Seasonal variation in sedimentation of organic materials in the St. Lawrence Island polynya region, Bering Sea

A polynya that forms each winter south of St. Lawrence Island in the Bering Sea is located near very high standing stocks of macrobenthic invertebrates on this Arctic continental shelf, Sampling during Or just after the ice melt in 1990, 1993, and 1994 revealed water-column, sediment particle and chemical distributions that were consistent with a high flux of organic materials to the sediments during the ice retreat. In contrast, lower phytoplankton biomass observed in late summer and fall of 1993, 1998, and 1999 was associated with seasonal declines in sedimentation indicators such as chlorophyll a in the water column and deposited on the surface sediments, lower occurrences of an atmospherically derived radionuclide beryllium-7 (Be-7) in surface sediments, and lower sediment oxygen demand. Moreover, observations under late-winter conditions in April 1999 did not support a direct, continuous linkage between water-column production when the winter polynya is active and the very high benthic biomass and productivity observed on this shallow (30 to 60 m) shelf. For example, deposition of recent water-column production to the sediments in April was significantly lower than that during and just after the ice retreat in May and June. This lower rate of deposition in April was inferred from proxy indicators including photosynthetically-competent chlorophyll extractable from surface sediments, the presence of Be-7 in surface sediments, and sediment oxygen demand, Despite high nutrient concentrations, water-column chlorophyll a concentrations were also uniformly lower (0.5 to 2 mg m(-3)) in April than in May and June, Measurements of the underwater light and ultraviolet radiation field and analysis of DNA damage to dosimeters indicated that the low chlorophyll a biomass is probably not influenced by UV radiation in late winter and early spring, The stable carbon isotope composition of organic carbon in surface sediments suggests that open-water production in the polynya, relative to the adjacent production of sea-ice algae, is not large enough to influence the isotopic composition of organic materials in surface sediments near the polynya.