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Sediment Traps

Sediment trap recovery onboard RV Maria S. Merian during expedition MSM134. @J.C. Faust

Abbildung zeigt
Two sediment traps on deck. Photo: MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences, Uni­versity of Bre­men; M. Klann
Illustration shows
Deployment plan of a sediment trap mooring. Scheme: MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences, Uni­versity of Bre­men; G. Ruhland
Recovered sediment trap samples
Recovered sample bottles. Photo: MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences, Uni­versity of Bre­men; M. Klann

Sediment traps

Sediment traps as used in a marine science context are devices that intercept the flux of descending particles in the water column. Sediment traps can be deployed for a short time interval as drifters or as part of a mooring with fixed location for many months or even years. The sediment traps used in mooring are usually large, about 2 m high, funnels with an aperture size of 0.5 m² mounted in a frame. Below the funnel sits a motorized carousel with 20 or more sample bottles that activates in programmed intervals, advancing the carousel and thus changing the sample bottle collecting the flux entering the funnel. Before deployment the sample bottles are filled with artificial hyper-saline seawater and poisoned with mercury chloride. The increased density reduces mixing of the poisoned water with the ambient seawater and the poison prevents the degradation of collected organic material by bacteria and other organisms. It also kills any small zooplankton organisms that might have entered the sample bottle actively. A grid is usually mounted in the aperture of the funnel to prevent larger organisms such as small fish from entering the device.

Sediment trap moorings

A sediment trap mooring consists of one or more sediment traps mounted on a single mooring line fixed to the seafloor by a ground weight. The mooring line is held upright in the water column by floatation devices mounted in several positions along the mooring line including the very top. Next to the sediment traps a variety of sensors, such as temperature and conductivity meters or acoustic doppler profilers for current measurements can be installed on the mooring line. Just above the bottom weight one or two acoustic releases is installed in the mooring line. These devices separate the mooring line (and themselves) from the ground weight after receiving the correct acoustic signal from a research vessel. After a successful detachment from the bottom weight, the whole assembly rises to the surface and can be collected by the research vessel. To assist in this procedure the top buoy of the mooring is usually equipped with a satellite transponder that activates upon emergence and transmits a positioning signal that can be followed by the research vessel.

Cape Blanc sediment trap observatory

MARUM maintains a set of sediment trap moorings since 1988 to study the fluxes of marine particles in the productive waters off Cape Blanc in Mauretania, West Africa. The material and data derived from the Cape Blanc sediment trap observatory is available to a consortium of researchers at MARUM (and also other institutes via collaboration) and has so far been employed in dozens of publications covering a broad range of flux associated topics such as quantitative investigations of decadal-scale changes in productivity, lithogenic flux and the composition of marine plankton. Particularly in a time marked by unprecedented environmental challenges, sediment trap records serve as a crucial indicator of change.

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The location of the Cape Blanc sediment trap observatory.