Abstract:
The factors affecting the timing, quantity and fate of southeastern Bering Sea primary production, including coupling to higher trophic levels, were investigated via sinking particles collected using sediment traps. Stable carbon and nitrogen isotopic composition and lipid composition of zooplankton and sinking particles were measured for middle (M2) and outer (M3) shelf samples collected during 1997-2000. The quantity collected by the M2 sediment trap was high in late summer and fall, as well as during spring blooms, and was much greater than that collected at M3. M2 zooplankton and trap samples were enriched in ¹⁵N and ¹³C over those from M3. This could be explained by greater primary productivity over the middle shelf, associated with consumption of more of the available inorganic nitrogen. M2 sediment trap samples contained more fatty acids typical of diatoms, while M3 samples contained more fatty acids typical of zooplankton. Diatoms were much more numerous in the M2 than the M3 trap. Cholesterol was the dominant sterol, indicating that much of the material in the traps was fecal matter. During two of the years sampled, 1997 and 1999, ice edge blooms occurred from late April to early May. Ice receded earlier in 1998 and 2000, so phytoplankton bloomed in open water in late May. Lipids indicating greater phytoplankton input were high in the M2 trap during the ice-edge bloom years. Conversely, in 1998 and 2000, there was greater coupling between phytoplankton and zooplankton, much of the material collected was fecal pellets rather than intact diatoms and lipids were more characteristic of zooplankton sources. In zooplankton some monounsaturated fatty acids decreased sharply between February and April, reflecting mobilization of lipids for egg production. A polyunsaturated fatty acid characteristic of prymnesiophytes was elevated in winter and spring 1998 zooplankton, resulting from grazing of the 1997 coccolithophorid bloom. Overall, the results indicate that primary productivity is greater at M2 than at M3. Much of the annual primary production occurs outside of the April-May spring bloom period. Grazing of primary production is greater at M3 than M2, and at M2 more primary production is grazed in warm than cold years.
