Oceanographers studying the chemistry of seawater in three dimensions need samples from various depths. A hydrocast carousel is the workhorse that collects these sips from the deep. Attached to a hydrowire cable (1), the carousel dives off the side of a vessel and can reach depths of a mile or more beneath the surface.
Before deploying the sampling gear, researchers program a small computer housed in an aluminum cylinder (2) with the depths at which they want to collect samples. While the carousel is in the water, a Conductivity/Temperature/Depth (CTD) profiler (3) continuously records temperature, salt content, and depth.
When the CTD indicates that the sampler has reached a requested depth, an electromagnet in the latch mechanism (4) releases the caps of a Niskin bottle (5), sealing a water sample inside. In some cases, an attached fluorometer (6) collects data on the water’s chlorophyll content, helping researchers understand the vertical distribution of photosynthetic plankton.
Once the carousel is back on board, researchers analyze the water samples to determine their chemical properties such as the amount of dissolved oxygen gas and the levels of biologically important nutrients, including phosphates, sulfates, and nitrates.
Hydrocasts will provide samples for GEOTRACES, a decadelong international program to measure trace elements like iron and cobalt in the oceans.