After all the struggles that we’ve had in the past the day has finally come – the day of the first deep water addition. When Jan asked me this morning whether I wanted to join in the afternoon for the deep water addition itself I immediately said yes. After all the waiting and the calculations I now wanted to go through with it to the end. I had no idea what I was going for ….
Any guess what those magic numbers on my arm could mean?
To understand all the complications, at first I have to take you through a bit of theory: The goal of this experiment is to simulate a continuous upwelling situation. Therefore, we want to add 4% of previously collected deep water to each mesocosm every second day, which is around 300 L per mesocosm. As the deep water doesn’t contain the required amounts of nutrients and as the experimental setup requires different silicate to nitrate ratios in each mesocosm, some inorganic nutrients need to be added as well. The exact amount of added nutrients depends of course on the nutrients that are already contained in our previously collected deep water. Earlier measurements of the deep water tanks have also revealed that the concentrations in the different tanks are not completely identical.
It takes team work to lower the spider into the mesocosms.
Therefore the day started by remeasuring the nutrient concentrations of the three tanks assigned for today’s deep water addition. For one deep water addition three cubic meter tanks (containing different nutrient concentrations) are required. To make sure that each mesocosm gets identical deep water the plan is to add roughly 100 L of each tank to each mesocosm. We calculated and prepared the nutrient solution accordingly. And by 3 pm we were ready to go down to the pier. We were a team of five: Ulf, Jan, Andrea, Micha and me. In theory the task was easy: Just take 103 L of each deep water tank, put it in each mesocosm, add the nutrients as well and use the spider to distribute the nutrient enriched water evenly in the mesocosms.
First of all the so called “spider” is very heavy as well as unwieldy and is connected to a 50 m tube which is connected to a flow meter and a tank. It takes at least four people and quite a lot of time and effort to install the spider in one mesocosm or to take it out and move it to the next one. That being said we soon realized that it wouldn’t be possible to switch between the deep water tanks while pumping and finish the mesocosms one after another. The solution (you’ve probably guessed it already) was to empty the deep water tanks one after the other which meant we basically did 24 deep water additions (instead of 8) and all the moving around of the spider that was coming with it. Another problem was, that the flow meter we used to measure the added deep water was sitting right behind the tube. As soon as the flowmeter was showing the magical number of 0,1033m3 we would stop the pump but most of the deep water was still sitting in the tube connecting the pump and the spider. To get the water out of the tube into the mesocosm someone had to perform the “tube dance” after each addition: disconnect the tube from the pump and by lifting the tube over ones head while running along the tube pouring all the remaining water in the mesocosm.
The tube dance performed by Micha.
With all that running back and forth, the connecting and disconnecting it had gotten late and sunset was approaching fast. We were cold, hungry and tired already working for 5 hours. Luckily Carsten agreed to bring down warm jumpers and get us some food from a nearby restaurant, while we continued working in the dark.
The late-night delivery service was greeted with joy.
Everyone was tired and Jan and me both wetted our shoes while finishing off the last mesocosm. In the end it was half past ten when we finished. PLOCAN had closed by that time so all our belongings had to stay there overnight. I tumbled home while the others discussed how to improve the deep water addition for the remaining experiment. An addition taking about 7 hours every second day just wasn’t an option!
UPDATE: The team is now proud owner of a three cubic meter tank which allows the premixing of the deep water and an addition of 300 L per mesocosm in one go. After the nightmare of the first addition the second addition two days later took roughly 2.5 hours and was done in broad daylight with the sun accompanying our work.