There are many different types of oceanographic equipment, some as simple as a black-and-white disk that is lowered into the water until it disappears (secchi disk), some sophisticated enough to count cells in a stream of water (flow cytometer). This page will give you an introduction to some of the equipment that we use on our cruises at sea.
A CTD is an instrument that is standard for every oceanographic research vessel. It stands for Conductivity, Temperature, Depth, as those are the three main characteristics it measures. However, CTDs usually have many sensors, and can measure many characteristics of the water column.
Common sensors include:
- Conductivity (to measure salinity)
- Pressure (to measure depth)
- Dissolved oxygen
- Fluorescence (to measure chlorophyll as an indicator of photosynthesis)
- PAR (Photosynthetically Active Radiation, a measure of sunlight)
- Transmissivity (a measure of turbidity / how many particles are in the water)
Some CTDs can store the data they collect on interior memory (see the Incubator section below), but the large ones are deployed using a ship’s crane, on a conductive wire that sends the data directly to a computer onboard. The data can be viewed in real-time as profiles, and saved to be viewed later. Real-time viewing of the water profiles allow scientists to determine where to collect water samples (e.g. surface fresh layer; chlorophyll maximum; anoxic bottom water) or deploy other instruments.
Although CTDs can be deployed independently (see the Incubator section below), most CTDs on research vessels are contained within a rosette of Niskin bottles. Niskin bottles are tall cylinders that are arrayed vertically within the protective metal cage that encloses the entire rosette. They have caps top and bottom that are connected to a bungee that keeps them tightly closed. Before launching the CTD / rosette, the caps are opened and connected to a release mechanism that can be triggered from the control station onboard the ship. When the rosette reaches the desired depth, the marine tech or scientist on board pushes a button and the first Niskin bottle will close, collecting a water sample. Since the bottles are vertical and the caps close tightly, the water sample is from a specific, known depth, and it will not get contaminated with other water as it is brought up to the surface. Once onboard, water can be collected from the bottles through a small valve. Rosettes can contain any number of Niskin bottles; big ones have more than 20.
Sediment traps look like giant funnels of mesh fabric. Above the cone are two metal rings that are connected by heavier fabric; below the cone is a cod-end which is a semi-enclosed (there are very fine mesh screens to let water through), detachable container. A trap is deployed in the water column at a chosen depth, and left for however many hours the study requires. During the time that it is deployed, it collects sediment that is falling down through the water column. The rings at the top of the net hold it open, and the cone concentrates the sediment falling down in that area into the cod-end. When the vessel returns to the trap, before lifting it back onboard, a messenger is sent down the line from the surface float. It trips a mechanism that was originally holding the net upright, causing the upper rings to collapse to one side, effectively closing the net so that the collected sediment doesn’t get contaminated with other material when it’s hauled back up through the water column.
Once onboard, the sediment collected in the cod-end is further concentrated by settling, after which it can be used for experiments. The sediment consists of fish poop, dead plankton or algae, and anything else that is heavier than water and unable to swim. A lot of it contains carbon compounds, which can be used as nutrients for any bacteria or other organisms in the water.
The incubator we use on our cruises incorporates a CTD and a sediment trap. The incubator itself is a small metal cage with two cylinders (incubation chambers) that are threaded on one end and have ball-closures on the other. Two small CTDs screw into the bottom of the incubation chambers, to measure water characteristics within them. A sediment trap is suspended over one of the chambers; the other is just open to the sea.
Burn-wires are attached to the ball-closures in the incubation chambers; these can be programmed to “burn” at any time; when that happens, the wire disintegrates and releases the ball, sealing the chambers. Syringes can also be prepped and spring-triggered with burn wires, to inject material into the incubation chambers at some point during the experiment.
The incubator setup is deployed like a regular sediment trap, at a desired depth. The sediment trap funnels sediment into one incubation chamber, while the other chamber is open to the ambient water. At some point the balls close, and the chambers become isolated systems. The syringes can contain any sort of material; more sediment, nutrients, either standard or isotope-labeled for later detection. During the entire experiment, the CTDs are measuring the water within the chambers. In particular, dissolved oxygen and pH are indicators of the rates of respiration occurring in the two chambers.