Silicon dynamics within an intense open-ocean diatom bloom in the Pacific sector of the Southern Ocean

An intense diatom bloom developed within a strong meridional silicic acid gradient across the Antarctic Polar Front at 61 degreesS, 170 degreesW following stratification of the water column in late October/early November 1997. The region of high diatom biomass and the silicic acid gradient propogated southward across the Seasonal Ice Zone through time, with the maximum diatom biomass tracking the center of the silicic acid gradient. High diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of January 1998 (ca. 70 d) driving the gradient over 500 km to the south of its original position at the Polar Front. The bloom consumed 30 to > 40 muM Si(OH)(4) in the euphotic zone between about 60 and 66 degreesS leaving near surface concentrations <2.5 muM and occasionally <1.0 muM in its wake. Integrated biogenic silica concentrations within the bloom averaged 410 mmol Si m(-2) (range 162-793 mmol Si m(-2)). Average integrated silica production on two consecutive cruises in December 1997 and January 1998 that sampled the bloom while it was well developed were 27.5 +/- 6.9 and 22.6 +/- 20 mmol Si m(-2) d(-1), respectively. Those levels of siliceous biomass and silica production are similar in magnitude to those reported for ice-edge diatom blooms in the Ross Sea, Antarctica, which is considered to be among the most productive regions in the Southern Ocean. Net silica production (production minus dissolution) in surface waters during the bloom was 16-21 mmol Si m(-2) d(-1), which is sufficient for diatom growth to be the cause of the southward displacement of the silicic acid gradient. A strong seasonal change in silica dissolution: silica production rate ratios was observed. Integrated silica dissolution rates in the upper 100-150 m during the low biomass period before stratification averaged 64% of integrated production. During the bloom integrated dissolution rates averaged only 23% of integrated silica production, making 77% of the opal produced available for export to depth. The bloom ended in late January apparently due to a mixing event.Dissolution: production rate ratios increased to an average of 0.67 during that period indicating a return to a predominantly regenerative system. Our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around Antarctica also occur in the deep ocean near the Polar Front. The bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of Antarctica implying a role for such blooms in the formation of those sediments. Comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the Southern Ocean, which have been corrected for sediment focusing, indicate a burial efficiency of less than or equal to4.6% for biogenic silica. That efficiency is considerably lower than previous estimates for the Southern Ocean. (C) 2001 Elsevier Science Ltd. All rights reserved.