Pushing the boundaries of hydrography research
Order for Water Sampling at Sea: a EuroGO-SHIP Best Practice
One of the first and most important lessons a chemical oceanographer must learn and master while observing on a research vessel is the correct order for water sampling when opening Niskin bottles within the Rosette frame. However, that doesn’t mean everybody remembers the right sequence of steps. The EuroGO-SHIP team has produced a short visual reminder to underscore the necessity of following the right order.
Why is important?
Variables that will be sampled and then measured are differentially sensitive to two main issues, warming and air-sea exchange contamination. For example, the headspace left in the Niskin bottle while withdrawing the water is a source of contamination altering seawater dissolved gasses, and warming should be avoided as it will also alter gasses. Therefore, not only should sampling be done in order, starting from the Niskin bottle from the deepest depth, but also quickly and thoroughly.
For certain cruises, for example a GO-SHIP cruise, when there are many variables to be sampled and many people around the Rosette, it is recommended that a coordinator is appointed—a research assistant or a CTD technician—to take an overarching responsibility for sampling sheets, to run checks for any direct issues, such as leaking in the Niskin bottles, and then to assure the sampling order, the identification of the samples, who does what and so on, filling the log sheets accordingly. In other words, somebody to hold the baton and conduct the orchestra of scientists and technicians.
- First: Transient Tracers because chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) have a very small concentration in the deep ocean compared to the atmosphere, and is thus very sensitive to air-sea exchange. In the upper layer and surface the concentrations are about the same as in the atmosphere, but the reason it is first is because tracers are very sensitive to air-sea exchange, similar to oxygen. Since tracers are used as age markers and any contamination of “younger” water easily adds uncertainty.
- Second: Dissolved Oxygen, to avoid the fast air-sea equilibration; and it’s important to wear gloves as chemicals are used directly after the sampling.
- Third: CO2 system variables as Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) because while they are less sensitive to air-sea exchange (albeit, still sensitive), they are usually sampled in the same bottle.
- Four: pH also belongs to the CO2 system variables, but because it is sensitive to air-sea exchange and warming, it is usually sampled independently from DIC and TA.
- Five: Inorganic nutrients are next—always wear gloves when sampling to avoid contamination from sweat and grease from the hands.
- Six: Finally, Salinity samples require careful handling after drawing seawater; the sampling bottles must be cleaned and dried to avoid any formation of salt particles while storing the samples.
An important general tip is to ensure that there is enough water for all sampled variables.
Each variable is tricky, each variable needs a sampling, a conservation, and an analytical and quality control procedure, i.e., a best practices protocol to assure the quality of the final data that is conveyed to data centers’ hydrographic and biogeochemical data
Special thanks for sharing these important best practice tips to:
Caroline Cusack
Work Package: WP 2, 3, 4 and 5
Organisation: Marine Institute, Ireland
Marta Álvarez
Work Package: WP 2, 3 and 5
Organisation: Spanish Institute of Oceanography, Spain
