Collecting samples at sea#

Planning and collecting water samples#

  • Plan in advance how to obtain salinity samples for calibrating the CTD casts.

  • If your cruise stations are all in shallow waters, consider whether you can afford “calibration casts” in deep waters at the start and end of the cruise.

    • Start calibration cast is more critical than end cast.

  • In general, consider that Niskin bottles average over ~5 m. For accurate comparisons with CTD, we therefore want to sample salinities in areas of the profie where salinity varies slowly with depth (typically below the pynocline, and usually below ~200 m depending on the hydrography).

  • Collect at minimum one sample per cast to >200 m.

  • For deep (<1000 m) stations, collect a salinity sample at least every 1000 m.

Preparing salinity sampling bottles#

  • Rinse bottles prior to use in warm but not too hot water (if the water is too warm it will damage the inner coating).

    • Tap water can be used only if limestone is not affecting the groundwater, if it does use distilled milli-Q water.

    • Do not use bottles with a damaged inner coating (seen as a slightly coloured thin film inside the bottle).

    • After cleaning, store bottles upside down in the crates to avoid deposition of particles.

    • To minimise the need for re-cleaning, place the crates in a plastic bag before transport/storage on open pallets.

Sampling from the Niskin bottles#

  • Rinse bottles (fill with water from the rosette, shake gently, empty the bottle and fill again) three times during sampling before collecting the sample.

  • Fill the sample bottle to the shoulder, leaving some air to allow for thermal expansion.

  • Dry the bottle top with lab towel before sealing the bottle insert and lid.

    • There is no massive rush to get screw taps on.

  • Seal the bottle by instering a plastic insert and screwing on the outer lid.

  • Dry the mouth and neck of the bottle after sealing.

    • This prevents salt crystals from forming on the top of the bottle, potentially contaminating the sample when it is opened for analysis later.

  • Leave bottles for > 24h in the room where salinometer is located for temperature stabilization before processing.