*1 Themocline: The specific gravity
(density) of seawater is mainly determined by water temperature and salinity,
and the specific gravity of seawater with high water temperature and low
salinity is light, while that of seawater with low water temperature and high
salinity is high. When the ocean surface becomes warmer due to solar radiation,
a discontinuous layer of density due to the difference in water temperature
develops between the warmer water and the cooler water of the deep sea. This pyconocline
caused by the difference in water temperature is called the “thermocline”. The
fact that when you heat a bath, the surface is warm while the bottom remains
cold is an example of the density difference in water caused by the difference
in water temperature.
The area shallow of the thermocline is
called the mixing layer, where seawater mixing occurs, but the density
difference between the shallower and the deeper the thermocline prevents the
mixing of seawater beyond the thermocline. Due to this difference in the mixing
of seawater between the shallower and the deeper the thermocline, the
concentration of nutrients such as nitrogen and phosphorus, which are essential
elements for phytoplankton to carry out photosynthesis, differs greatly between
the depths below the thermocline and the depths above the thermocline.
Nutrient concentrations in the shallow part
of the thermocline are low because phytoplankton consumes nutrients through
photosynthesis, while nutrient concentrations in the deep part are high because
phytoplankton consumes less nutrients and bacteria decompose organic matter.
