The seasonal changes in the Celtic Sea primarily revolve around the development of water column stratification in spring and when it breaks down in late summer to early autumn. Right now in March, the Celtic Sea is fully mixed, however with the days getting longer and warmer (we hope), the surface of the Celtic Sea is also warming. As the surface warms its density decreases and the water becomes lighter compared to the colder waters below which don’t have access to the suns heat. (Fig 1.) To help watch for these changes we have a daily set of sea surface chlorophyll and temperature satellite images sent from the NEODAAS team at PML to the ship, and any developments of blooms and changes to the temperature can be seen as they occur.
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Fig. 1: Temperature profiles in the mid latitudes in the ocean. Dashed (- - - -) line is for the winter and the continuous line for the summer season.Source: https://nptel.ac.in/ |
This will
eventually result in the creation of two distinct bodies of water, with a warm
surface layer resting above a colder layer below, much like a cocktail which
often have two or three coloured layers sitting on top of one another.
As well as
causing the onset in stratification, the increase in temperature and sunlight
also causes a truly massive increase in the number of phytoplankton in an event
known as a plankton bloom [many plankton blooms are so large they can be seen
from space! (see Fig.2)]. This results in a feeding frenzy as zooplankton (Fig. 3) numbers surge and they are in turn eaten by other organisms, passing the
energy down the food web.
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Fig. 2: Plankton Bloom in the Celtic Sea. Captured by the Envisat's Medium Resolution Imaging Spectrometer (MERIS) on 23 May 2010. Credits ESA |
The
phytoplankton bloom starts just before the onset of stratification, and then
continues in the surface layer as the water there is warmer and receives much
more sunlight. Eventually the phytoplankton will use all of the nutrients
available in the surface layer and most of the plankton will die off. When this
happens their cells will fall through the water column, causing a large increase
in the biological material available on the seabed.
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"Copepodkils". Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Copepodkils.jpg#/media/File:Copepodkils.jpg |
When
stratification breaks down at the end of summer, the water column in the Celtic
Sea is again fully mixed. The bottom layer of water is still nutrient rich and
these nutrients are also mixed into the surface of the water column, and become
available for photosynthesis. This causes a smaller phytoplankton bloom at the
end of summer before the days darken, and the cycle is complete.
