Biogeochemical responses to late-winter storms in the Sargasso Sea, II: Increased rates of biogenic silica production and export

Previous studies measuring biogenic silica production in the Sargasso Sea, all conducted when no phytoplankton bloom was in progress, have reported a mean rate of 0.4 mmol Si m(-2) d(-1) and maximum rate of 0.9 mmol Si m(-2) d(-1), the lowest rates yet recorded in any ocean habitat. During February/March of 2004 and 2005 we studied the effects of late-winter storms prior to seasonal stratification on the production rate, standing stock and vertical export of biogenic silica in the Sargasso Sea. In 2004, alternating storm and stratification events provided pulsed input of nutrients to the euphotic zone. In contrast, nearly constant storm conditions in 2005 caused the mixed layer to deepen to similar to 350 m toward the end of the cruise. Biogenic silica production rates in the upper 140 m were statistically indistinguishable between years, averaging similar to 1.0 mmol Si m(-2) d(-1). In early March 2004, a storm event entrained nutrients into the euphotic zone and, upon stabilization, vertically integrated biogenic silica in the upper 140 m nearly doubled in 2 days. Within 4 days, 75-100% of the accumulated biogenic silica was exported, sustaining a flux to 200 m of similar to 0.5 mmol Si m(-2) d(-1) (4 x greater than export measured during February and March in the mid-1990s). In 2005, destabilization without stratification increased biogenic silica flux at 200 m up to two-fold above previously measured export in late winter, with little or no increase in water-column biogenic silica. Despite comprising <5% of total chlorophyll, diatoms accounted for an estimated 25-50% of the nitrate uptake in the upper 140 m and 35-97% of the particulate organic nitrogen export from the upper 200 m during both cruise periods. These previously unobserved brief episodes of diatom production and export in response to late-winter storms increase the estimated production and export of diatom-derived material in the Sargasso Sea in late winter by >150%, and increase estimated annual biogenic silica production in this region by similar to 8%. (C) 2009 Elsevier Ltd. All rights reserved.