9/30/1997
Susan M. Henrichs
Objective and approach:
The project objective is to test the hypothesis that the temporal variation of the quantity and composition of sinking particles depends on interactions among weather, climate, and geographic location over the Southeast Bering Sea shelf. Since the composition of the sinking organic matter reflects the pelagic food web, the research will improve understanding of grazing of ice-edge and open water primary production and of the transfer of water column production to the benthos.
Moored sediment traps are being used to examine temporal variability, on time scales of weeks to years, in the source and quantity of sinking particles. A swimmer (zooplankton)-excluding trap designed to minimize trapping efficiency artifacts (Peterson et al., 1993) was chosen for this study. The sediment trap samples will be analyzed for carbon and nitrogen stable isotope composition. The stable isotope composition of sinking particles is expected to reflect the rate of photosynthesis, extent of nutrient depletion, and the trophic level of animals supplying particulate matter to the trap. The sediment trap samples will also be analyzed for wax esters, triacylglycerols, and sterols, lipid biomarkers, which could allow identification of the plant and animal sources of trapped material. Stable isotope and lipid composition of zooplankton and phytoplankton samples collected near the mooring sites are being determined to facilitate source identification.
Progress to date:
Two traps were deployed in late April, 1997, at sites M2 (56 degrees 53' N, 164 degrees 02' W) and M3 (56 degrees 04' N, 166 degrees 20' W), and recovered in late September, 1997. One trap (at M3) was significantly damaged during the bad-weather recovery. That trap contained no usable samples, apparently because of a motor failure. Eleven samples, collected at intervals of 1 or 2 weeks, were recovered from the other trap at M2. A third sediment trap, funded through a cooperating research project (NOAA-CIFAR, "Composition of Sinking Organic Matter at the Ice Edge of the Southeast Bering Sea", S. Henrichs, P.I.) was deployed at site M2 in late September, 1997; this trap will collect samples at biweekly to monthly intervals during the winter and early spring.
Phytoplankton and zooplankton samples were collected at the five "X" stations near each of the sites M2, M3, and M4 (57 degrees 52' N, 169 degrees 12'W) during June, 1997, and subsamples of single zooplankton species were sorted from these samples. Zooplankton samples were also collected near the mooring sites during September, 1996 (provided to this project by Dr. Jeff Napp); February, 1997; and April, 1997. Bad weather prevented sampling during September, 1997. The analysis of plankton samples for stable isotope and lipid composition is in progress.
Reference: Peterson, M. L., P. J. Hernes, D. S. Thoreson, J. I. Hedges, C. Lee, and S. G. Wakeham. 1993. Field evaluation of a valved sediment trap. Limnol. Oceanogr. 38: 1741-1761.