Laser counting in bouncy seas

Latitutde: 60°18′ N
Longitude: 001°00′ E
Water temperature: 7.6
Sea state: waveheight 1.5 meters
by Sunnje Basedow, aboard FS Meteor

Over the last 48 hours we have crossed the Faroe-Shetland Channel, and are relatively sheltered by the Shetland Islands to the west of us.

Waves crashing over the side is a regular event while sampling the North Atlantic.

We have been deploying the Laser Optical Plankton Counter (LOPC). It is a really great instrument for biological oceanographers. It automatically counts and measures all plankton particles in the water, producing volumes of easy to process biological data that do not require tedious counting under the microscope. With the LOPC we can collect data with the same high resolution in space and time than physical oceanographers have been able to collect since the last century.

Biologist Sunnje Basedow assists with deployment of the LOPC frame just before a wave crashes on deck.

Matching up physical and biological data allows us to separate the effect of the physical environment (e.g. water currents) from ecological processes (e.g. how plankton chooses their preferred position in the water column). We then know in much greater detail which factors determine the distribution of zooplankton on small and large scales, and can better predict more accurately how the marine system will react to future changes.

This is how the LOPC works. A metal frame has a laser source on one side, and photo diodes at the other. When the LOPC is towed through the water, plankton passes between the laser and the diodes, and is hit by the light emitted from the laser. The photo diodes on the other side receive the light that is not absorbed by the plankton, and measures the particles shadows. Based on this information, we calculate the size and transparency of all particles suspended in the water. In combination with a few net samples from the MOCNESS (see Anneke Denda`s entry), we can assign zooplankton groups to particle sizes detected by the laser beam.

The only way to deal with a wave of cold water spilling on deck is ... to smile!

Where is FS METEOR today? © The sea surface temperature around Faroe-Shetland Channel. Satellite imagery © NEODAAS/PML/NERC  Chlorophyll A around FS METEOR as seen from orbit. © NEODAAS/PML/NERC

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