Javier from La Guardia starts his watch

Suddenly things are happening, and it was about time too because we start to feel a bit restless after so many days waiting for the action to start. This morning we saw a whale blow and take a dive very close to the Vizconde, and we have seen three freighters. The depth is increasing again, so we are leaving behind the Atlantic Ridge. The Captain estimates we will start fishing on Monday morning. Today we have a stronger wind than yesterday but overall this has been the best trip to the Grand Banks in years.

One of the freighters we saw this morning

Nair and e-Teo have kept themselves busy this morning taking the CTD out of its box and making it ready. We will start the survey - and hopefully finish it - with this data logger, which has more sensors than the spare. We have already discussed this, but let's go through it again: the CTD data logger is the equipment we throw overboard to collect, among other things, data on conductivity, temperature and water depth. From the English initials of these three words comes the acronym CTD. Have you noticed how much we like throwing over board very expensive equipment? - starting with the fishing gear itself...

Here is Nair doing some hi-tech fix with high quality duct tape

The conductivity indicates the salinity of the water. Those of us who studied with the old system learned in Physics class (it was a time in which there was no fear of calling the different disciplines by their true name) that sea water is an electrolyte, i.e. it conducts electricity. And the more saline, the better. For those of us who splash in the sea, whether we are biologists or oceanographers, this information is very interesting, because not all the water in the oceans is the same. The seas and oceans are composed of very diverse water masses in continuous movement that do not mix, because different salinities and temperatures translate into different densities: the denser water masses sink to the bottom and the less dense water masses circulate above them. The wind plays a key role, moving the water masses on the surface, from the equator to the poles. There the water cools and sinks to the bottom. You can find more information on these phenomena by searching in the internet "global ocean circulation", and I recommend it because it is an exciting subject and closely related to climate. Sometimes you can see where two bodies of water converge, as in river mouths, even on the high seas. But sometimes it is necessary to collect data with these sounders to detect the different water masses.

I said that the sensor measures depth, but physicists must be laughing. Actually many data loggers measure the pressure, and depth is deduced from it. To me the approximation that an atmosphere is equal to a meter suffices but for certain studies more precision is needed.

The CTD data logger. Temperatura= temperature, Conductividad= conducivity, Presión=pressure, Fluorescencia=fluorescence

Our data logger also includes a fluorescence sensor, which records data on chlorophyll content (photosynthetic systems emit fluorescence) and dissolved oxygen. These data can be used to estimate primary productivity, which as you know is due to phytoplankton. As for dissolved oxygen, it is not an issue in this survey, but in coastal waters it can be a problem. Many processes occur in water that produce or consume oxygen, and if consumption exceeds production it may led to anoxia, or lack of oxygen. And that is a problem. For example, increasing the temperature of water and dissolved organic matter leads to a decline of dissolved oxygen in the water.

To wrap up, this data logger will take measurements of temperature, conductivity, pressure, fluorescence and dissolved oxygen at one second intervals (equals to 1 m depth intervals). When we cast it at 100m it will take a hundred measurements and when we cast it at 1200m, it will take 1200 measurements of each parameter.

When we start using the CTD, after each cast Nair will have to download the data and check the profile obtained to confirm that the cast has been good. If so, she will have a plot with temperature and salinity curves without interferences. If not, we'll have to curse a little bit until we solve the problem. Well, they -Nair and e-Teo- will have to curse, after all it is them who are in the CTD arena.

Nair will give me at the end of the survey 115 files that we call "hex" for their extension and to which in fact the English word "hex" is very fitting. If you open them in a simple text application you see a heading with very detailed information on the cast followed by something looking like gibberish. There is one hex file per cast.

A hex file from hell

These files are sent to a colleague in Madrid (physicist / oceanographer) along with additional information on the sounder configuration. She prepares and loads them into a European oceanographic data portal called SEADATANET. When I get the data from that website they are in an intelligible format that I can export into another program to analyse them.

The SEADATANET portal

A text file with data from a CTD cast. First column is depth, third is density, fifth is temperature and seventh is salinity.

So as you can see this small piece of equipment can keep quite a number of people very busy indeed. But during this survey the CTD is going to behave. Otherwise, we'll throw it overboard.