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The KOSMOS spider

Getting the volume right

 Anna Sophie Fiesinger Sep 10, 2019 2 min read

As we have seen from previous blog posts, this experiment doesn’t like to keep us on schedule. According to plan we were supposed to add the deep water treatment to each mesocosm tomorrow. Therefore, the ship we had managed to charter for getting the deep water left Taliarte harbour yesterday morning and came home with having accomplished the task after 14 rough hours at sea. However, the deep water is added to each mesocosm as a percentage of the whole volume of the mesocosm as to ensure replicability among them. Unfortunately, up to date we do not know the exact volume of each mesocosm. Let me tell you why …

Before starting the experiment we had to fill the mesocosms. As we did that some of our flow meters that were supposed to measure the flow rate and the volume of the water flowing into each mesocosm gave us a little headache. Some reset themselves during the filling with no way of knowing where they had left off and how much water was already in the mesocosm. One other shut down completely without a warning. Since we had the mean flow rates for each mesocosm before and after the resets, we could roughly estimate the amount of water pumped into those mesocosms where the flow meters had problems. But is that good enough to rely on for this experiment? The past days - unnoticed by many - the heads of the experiment sat together debating how to solve the problem of the volume determination of each mesocosm. This morning we were let in on all the debates.

Voume determination(1) Deionized freshwater is added into the mesocosms with the so-called spider, which is slowly moved up and down and thereby distributes the injected water evenly throughout the enclosed water body.

Sitting neatly at home was the equipment used in previous mesocosm campaigns for volume determination: the approach uses addition of salt to the mesocosm water. Salt changes the chemical and physical properties of water. Thus, when adding a small amount of salt brine (a highly saline solution) to the mesocosms and precisely measuring the salinity before and after adding the solution we can back calculate the volume that was needed to generate this change in salinity, i.e. the initial volume of the mesocosm. Unfortunately, as we didn’t have the equipment at hand we needed to revert to a different method. By adding deionized freshwater to a body of water you can basically do the same as with the salt brine. The fresh water dilutes the seawater and lowers its density, whereas the addition of salt does the opposite. Through this method the volume of each mesocosm was determined to be between 9.5 - 10.5 m3. But hold on. Something doesn’t quite add up there. According to the manufacturers, each mesocosm is able to hold about 8 m3 of water. So what happened? Did the fresh water not properly mix with the rest of the water? Could that explain the offset? This afternoon our scientific divers went down to measure precisely the dimensions of each mesocosm. Because the mesocosm bags are fully filled, the walls are completely straight and have an almost ideal geometric shape. So knowing their dimensions, we could calculate the volume. It turns out to be close to estimates from the flow meters and to the 8 m3 the manufacturer claims they are able to hold. So what is it that happened with the fresh water addition? We might be back with the solution of that mystery. Or we might never find out. Stay tuned to find out more about the struggles we face as scientists when our equipment doesn’t do what we want it to do. But don’t be alarmed, we will show you the beautiful sides of being a scientist as well. And let me tell you, there are many!

Volume determination(2) The spider at the start.