Did you already manage to analyse the CTD data today? This question Andrea get’s to hear quite frequently. The plots she sends around after analysing the data taken with the CTD probe are anticipated with great suspense. How are the mesocosms doing today?
CTD stands for conductivity, temperature, and depth. Conductivity measurements serve to determine seawater salinity. In combination with temperature this yields seawater density. By measuring hydrostatic pressure the instrument knows at which depth it is. Our CTD probe also houses some other useful sensors, namely for light intensity, pH, oxygen concentration, turbidity, and chlorophyll fluorescence. The latter provides an indication of phytoplankton biomass. On every sampling day the probe is lowered into each mesocosm collecting vertical profiles of each of these parameters.
Last Thursday’s CTD profile. Black lines show profiles of the fjord.
For the mesocosm team the CTD profiles serve as a kind of electrocardiogram for our big babies. They tell us precisely the condition of the enclosed water. Whether it is well mixed or stratified, how our CO2 manipulation worked, how the replicate pH treatments compare to each other, whether or not the phytoplankton bloom is starting, how much light enters our mesocosms, whether there are leaks in the mesocosm bags and so on. The critical inspection of the CTD profiles at the end of each day often causes anxious faces when new problems arise or existing problems could not be solved.
When inspecting last Thursday’s profiles we all cheered in joy. An epical profile, as Lennart put it. All ten mesocosms were well mixed and with similar starting values, the two small leaks we had spotted earlier that week and tried to fix were tight, the phytoplankton biomass was slowly increasing, and above all, the three-step CO2 manipulation worked out just perfectly. When plotted together, the pH profiles of the five CO2-enriched mesocosms came out as one line, right on top of each other. Ten days after we had decided to open the mesocosms and start from scratch, we had finally made it back into the race. It was time for a beer, or two, or …. well, you got the message.