Reigstad, Marit¹, Tatjana Ratkova², Kalle Olli³ and Paul Wassmann¹

¹Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway, Tel: +47 776 44525, Fax: +47 776 46020, E-mail: maritr@nfh.uit.no, ²Shirshov Institute of Oceanology, Academy of Sciences of Russia, Nakhimovsky Avenue 36, 117851 Moscow, Russia and ³Institute of Botany and Ecology, University of Tartu, Lai 40, 51005 Tartu, Estonia

On the regulation of export production in the twilight zone and the significance of small cells for the vertical POC-flux

The current lack of adequate investigations of the vertical export above the 200-500 m depth, where the majority of the long-term sediment traps have been deployed, result in significant difficulties to understand and model the carbon flux. The quatitatively important processes shaping the export production from the euphotic zone to the vertical flux measured at depth, takes place in the upper layer, often over very short distances. This upper region has been called the twilight zone.

Using high resolution sediment trap deployments (short term deployments every 10-50 m, from 30-200 m) we show that the curvature of the C-export production is variable, reflecting the physical and biological regime in the upper layer. The grazer community as well as the depth of the upper mixed layer is of importance determining the curvature and the C–flux reduction below the euphotic zone. In the central Barents Sea, vertical POC-export decreased from 1000 to about 300 mg POC m^-2d^-1 over a 30 and 60 m depth interval in shallow mixed Arctic and deeper mixed Atlantic water masses, respectively.

Larger and fast sinking particles have long been considered the main contributors to vertical export production. Their logic appearance and visible dominance in the sinking material has supported this view. However, through close examination of the settling material we have found that small plankton cells like flagellates can contribute significantly, and even dominate, the vertical carbon flux. Their role in the vertical carbon export is strongly underestimated.