Life in the ocean depends on the mixing of warm upper layers with cooler lower layers in autumn and winter. This recharges surface water with the nutrients essential for phytoplankton growth. The diagram illustrates the changes in ocean temperature at increasing depths. During summer (December) there is a thin layer of warm water above the nutrient-rich cool water. As autumn progresses (May) the warm water begins to mix with the cooler water. By the end of winter (September), the nutrients have reached the surface. The cycle starts again when summer heat warms up surface waters.
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