How do we collect and analyze ocean data?
Oceanographers use a number of different instruments and techniques to collect data. Below are some of the instruments and techniques most commonly used:
(Underlined terms on this page provide opportunities for you and your students to create your own hyperlinks. See Adopt a Link! for more information.) CTD Rosette
CTD stands for Conductivity, Temperature, and Depth recorder. It is an electronic instrument that continuously records the salinity (by measuring conductivity), temperature, and depth (by measuring pressure) as the instrument is lowered on a hydrowire from the ship. The CTD is attached to a frame fitted with a number of large water-collecting bottles called Niskins; the instrument together with the bottles is called the CTD rosette. The Niskin bottles are designed so that they have lids at both ends. They are sent down open, and triggered to close electronically at the depth that water needs to be collected. This way, water can be sampled at many different depths throughout the water column, and kept separate from water collected in other bottles at other depths.
BATS technicians analyze the water that is collected from the CTD for many different things such as oxygen, nutrients, plant pigments, and bacteria abundance.
Plankton Nets
Plankton, Greek for "drifter", are small plants (phytoplankton) and animals (zooplankton) that drift with the ocean's currents. They form the bottom of the food chain in the sea and are very important in ocean food webs. Oceanographers use nets to catch these small creatures and study them. The nets are of a much finer mesh than fish nets, as the mesh openings must be small enough to concentrate the plankton while still allowing water through. Phytoplankton nets have a very small mesh opening (about 36/1000 of a mm) and zooplankton nets have larger meshes (about one 1/3 to 1/2 of a mm). The nets are attached to the hydrowire and towed behind the ship. Plankton tows can be done at any depth or time of day, and can be used with opening/closing mechanisms to enable them to collect at a desired depth.
Sediment traps are cone-shaped or cylindrical collectors that catch detritus that sinks down from the surface ocean to the deep sea. This material is made up of dead phytoplankton and zooplankton, the feces of zooplankton and fish, and many other different kinds of detritus. This material, often termed marine snow, is an important food source for organisms that live in the deep sea, as
well as a mechanism for transporting material from the surface waters to the deep sea where it is eventually decomposed by bacteria. The sediment traps are attached to a line that has floats on the surface and a weight at the bottom to keep it vertical. Some sediment traps have subsurface floats and a bottom weight that actually rests on the sea floor. After several days or weeks, oceanographers recover the traps, weigh the particulate material therein, and analyze the material's chemistry. The quantity of material divided by the collection area and the time the traps were deployed gives the particle flux.
Primary Production ArrayOne important measurement that oceanographers make is the rate of plant photosynthesis in the sea, also known as the rate of primary production. During photosynthesis, phytoplankton take up carbon dioxide that is dissolved in sea water. These tiny marine organisms thus provide the entry point for carbon into the marine food chain. Knowing how fast the phytoplankton grow gives oceanographers an idea of how much carbon they take up, and how rapidly.
To measure the rate of plant photosynthesis at the BATS site, the technicians collect water samples just before sunrise at various depths in the top 140 meters of the ocean. (Below this depth there is generally not enough light for photosynthesis.) They then pour these water samples into transparent bottles and add a tiny amount of radioactive tracer. Attaching the bottles to a line, they release them overboard at the depth from which the water in the bottle was originally collected. At sunset, they retrieve this floating array of bottles, and filter the water to collect any small phytoplankton. They then use a scintillation counter to measure the amount of radioactive tracer that the phytoplankton cells have incorporated during the day. The quantity divided by the number of hours that the bottles were deployed gives the rate of photosynthesis or rate of primary production.
For some purposes, robotic vehicles provide a more efficient, cheaper, and safer means of collecting scientific data. The Autosub is an experimental sea-going robot designed to collect data from the ocean. Engineers are developing the Autosub for many of the same reasons that they developed unmanned spacecraft to collect data from Earth orbit or from the surface of other planets. The ocean surface, like the surface of other planets and moons, can present formidable obstacles to human exploration. Scorching or freezing temperatures, high winds, and large waves can sometimes conspire to make traditional ocean research uncomfortable and even dangerous. Just imagine trying to collect data from the deck of a ship while battling 30-knot winds, 5-meter waves, and sea sickness! The ocean is also vast and deep. Robotic vehicles provide a means to explore large areas of the ocean's surface and depths relatively quickly and inexpensively. Satellites can only view the upper few meters of the ocean's surface.
The
Autosub is being developed by ocean engineers at the Southampton Oceanography Centre in England. A set of 2,500 D-cell batteries power its engine and electronic systems, allowing the vehicle to travel as far as 200 kilometers, at depths of up to 500 meters. Engineers can fit the Autosub with a number of different electronic sensors for measuring temperature, salinity, nutrients, and other ocean properties. They can also program a route into the Autosub's onboard navigational computer, then launch the vehicle from a ship or from land. During its voyage, the Autosub periodically rises to the surface and uploads its data via satellite to researchers who are safe and dry on shore. After the Autosub completes its journey, the vehicle is recovered and prepared for its next mission.Robotic vehicles will never completely replace manned seacraft. There are just too many research situations that require human adaptability and intelligence. But in the future, whole fleets of robotic submarines could be used to gather data from all over the world's oceans, particularly from areas that would otherwise be too expensive or dangerous to study. Scientists could use these data directly, and to help them interpret the data collected from ships and satellites.
Probably 95 % of oceanographic research involves filling bottles and filtering water! At BATS, technicians filter water to catch phytoplankton and extract their photosynthetic pigments to get a measure of plant biomass in the sea. They also filter water to catch small bacteria to count them, or to measure the total amount of particulate matter in the water. Sometime water is filtered to exclude the solid particles, so that dissolved nutrients (e.g. nitrate and phosphate) can be measured.
