Instruments 1 or: Everything you Never Wanted to Know About CTDs

We’ve spent about a week onboard, and as I’m settling into shipboard life, I’ll try to post more often.

The most used sampling instruments are the CTD (conductivity, temperature, depth) rosettes. These have two functions – collecting water samples and measuring some basic parameters within the water column. As the instrument is lowered, it constantly records salinity, temperature and depth information, along with data on light and chlorophyll levels, which we use to decide where sample. Usually, we sample at the surface, the deepest point in the cast (anywhere from 200 to 2,500 meters), the chlorophyll max, and some predetermined light levels.

The samples are collected with Niskin bottles. The tops and bottoms remain open through the downcast, where we use the data to decide on the best depth sample. The bottles are individually closed when electronic actuators are tripped, which releases the end caps, sealing the bottle tight. On deck we sample through a spigot into our various sampling bottles. We have two CTDs - one with 24 ten liter bottles, and one with 12 thirty liter bottles.

When we’re sampling we need to follow a specific order. First the researchers studying dissolved gasses get to sample, because as the bottle empties, there’s more air to contaminate the samples. Other chemistry, such as dissolved organic matter comes next, since it’s easy to contaminate the spigots. Then we collect samples that need to be processed quickly, such as chlorophyll a, and organisms that need to stay alive for experiments. Finally, samples that need as much water as possible, aren’t time dependant, or aren’t easily contaminated get whatever remains.

Most scientists onboard use water from the CTDs, and that can lead to some drama when too many people want water from a limited number of CTD casts. Tish Yager, the head scientist, has done a wonderful job making sure everyone who needs water gets it, sometimes adding other CTD casts to keep everything running smoothly. For example, earlier this week, a CTD cast ran out of water before we could sample for biogenic silica or TEP (transparent exopolymer particles), two parameters that the carpenter lab uses. Now we have our own CTD cast every station, which other groups have begun to take advantage of and to get some of the water that remains. As people get into a more consistent work schedule, these problems will be less common.