Water Column Stratification

Stratification of ocean water is a naturally occurring phenomenon that is important to the structure, circulation and productivity of the oceans.1-3 The formation of vertical stratification in the water column is a consequence of water masses with different densities. Water density is strongly influenced by temperature and salinity; with less dense, warmer surface waters floating on top of denser, colder waters. The boundary between the warmer and cold waters is called the thermocline. Water is unable to passively mix across this layer, but wind, upwelling, down-welling and storms help move water across the boundary. For instance, winter storms create turbulent mixing between water layers. Mixing is critical for ecosystem productivity because it brings nutrients to the surface and oxygen to deeper waters. During springtime warming, the waters stratify, trapping phytoplankton near the surface, resulting in a spring bloom in the nutrient rich water. The spring bloom in turn provides food for many marine animals and plays an important role in the global carbon cycle.4-6

While stratification of the water column is important, prolonged or strengthened stratification can have negative impacts. As the temperature of coastal waters increase, the thermocline becomes a more powerful boundary, making it more difficult for the nutrient rich waters to reach the surface. This potential reduction in upwelling and mixing can result in local or widespread biomass loss and changes in species composition. Between 1951 and 1993 zooplankton biomass off Southern California decreased by 80% as a result of warming surface waters. In some areas the water temperature rose by 1.5oC and restricted coastal upwelling and nutrient availability.7

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