Having the task of measuring the nutrients feels like a privilege. By mixing some chemicals (for some of which I still find it hard to pronounce the name), you can quantify the concentrations of inorganic nutrients. And those are the key drivers in our experiment here in Gran Canaria. My task has actually three steps: First I filter the samples, second I mix the chemicals and third I measure the absorbance with a spectrophotometer - a big machine which in our lab is considered as a spoiled old lady, who works whenever she feels like it, but when she does it is amazing.
After doing the nutrient measurements for some time you start getting a feeling for the nutrient concentrations just by looking at the colouring of the tubes. So it seemed I could almost do without the spoiled old lady. This worked well at least for the first few additions of nutrient-rich deep water into our mesocosms. When the water samples arrived in the mornings after the additions, the agents used for silicate measurements coloured the samples nicely and in accordance with the treatments we had applied. Things seemed straight-forward.
Test tubes for silicate measurements: the pinker the merrier for the diatoms.
But then suddenly the colouring vanished, although the same deep water addition was applied just as before. What had happened? Did my reagents go bad? Did something go wrong with the sampling? In search for an explanation, I went next door and bothered my microscopy friends, who took an extra sample from one of the mesocosms which should have lots of silicate. A quick look at the sample under the microscope gave the answer. Diatoms, the “big guys”, had hugely increased in their abundance. They completely dominated the phytoplankton community, particularly in those mesocosms that receive high doses of silicate. Diatoms need silicate to build their glass houses that protect them. So obviously they had taken up all the silicate - and along with it the nitrate and phosphate added with the last deep water injection- overnight. To the extent that my test tubes remained colourless the next morning. So from now on I my test tubes may stay colourless, but who knows. At least it means that the nutrients have entered the food web. So now I’m curious to hear from my friends how it changes the plankton community and what effects the different silicate treatments have for the food web. The exciting part of the experiment has just only begun.
Where did the silicate go?