the high time in any biological field station, buzzing of scientists on their
mission to catch the growth season of nature. The common look of a summer field
biologist: sweaty and sunburned, likely dressed in shorts. All of them? Well……
no. There is a small group of marine scientists that provoke puzzled looks. Their
general appearance winter to summer: double soled fur boots, snow trousers and
woolen head. Pale faces, cold fingers clasped around a pot of hot tea. Weirdos.
I am one of
those thickly wrapped people and I am happy to announce that we are neither
ill nor weirder than everyone else (at least not much); yet, working on biogeochemical processes in
marine sediments means to do this under conditions as close to nature as
possible with in situ temperature as a crucial factor. And as the temperature
at the bottom of the sea is usually cold, so it is in the climate controlled lab.
Summer at Tvärminne Zoological Station: while surface waters warm up (upper pics), bottom waters stay cold - so does my lab (lower pics).
Now you may
say: “Come on, you are not working on open ocean sediments! Get a grip - you
are working at the coast, the water column is much shallower there, shouldn´t also the
bottom water warm up during summer at only 30 m water depth?”
Thanks to stratification
of the water column: not much! While the water column in winter and spring is
mixed from surface to bottom due to colder air and thus water temperatures and frequent storms, it becomes stratified when increasing air temperatures
warm up the surface waters. Warmer water has a lower density than colder water,
resulting in a layering of the water column with warm, light surface waters on
top of cold, heavy bottom waters. These layers don´t mix without strong
external forcing (like a heavy summer storm), which means…..it stays chilly at
the bottom of the sea.
follow the establishment of this year´s summer stratification at Storfjärden
(33m), Finnish coast in our monthly salinity and temperature profiles (data: group Hietanen, unpublished).
In April, the water column is still roughly mixed with only a slight difference in temperature between sea surface and bottom (brrrr, 1.5°C ....). Profiles are taken with a CTD probe (conductivity-temperature-depth).
1 month later in May, surface waters are already 8°C and the water column is layered into a warm surface and a cold bottom layer. This bottom layer becomes even colder one month later, while surface waters become warmer and warmer. Now, in August, bottom waters had reached about 5°C, which felt nearly warm in the climate control lab.
Let´s see how it will be in September and October: a cooling of air temperature will also cool surface waters, weaken the stratification and with the help of a storm break the layering, so that water masses mix again. Maybe some time in autumn I might be able to take off my woolen head.