Project Title: Isotopic and Biomarker Composition of Sinking Organic Matter in the Southeast Bering Sea: Indicators of Food Web Structure

Principal Investigator: Susan M. Henrichs, Institute of Marine Science, University of Alaska, P.O. Box 757220, Fairbanks, AK 99775-7220.

Participating Investigator: Stacy Smith, Graduate Program in Marine Science and Limnology, University of Alaska, P.O. Box 757220, Fairbanks, AK 99775-7220.

Objectives and methods: 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. The sediment trap samples recovered so far have been 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. During Fall, 1998 the sediment trap samples will also be analyzed for wax esters, triacylglycerols, and sterols, biomarkers which could allow identification of the specific 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.

Two traps were deployed in late April, 1997, at sites M2 (56^o53' N, 164^o02' W) and M3 (56^o04' N, 166^o20' 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 and successfully recovered in February, 1998 with eleven, biweekly samples. The sediment traps were redeployed at M2 and M3 in February, 1998, and will be recovered in October, 1998.

Phytoplankton and zooplankton samples were collected at the five "X" stations near each of the sites M2, M3, and M4 (57^o52' N, 169^o12'W) during June, 1997 and May, 1998. 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 1998; and April, 1997 and 1998. The analysis of plankton samples for stable isotope composition has been completed and lipid analysis is in progress.

Results and Discussion: From April 22 to May 6, the identifiable constituents of the sediment trap sample were mostly diatoms, including Fragilariopsis oceanica, along with some small flagellates. From May 6 to May 20, diatoms were again the main component, but Leptocylindrus spp. were the most numerous. From May 20 to July 15, the sample contained few intact diatoms and more amorphous material, and the quantity of material collected was much less. The d¹⁵N of the trapped material was relatively low from April 22 to May 6, 12.2 â, maximized at 15.6 â from May 20 to 27, then declined to about 13.5 â from June 17 to July 15. The d¹³C decreased from an average of -20.5 â in the April 22 to May 20 samples to ó21.9 â in the July 1 to 15 sample. Overall, these patterns indicate that the trap collected substantial intact phytoplankton in late April and early May, following the spring bloom. Late May through early July material probably contains more zooplankton-derived organic matter. Middle shelf Thysanoessa raschi had d¹⁵N of about 10.2 â and d¹³C of ó22.1â; Calanus marshallae had d¹⁵N of about 12.7 â and d¹³C of ó24.0 â; Sagitta elegans had d¹⁵N of about 15.5 â, since it is carnivorous, and d¹³C of ó20.4 â. In addition to trophic level effects on d¹⁵N of sediment trap material, nutrient depletion could have contributed to increasing values. However, the zooplankton isotopic composition did not differ between Feb., 1997 and June, 1997, making this explanation less likely.

Links between sediment trap and plankton isotopic composition will be easier to discern if trap samples are successfully recovered from M3 this fall. The zooplankton near M3 are 2-3 â lighter in d¹⁵N than those near M2 and M4, consistent with a broad regional pattern observed earlier and attributed to increasing nutrient depletion shoreward (Schell, D. M., B. A.Barnett, and K. Vinette, 1998, Mar. Ecol. Prog. Ser. 162:11-23). Also, the isotopic data suggest that d¹⁵N of zooplankton were lighter in 1998 than 1997, perhaps related to the lack of stratification and delayed spring bloom in 1998.

The July 15 to 29 and July 29 to August 12 samples contained large numbers of intact diatoms, comparable to the spring samples, and the amount of organic carbon collected by the traps was also similar. The diatoms were mainly Detonula spp., and Odontella aurita was numerous in early August. Small flagellates were also abundant in both samples. The d¹⁵N increased slightly, to 14.2 â, in the early August sample, while the d¹³C increased to ó20.8 â in the late July sample. The August 12 to September 1 sample had similar isotopic composition, but fewer diatoms, with flagellates numerically dominant. This suggests that the late July-August isotopic composition mainly reflects that of the flagellates, which appeared to be heterotrophic as they lacked chlorophyll.

Through September 1 no coccoliths were observed in the M2 sediment. However, coccoliths were found in sediment trap samples representing September 22 to 29 and September 29 to October 13. Coccoliths of Emiliania huxleyi were abundant in both samples, giving them a "milky" appearance distinct from the brownish coloration of samples collected at other times. The coccoliths were much more numerous in the September 29 to October 13 sample; the September 22 to 29 sample had some pennate diatoms and fecal pellets as well. The September occurrence of the coccoliths in trap samples, rather than earlier, suggests that there was little grazing of the summer bloom. The trap collection of coccoliths appears coincident with the onset of fall mixing of the water column. The October 13 to 27 sample did not contain obvious coccoliths when examined by SEM, but this sample contained a lot of reddish-brown, amorphous material which might have obscurred the coccoliths if they were not very numerous. This sample also contained numerous flagellates. The October 27 to November 10 sample contained a small number of flagellates, diatoms and amorphous particles. The November 10 to 24 sample contained a large amount of resuspended bottom sediment, due to a very severe storm which occurred during that period. The trap was moored at about 35 m depth in a 70 m water column, so the sediment resuspension event was apparently quite massive. Subsequent samples through December 22 contained broken diatom tests, which probably also were resuspended from the sediment. Samples collected from December 22 to February 16 contained almost nothing.

The d¹⁵N of the September 22-29 sample was unusually light relative to earlier and later samples. This may represent an influx of relatively light, "new" nutrients to the phytoplankton due to fall mixing, as well as the sinking of phytoplankton (coccolithophorids and pennate diatoms) observed in the traps. Subsequent samples had heavier d¹⁵N, ranging from 11.9 to 13.2 â, but these values averaged about 1 to 2 â lighter than those found in summer trap samples. The d¹³C values in the fall-winter samples, ranging from ó21.7 to ó22.4 â, also averaged about 1 â lighter than summer samples.

During the fall, the sediment traps collected a quantity of organic matter that was comparable in magnitude to that collected during Spring 1997. Because only the 1997 data are available, it is not clear whether this linked to the unusual coccolithophorid bloom, or whether autumn conditions result in substantial sinking particulate material in more normal years as well.

Material collected by the sediment traps clearly reflects primary and secondary production processes in the water column. Further, the traps reveal events that were not apparent from time-limited shipboard sampling or other moored instruments, such as the apparent diatom bloom in late July.

Publications and presentations:

Goering, J. J., S. M. Henrichs, T. Rho, S. Smith, T. E. Whitledge, C. T. Baier, R. D. Brodeur, D. M. Blood, J. M. Napp, J. J. Cullen, R. F. Davis, J. D. Schumacher, P. J. Stabeno, G. L. Hunt, Jr., and G. L. Swartzman. 1998. Southeast Bering Sea Carrying Capacity (SEBSCC): Ecosystem dynamics research in a marginal sea. 7^th PICES Meeting, Fairbanks, AK (Abstract submitted).

Napp, J. M., C. T. Baier, R. D. Brodeur, J. J. Cullen, R. F. Davis, M. B. Decker, J. J. Goering, C. E. Mills, J. D. Schumacher, S. Smith, P. J. Stabeno, T. C. Vance, and T. E. Whitledge. 1998. The 1997 eastern Bering Sea shelf-wide coccolithophorid bloom: Ecosystem observations and hypotheses. AGU/ASLO Ocean Sciences meeting (Abstract).

Smith, S., and S. M. Henrichs. 1998. Isotopic and microscopic analysis of sinking particulate matter on the Bering Sea middle shelf. 49^th AAAS Arctic Science Conference (Abstract submitted).

Smith, S., and S. M. Henrichs. 1998. Plankton collected by a time-series sediment trap deployed in the southeast Bering Sea during 1997. 7^th PICES Meeting, Fairbanks, AK (Abstract submitted).