Carin Ashjian
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SHEBA | SBI Food Webs | SBI Shelf-Basin Exchange

Shelf-Basin Exchange of Plankton and Biogenic Material between the Chukchi and Beaufort Seas
Carin J. Ashjian and Scott M. Gallager, Woods Hole Oceanographic Institution
Mark C. Benfield, Louisiana State University

Transfer of organic material and organisms from Arctic shelves to the deep Arctic basin may impact significantly the biogeochemical characteristics, and ecosystem function and structure, of both Arctic basin and the Arctic shelf ecosystems. To date, little work has been done relating the influence of physical processes such as eddy formation and canyon exchange on biological distributions and the potential cross-shelf transport of organic material in the Arctic Ocean. We hypothesize that transport of biogenic material and organisms between the two regions by mesoscale processes is the dominant mode of exchange between shelf and basin. This cannot be verified through the study of physics alone; we must describe and explain what is found in the advecting water. For zooplankton, physical processes and features interact with biological processes, such as behavior, to produce the observed abundances and distributions. It is critical to increase our understanding of the biological-physical coupling in the shelf-slope region in order to better predict the potential impact of large-scale changes in climate on the Arctic ecosystem.

We are describing the role of mesoscale physical processes in the exchange or transport of biogenic material and plankton populations between the Chukchi Shelf and neighboring Beaufort Sea as part of the Shelf Basin Interactions Phase II field program (SBI). Our project has two distinct yet interrelated components.

For the first component, we are describing the high resolution vertical distribution, abundance, taxonomic and size composition of plankton and particles and aspects of behavior of plankton across transects of the Beaufort/Chukchi Seas shelf-basin interface using a self-contained Video Plankton Recorder (VPR). The VPR is essentially an underwater microscope that photographs the plankton and particles in the water column as the instrument is lowered from the ship. Plankton distributions and abundances are considered relative to hydrography (from a CTD) and velocity (from a ship-mounted acoustic Doppler current profiler) to describe what is in the water and in which direction the water and plankton are moving. We participated in two six week cruises to this region on the USCGC Healy during the spring and summer of 2002 and collected vertical profiles from over 40 stations. A second set of cruises is planned for spring/summer of 2004. During September of 2003, I will be conducting closely spaced vertical profiles using the VPR in collaboration with Bob Pickart (WHOI) to obtain high-resolution horizontal descriptions of plankton and hydrography along a line of moorings on the Beaufort Sea shelf break (Arctic Edge website).

In the second component, we will describe the abundance of zooplankton on the outer shelf and upper slope and in mesoscale physical features that exchange water and biogenic materials between the two regions over two annual cycles using the absolute backscatter intensity data from an array of eight moored calibrated acoustic Doppler current profilers (ADCPs). Mark Benfield calibrated the ADCPs prior to deployment of the instruments. The moorings have been in place for nearly a year. This fall (September), the moorings will be recovered and the data downloaded.

This project is funded by the National Science Foundation and the WHOI Access to the Sea program.
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