"BEAWIS" the Bottom Water Sampler
The BWS (see figures above and below) is a new sampling device which was build with funding from the B2-project in cooperation with the company KUM from Kiel. It was especially designed to collect water samples from different heights above the seafloor which are not in reach of a standard rosette-watersampler. It consists of a three-footed frame with an additional central axis which is revolvable against the frame. Five 5-liter Niskin-bottles are horizontally attached to this axis and can be shifted between 10 and 120 cm above ground. The whole device is attached to a rope from the ship via a revolvable connector and both the frame and the inner axis have current-sails which turn the Niskin-bottles directly into the current. This is necessary to avoid any influences of turbulences developing from parts of the frame on the particle-flow. A burnwire-system connected to a timer which is programmed before deployment closes the Niskin-bottles in a specified time after bottom contact.
During the cruise M57/2 we used a waiting time of 45 minutes to make sure that all the material which was resuspended in succession of the deployment of the device was redeposited or drifted away.
30 m above the BWS a pinger was attached to the rope. The signal of the pinger made it possible to stop the BWS for a couple of minutes at 5 to 10 m (depending on the wave and wind conditions) above the seafloor to give it some time to align with the current conditions close to the seafloor.
Another 20 m above the pinger two floats were attached to the rope, which made it possible to let the rope hang loosely in the water without the risk of the slack rope winding around the BWS. By that it is assured that a movement of the ship doesn't directly result in a movement of the BWS which would go along with an artificial resuspension event.
The BWS is additionally equipped with a SEABIRD SBE 19 CTD profiler (from the division of General Geology, University of Bremen), a NORTEK AQUADOPP current-profiler, a PHOTOSEA 1000 photo-camera system (MPI Bremen) and a MCLANE insitu-pump (from the division of Marine Geochemistry, University of Bremen).
The current-profiler was attached very close to the bottom of the BWS-frame looking upwards into the water column. It can resolve the current-speed and direction in up to 50 cells of 10 cm size, which is important to estimate the transport-velocity of the particles and to calculate the critical shear-velocity which is crucial to estimate the resuspending forces affecting the uppermost sediments. By evaluating the orientation-information from the compass and tilt-sensor of the current-profiler it will be also possible to see if the BWS really turned into the direction of the current while sampling the water.
The position of the CTD was about 50 cm above the seafloor. Aside of the pressure-, temperature- and salinity-sensor there were also an oxygen- and a SEATECH transmissometer connected to the CTD. The figure above shows the depth (converted from the pressure data) and transmission data over the whole duration of a BWS deployment and illustrates the overall process.
The transmissometer was attached to the frame of the BWS about 25 cm above the ground. Beam transmission is a measure of the percentage attenuation of a light signal along its 0.25 m long way from the emitter to a sensor. It operates with light at 660 nm (red) wavelength. The light transmission is directly linked to the particle content of the water. Light transmission in perfectly clean water is 91.3 % for this instrument (information given by the manufacturer). The relative measurements of the transmission cannot be compared directly between different deployments. They will be empirically calibrated by the particle content on the filters later on. But the information is sufficient for a first overview and the localization of the nepheloid layers. It was also used to make sure that the artificially resuspended particle-cloud had disappeared before the closing of the Niskin-bottles. This cloud usually vanished after 5 to 15 minutes depending on the softness of the seafloor, so that the closing of the Niskin-bottles, 45 minutes after the bottom contact of the BWS, was not affected by this process.
The photo-camera system made up to 30 pictures of the lowermost sampling-bottle of the BWS and the surrounding seafloor. One picture was taken per minute or every second minute from the time the BWS reached the seafloor. These photos will also help us to make sure that the resuspended particle cloud had drifted away before the closing of the bottles. With the pictures of the seafloor-structure the relative importance of macrobenthic activity can be assessed. We will also achieve visual informations about the particle environment close to the bottom (e.g. existence of marine snow) and might get a rough estimate on particle-size and quantity of large aggregates.
For further informations concerning the bottom water sampler don't hesitate to contact Maik Inthorn.
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