Imagine you fill a plastic bag from your grocery store with seawater and place it in a bathtub full of tab water. What will happen?
Well, due to the salt dissolved in seawater it is heavier than freshwater. Hence, the bag of seawater in your bathtub will sink. The higher the salt content, the heavier the water. The salt content of seawater, which oceanographers call salinity, can be determined very precisely by measuring its electrical conductivity. In doing so we determined the salinity of the water enclosed in our mesocosms to be 33.3. This tells us that per litre of water there is approximately 33.3 grams of salt in our mesocosms. This is great, because this salinity is typical for oceanic waters. It means that the seawater conditions we have in our mesocosms is representative for much of our world’s oceans.
Well, it turns out that the day we closed our mesocosms, happy about its open ocean salinity, was quite unusual for the fjord. The same night the currents in the fjord turned direction and the next morning the salinity in the fjord was 28. Brackish water, probably a mix of low salinity Baltic Sea water and the more saline North Sea water, was flowing into the fjord. This trend continued and the day after it was down to 23. Ten units difference between inside and outside our bags means that each litre of water in our mesocosms is about 10 grams heavier than the surrounding fjord water. Multiply this by 55.000 litres in the mesocosms and you end up with 550 kg of extra weight. Well, imagine what this did to our mesocosms.
Mission impossible: Because the heavy water expanded the lower parts of the mesocosm bags, their top ends shrank. Too narrow for the cleaning ring to slip through.
As much of the salinity change happened over night, it took us by surprise. The next morning “Wassermann” rushed out to attached extra buoyancy to the mesocosms. In a first step three 100 l buoys were attached to each mesocosm and pulled below the sea surface. As the day continued and lower salinity water kept flushing into the fjord, the extra 300 litres of buoyancy turned out to be insufficient. Another three buoys per mesocosm were attached deeper at the floating frame and filled under water with compressed air. Finally our mesocosms had enough extra buoyancy to carry our precious marine waters.
Is this it? Have we seen it all now? Who knows. Stay tuned.