Catching the Rain: Sediment Trap Technology

Article excerpt

WHOI Senior Engineer Ken Doherty developed the first sediment trap in the late 1970s for what has come to be known as the WHOI PARFLUX (for "particle flux") group. Working closely with the scientific community, Doherty has continued to improve sediment traps for two decades, and these WHOI-developed instruments are widely used both nationally and internationally in the particle flux research community. Because ocean particle flux research encompasses nearly all areas of biogeochemistry, the trapped samples are shared by many programs to take advantage of interdisciplinary cooperative research. This is one reason why the trap opening must be relatively large, to collect enough samples to share with many programs. Many time-series sediment traps have openings at least half a meter square, which results in an instrument about one meter in diameter - time-series traps may be the largest instruments routinely deployed along moorings. However, we do expect that improved analytical methods will allow future reduction in the size of time-series traps used for multidisciplinary research.

Sediment traps require great mechanical strength, as they are often deployed for as long as a year or more, and then redeployed immediately after recovery of samples. Titanium is used extensively where strength is a critical requirement, such as in the structural frame and in the chamber that houses electronics under great pressure. Titanium is light yet as strong as steel and has virtually no reaction to seawater. Various industrial plastics that are very stable in the deep ocean are also used. The opening and closing of a trap requires a reliable mechanism with many moving parts. The shorter the open period, the better the temporal resolution. However, a shorter collection period requires more collection cups, and there is a limit to the number of cups a trap can accommodate. For a given trap diameter, a short open duration catches less volume of sample. Balancing the need for high temporal resolution with the engineering limitations, many scientists have set a unit duration of about two weeks, and cover a year with some 20 open periods. Sampling cups are opened sequentially: As one cup completes its open period, it rotates away from the base of the funnel opening and a new cup moves to the position, allowing no break in the collection of raining particles. Other than the one that is open, cups are sealed from the ambient water until recovery. In an effort to find a method to keep organic-matter samples in the trap in the best possible condition until they can be retrieved, scientists have tested many preservatives, and a formalin solution is now considered best for general purposes.

Through control technology, Doherty's design allows a scientist to program the entire open/close schedule into a trap's microcomputer from a laboratory PC before deployment. This enables the scientist to use, for example, periods of exactly equal duration throughout the experiment, to open/close frequently during an expected bloom season, or to sample for longer periods in winter when the ocean is less active. …