes) redistributes nutrients and oxygen.